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Why do we need abstract methods?

I have been studying abstract methods lately and I can't understand why do we need them?
I mean, after all, we are just overriding them. Do you know its just a declaration? Why do we need them?
Also, I tried understanding this from the internet and everywhere there's an explanation like imagine there's an abstract class human then there're its subclasses disabled and not disabled then the abstract function in human class walking() will contain different body or code. Now what I am saying is why don't we just create a function in the disabled and not disabled subclasses instead of overriding. Thus again back to the question in the first paragraph. Please explain it.

One of the most obvious uses of abstract methods is letting the abstract class call them from an implementation of other methods.
Here is an example:
class AbstractToy {
protected abstract String getName();
protected abstract String getSize();
public String getDescription() {
return "This is a really "+getSize()+" "+getName();
}
}
class ToyBear extends AbstractToy {
protected override String getName() { return "bear"; }
protected override String getSize() { return "big"; }
}
class ToyPenguin extends AbstractToy {
protected override String getName() { return "penguin"; }
protected override String getSize() { return "tiny"; }
}
Note how AbstractToy's implementation of getDescription is able to call getName and getSize, even though the definitions are in the subclasses. This is an instance of a well-known design pattern called Template Method.

The abstract method definition in a base type is a contract that guarantees that every concrete implementation of that type will have an implementation of that method.
Without it, the compiler wouldn't allow you to call that method on a reference of the base-type, because it couldn't guarantee that such a method will always be there.
So if you have
MyBaseClass x = getAnInstance();
x.doTheThing();
and MyBaseClass doesn't have a doTheThing method, then the compiler will tell you that it can't let you do that. By adding an abstract doTheThing method you guarantee that every concrete implementation that getAnInstance() can return has an implementation, which is good enough for the compiler, so it'll let you call that method.
Basically a more fundamental truth, that needs to be groked first is this:
You will have instances where the type of the variable is more general than the type of the value it holds. In simple cases you can just make the variable be the specific type:
MyDerivedClassA a = new MyDerivcedClassA();
In that case you could obviously call any method of MyDerivedClassA and wouldn't need any abstract methods in the base class.
But sometimes you want to do a thing with any MyBaseClass instance and you don't know what specific type it is:
public void doTheThingsForAll(Collection<? extends MyBaseClass> baseClassReferences) {
for (MyBaseClass myBaseReference : baseClassReferences) {
myBaseReference.doTheThing();
}
}
If your MyBaseClass didn't have the doTheThing abstract method, then the compiler wouldn't let you do that.

To continue with your example, at some point you might have a List of humans, and you don't really care whether they are disabled or not, all you care about is that you want to call the walking() method on them. In order to do that, the Human class needs to define a walking() method. However, you might not know how to implement that without knowing whether the human is or isn't disabled. So you leave the implementation to the inheriting classes.

There are some examples of how you'd use this in the other answers, so let me give some explanation of why you might do this.
First, one common rule of Object Oriented Design is that you should, in general, try to program to interfaces rather than specific implementations. This tends to improve the program's flexibility and maintainability if you need to change some behavior later. For example, in one program I wrote, we were writing data to CSV files. We later decided to switch to writing to Excel files instead. Programming to interfaces (rather than a specific implementation) made it a lot easier for us to make this change because we could just "drop in" a new class to write to Excel files in place of the class to write to CSV files.
You probably haven't studied this yet, but this is actually important for certain design patterns. A few notable examples of where this is potentially helpful are the Factory Pattern, the Strategy Pattern, and the State Pattern.
For context, a Design Pattern is a standard way of describing and documenting a solution to a known problem. If, for example, someone says "you should use the strategy pattern to solve this problem," this makes the general idea of how you should approach the problem clear.

Because sometimes we need a method that should behave differently in its instances.
For example, imagine a class Animal which contains a method Shout.
We are going to have different instances of this Animal class but we need to implement the method differently in some cases like below:
class Animal:
/**
different properties and methods
which are shared between all animals here
*/
...
method shout():
pass
class Dog extends Animal:
method shout():
makeDogVoice()
class Cat extends Animal:
method shout():
makeCatVoice()
dog = new Animal
cat = new Animal
dog.shout()
cat.shout()
So dog shouts like dogs, and cat shouts like cats! Without implementing the shared behaviors twice
There is a different behavior of shouting in these instances. So we need abstract classes.

Suppose you don't know about implementation and still want to declare a method then we can do that with the help of abstract modifier and making it an abstract method. For abstract method only declaration is available but not the implementation. Hence they should end with ;
Example:
public abstract void m1(); // this is correct
public abstract void m1(){ ... } // this is wrong
Advantage: By declaring abstract method in parent class we can provide guideline to child classes such that which methods are compulsory to implement.
Example:
abstract class Vehicle{
abstract int getNoOfWheels();
}
Class Bus extends Car{
public int getNoOfWheels(){
return 4;
}
}

If you want the short answer, think of this:
You have an abstract class Car.
You implement 2 classes that extend it, Ferrari and Mercedes.
Now:
What if you did one of the following, for the method drive(), common to all cars:
1) changed the visibility of the method,
2) changed the name of the method from driving to Driving,
3) changed the return type, from a boolean to an int
Think about it. It might not seem to make any difference right, because they are different implementations?
Wrong!
If I am iterating through an array of cars, I would have to call a different method for each type of car, thereby making this implementation of abstract useless.
Abstract classes are there to group classes with a common template, that share common properties. One way this helps would be the looping over the array:
Abstract methods ensure that all cars declare the same method,
and therefore, any object of a subclass of Car will have the method drive(), as defined in the abstract class, making the for loop mentioned easy to implement.
Hope this helps.

Is putting static methods in interfaces a good practice?

Let's say I have a method getAverageDifficulty in my class Difficulty that goes like this:
public static Float getAverageDifficulty (List<Difficulty> difficultyList) {
...
return average;
}
My method obviously needs to be static, since it makes no sense requiring an instance of Difficulty. Now let's say I'm using an interface IDifficulty so I can change my implementation later. Obviously, I do want my getAverageDifficulty method to be available from IDifficulty, and, as I can't declare a static method abstract, the obvious option is to put getAverageDifficulty directly in my interface. My question is: am I making a mistake by putting that static method in my interface, which contains other Difficulty methods waiting to be implemented? Should I not mix static and "normal" interface abstract methods? Should I create DifficultyHelper class and put my method there?

Ultimately, this is a matter of style. However, if the only purpose of the helper class is to hold this method (and maybe a few similar ones), then you will be saving yourself one class definition by placing them in the interface:
As of Java 8, the restriction that interfaces cannot contain static methods was eliminated, so there is typically little reason to provide a noninstantiable companion class for an interface [Effective Java, 3rd edition].

Static methods in interfaces are similar to the default methods except that we cannot override these methods in the classes that implements these interfaces.so in my opinion its not not necessary to write static methods in interfaces until you want to prevent overriding.

Implementing interface with many methods = huge class. Patterns come to rescue?

I have an interface with a lot of methods. (which i cannot split into different interfaces)
When i create a class that implements the interface i get a lot of methods in 1 class file.
Things get worst when the body of all these methods get larger -> the class file becomes huge and quite difficult to navigate.
Even with ide's like eclipse because the Outline window containing all classes get a vertical scrollbar because not all methods can fit in the outline.
Is there a pattern that prevents this from happening?

No there is no way to split the implementation in many classes.
But you can delegate from the implementing class to any other classes.
This will reduce the code in the implementation but the number of methods stay the same.

I am posting this reply after an answer is accepted, hoping that future comers might find it useful.
As simas_ch said:
No there is no way to split the implementation in many classes. But
you can delegate from the implementing class to any other classes.
This will reduce the code in the implementation but the number of
methods stay the same.
Once I worked on a rather huge application in which I had to define a Lifecycle interface, which contained many states and many functions that could create a hassle, so I came around with something like this:
You can create a class and make it abstract and implement most of the common functions.
public interface TheBigFunctional {
public void functionalA();
public void functionalB();
public void functionalC();
//....
public void functionalZ();
}
public abstract class FunctionalBase implements TheBigFunctional {
public void functionalA() {
aInternal();
}
protected abstract void aInternal();
// Rest of methods implementations.
// You may choose to skip those if you want child classes to implement them.
}
public class FunctionalTypeFoo extends FunctionalBase {
// Implementations.
}
public class FunctionalTypeBar extends FunctionalBase {
// Implementations.
}
There are many (good) ways to come around the sitation, but I am sharing what I did.

I do not quite understand why you cannot split the interface into multiple ones...
I surely would try to use inheritance, like:
First interface:
public interface FatherInterface {
String methodOne(String var);
String methodTwo(String var);
String methodThree(String var);
}
Second interface:
public interface SonInterface extends FatherInterface {
String methodFour(String var);
String methodFive(String var);
}
Third interface:
public interface SecondSonInterface extends SonInterface {
String methodSix(String var);
String methodSeven(String var);
}
And so on... Each interface inheriting from the preceding one.
As for class files becoming large, go with inheritance too.
Father class:
public class Father implements FatherInterface
Son class:
public class Son extends Father implements SonInterface
And so on...
Edit
If you cannot split the interface (as when given by third party), I would do the implementations of the methods by parts. That is, only some of them implemented in each class. Using abstract classes if needed (or leaving blank methods). Each class inheriting from the above and implementing some of the remaining methods.

Perhaps you could employ the strategy pattern on the big interface by implementing some of those methods from different classes. Then, when you wish to use any of those methods, simply call it from the class that implements your 'large' interface.
More on strategy pattern here: https://www.tutorialspoint.com/design_pattern/strategy_pattern.htm

In this case i would consider whether all methods in your class are well designed. Probably they do not have clear purpose and should be splitted per few.
You have to define clear purpose of some particular interface.
Besides, if you are using Java 8, consider ability to provide some default implementation for some methods.

Is there a pattern that prevents this from happening?
In Java:
Subtyping: If you can group sets of methods into different interfaces such that a clear hierarchical relation makes sense. This is what the accepted answer is assuming.
Interface Segregation: If you can group sets of methods into different sibling "domains" or "categories". Take a look at this example from jcabi-github. See how Github is this API's entrypoint, while exposing different sets of functionalities via successive "grouping" interfaces.

Abstract and Interface Method usage in my Class [duplicate]

What exactly is the difference between an interface and an abstract class?

Interfaces
An interface is a contract: The person writing the interface says, "hey, I accept things looking that way", and the person using the interface says "OK, the class I write looks that way".
An interface is an empty shell. There are only the signatures of the methods, which implies that the methods do not have a body. The interface can't do anything. It's just a pattern.
For example (pseudo code):
// I say all motor vehicles should look like this:
interface MotorVehicle
{
void run();
int getFuel();
}
// My team mate complies and writes vehicle looking that way
class Car implements MotorVehicle
{
int fuel;
void run()
{
print("Wrroooooooom");
}
int getFuel()
{
return this.fuel;
}
}
Implementing an interface consumes very little CPU, because it's not a class, just a bunch of names, and therefore there isn't any expensive look-up to do. It's great when it matters, such as in embedded devices.
Abstract classes
Abstract classes, unlike interfaces, are classes. They are more expensive to use, because there is a look-up to do when you inherit from them.
Abstract classes look a lot like interfaces, but they have something more: You can define a behavior for them. It's more about a person saying, "these classes should look like that, and they have that in common, so fill in the blanks!".
For example:
// I say all motor vehicles should look like this:
abstract class MotorVehicle
{
int fuel;
// They ALL have fuel, so lets implement this for everybody.
int getFuel()
{
return this.fuel;
}
// That can be very different, force them to provide their
// own implementation.
abstract void run();
}
// My teammate complies and writes vehicle looking that way
class Car extends MotorVehicle
{
void run()
{
print("Wrroooooooom");
}
}
Implementation
While abstract classes and interfaces are supposed to be different concepts, the implementations make that statement sometimes untrue. Sometimes, they are not even what you think they are.
In Java, this rule is strongly enforced, while in PHP, interfaces are abstract classes with no method declared.
In Python, abstract classes are more a programming trick you can get from the ABC module and is actually using metaclasses, and therefore classes. And interfaces are more related to duck typing in this language and it's a mix between conventions and special methods that call descriptors (the __method__ methods).
As usual with programming, there is theory, practice, and practice in another language :-)

The key technical differences between an abstract class and an interface are:
Abstract classes can have constants, members, method stubs (methods without a body) and defined methods, whereas interfaces can only have constants and methods stubs.
Methods and members of an abstract class can be defined with any visibility, whereas all methods of an interface must be defined as public (they are defined public by default).
When inheriting an abstract class, a concrete child class must define the abstract methods, whereas an abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined.
Similarly, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.
A child class can only extend a single class (abstract or concrete), whereas an interface can extend or a class can implement multiple other interfaces.
A child class can define abstract methods with the same or less restrictive visibility, whereas a class implementing an interface must define the methods with the exact same visibility (public).

An Interface contains only the definition / signature of functionality, and if we have some common functionality as well as common signatures, then we need to use an abstract class. By using an abstract class, we can provide behavior as well as functionality both in the same time. Another developer inheriting abstract class can use this functionality easily, as they would only need to fill in the blanks.
Taken from:
http://www.dotnetbull.com/2011/11/difference-between-abstract-class-and.html
http://www.dotnetbull.com/2011/11/what-is-abstract-class-in-c-net.html
http://www.dotnetbull.com/2011/11/what-is-interface-in-c-net.html

An explanation can be found here: http://www.developer.com/lang/php/article.php/3604111/PHP-5-OOP-Interfaces-Abstract-Classes-and-the-Adapter-Pattern.htm
An abstract class is a class that is
only partially implemented by the
programmer. It may contain one or more
abstract methods. An abstract method
is simply a function definition that
serves to tell the programmer that the
method must be implemented in a child
class.
An interface is similar to an abstract
class; indeed interfaces occupy the
same namespace as classes and abstract
classes. For that reason, you cannot
define an interface with the same name
as a class. An interface is a fully
abstract class; none of its methods
are implemented and instead of a class
sub-classing from it, it is said to
implement that interface.
Anyway I find this explanation of interfaces somewhat confusing. A more common definition is: An interface defines a contract that implementing classes must fulfill. An interface definition consists of signatures of public members, without any implementing code.

I don't want to highlight the differences, which have been already said in many answers ( regarding public static final modifiers for variables in interface & support for protected, private methods in abstract classes)
In simple terms, I would like to say:
interface: To implement a contract by multiple unrelated objects
abstract class: To implement the same or different behaviour among multiple related objects
From the Oracle documentation
Consider using abstract classes if :
You want to share code among several closely related classes.
You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
You want to declare non-static or non-final fields.
Consider using interfaces if :
You expect that unrelated classes would implement your interface. For example,many unrelated objects can implement Serializable interface.
You want to specify the behaviour of a particular data type, but not concerned about who implements its behaviour.
You want to take advantage of multiple inheritance of type.
abstract class establishes "is a" relation with concrete classes. interface provides "has a" capability for classes.
If you are looking for Java as programming language, here are a few more updates:
Java 8 has reduced the gap between interface and abstract classes to some extent by providing a default method feature. An interface does not have an implementation for a method is no longer valid now.
Refer to this documentation page for more details.
Have a look at this SE question for code examples to understand better.
How should I have explained the difference between an Interface and an Abstract class?

Some important differences:
In the form of a table:
As stated by Joe from javapapers:
1.Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can
have instance methods that implements a default behavior.
2.Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.
3.Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private,
protected, etc..
4.Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.
5.An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java
interfaces.
6.A Java class can implement multiple interfaces but it can extend only one abstract class.
7.Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a
main() exists.
8.In comparison with java abstract classes, java interfaces are slow as it requires extra indirection.

The main point is that:
Abstract is object oriented. It offers the basic data an 'object' should have and/or functions it should be able to do. It is concerned with the object's basic characteristics: what it has and what it can do. Hence objects which inherit from the same abstract class share the basic characteristics (generalization).
Interface is functionality oriented. It defines functionalities an object should have. Regardless what object it is, as long as it can do these functionalities, which are defined in the interface, it's fine. It ignores everything else. An object/class can contain several (groups of) functionalities; hence it is possible for a class to implement multiple interfaces.

When you want to provide polymorphic behaviour in an inheritance hierarchy, use abstract classes.
When you want polymorphic behaviour for classes which are completely unrelated, use an interface.

I am constructing a building of 300 floors
The building's blueprint interface
For example, Servlet(I)
Building constructed up to 200 floors - partially completed---abstract
Partial implementation, for example, generic and HTTP servlet
Building construction completed-concrete
Full implementation, for example, own servlet
Interface
We don't know anything about implementation, just requirements. We can
go for an interface.
Every method is public and abstract by default
It is a 100% pure abstract class
If we declare public we cannot declare private and protected
If we declare abstract we cannot declare final, static, synchronized, strictfp and native
Every interface has public, static and final
Serialization and transient is not applicable, because we can't create an instance for in interface
Non-volatile because it is final
Every variable is static
When we declare a variable inside an interface we need to initialize variables while declaring
Instance and static block not allowed
Abstract
Partial implementation
It has an abstract method. An addition, it uses concrete
No restriction for abstract class method modifiers
No restriction for abstract class variable modifiers
We cannot declare other modifiers except abstract
No restriction to initialize variables
Taken from DurgaJobs Website

Let's work on this question again:
The first thing to let you know is that 1/1 and 1*1 results in the same, but it does not mean that multiplication and division are same. Obviously, they hold some good relationship, but mind you both are different.
I will point out main differences, and the rest have already been explained:
Abstract classes are useful for modeling a class hierarchy. At first glance of any requirement, we are partially clear on what exactly is to be built, but we know what to build. And so your abstract classes are your base classes.
Interfaces are useful for letting other hierarchy or classes to know that what I am capable of doing. And when you say I am capable of something, you must have that capacity. Interfaces will mark it as compulsory for a class to implement the same functionalities.

If you have some common methods that can be used by multiple classes go for abstract classes.
Else if you want the classes to follow some definite blueprint go for interfaces.
Following examples demonstrate this.
Abstract class in Java:
abstract class Animals
{
// They all love to eat. So let's implement them for everybody
void eat()
{
System.out.println("Eating...");
}
// The make different sounds. They will provide their own implementation.
abstract void sound();
}
class Dog extends Animals
{
void sound()
{
System.out.println("Woof Woof");
}
}
class Cat extends Animals
{
void sound()
{
System.out.println("Meoww");
}
}
Following is an implementation of interface in Java:
interface Shape
{
void display();
double area();
}
class Rectangle implements Shape
{
int length, width;
Rectangle(int length, int width)
{
this.length = length;
this.width = width;
}
#Override
public void display()
{
System.out.println("****\n* *\n* *\n****");
}
#Override
public double area()
{
return (double)(length*width);
}
}
class Circle implements Shape
{
double pi = 3.14;
int radius;
Circle(int radius)
{
this.radius = radius;
}
#Override
public void display()
{
System.out.println("O"); // :P
}
#Override
public double area()
{
return (double)((pi*radius*radius)/2);
}
}
Some Important Key points in a nutshell:
The variables declared in Java interface are by default final. Abstract classes can have non-final variables.
The variables declared in Java interface are by default static. Abstract classes can have non-static variables.
Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..

It's pretty simple actually.
You can think of an interface as a class which is only allowed to have abstract methods and nothing else.
So an interface can only "declare" and not define the behavior you want the class to have.
An abstract class allows you to do both declare (using abstract methods) as well as define (using full method implementations) the behavior you want the class to have.
And a regular class only allows you to define, not declare, the behavior/actions you want the class to have.
One last thing,
In Java, you can implement multiple interfaces, but you can only extend one (Abstract Class or Class)...
This means inheritance of defined behavior is restricted to only allow one per class... ie if you wanted a class that encapsulated behavior from Classes A,B&C you would need to do the following: Class A extends B, Class C extends A .. its a bit of a round about way to have multiple inheritance...
Interfaces on the other hand, you could simply do: interface C implements A, B
So in effect Java supports multiple inheritance only in "declared behavior" ie interfaces, and only single inheritance with defined behavior.. unless you do the round about way I described...
Hopefully that makes sense.

The comparison of interface vs. abstract class is wrong. There should be two other comparisons instead: 1) interface vs. class and 2) abstract vs. final class.
Interface vs Class
Interface is a contract between two objects. E.g., I'm a Postman and you're a Package to deliver. I expect you to know your delivery address. When someone gives me a Package, it has to know its delivery address:
interface Package {
String address();
}
Class is a group of objects that obey the contract. E.g., I'm a box from "Box" group and I obey the contract required by the Postman. At the same time I obey other contracts:
class Box implements Package, Property {
#Override
String address() {
return "5th Street, New York, NY";
}
#Override
Human owner() {
// this method is part of another contract
}
}
Abstract vs Final
Abstract class is a group of incomplete objects. They can't be used, because they miss some parts. E.g., I'm an abstract GPS-aware box - I know how to check my position on the map:
abstract class GpsBox implements Package {
#Override
public abstract String address();
protected Coordinates whereAmI() {
// connect to GPS and return my current position
}
}
This class, if inherited/extended by another class, can be very useful. But by itself - it is useless, since it can't have objects. Abstract classes can be building elements of final classes.
Final class is a group of complete objects, which can be used, but can't be modified. They know exactly how to work and what to do. E.g., I'm a Box that always goes to the address specified during its construction:
final class DirectBox implements Package {
private final String to;
public DirectBox(String addr) {
this.to = addr;
}
#Override
public String address() {
return this.to;
}
}
In most languages, like Java or C++, it is possible to have just a class, neither abstract nor final. Such a class can be inherited and can be instantiated. I don't think this is strictly in line with object-oriented paradigm, though.
Again, comparing interfaces with abstract classes is not correct.

The only difference is that one can participate in multiple inheritance and other cannot.
The definition of an interface has changed over time. Do you think an interface just has method declarations only and are just contracts? What about static final variables and what about default definitions after Java 8?
Interfaces were introduced to Java because of the diamond problem with multiple inheritance and that's what they actually intend to do.
Interfaces are the constructs that were created to get away with the multiple inheritance problem and can have abstract methods, default definitions and static final variables.
See Why does Java allow static final variables in interfaces when they are only intended to be contracts?.

Interface: Turn ( Turn Left, Turn Right.)
Abstract Class: Wheel.
Class: Steering Wheel, derives from Wheel, exposes Interface Turn
One is for categorizing behavior that can be offered across a diverse range of things, the other is for modelling an ontology of things.

In short the differences are the following:
Syntactical Differences Between Interface and Abstract Class:
Methods and members of an abstract class can have any visibility. All methods of an interface must be public. //Does not hold true from Java 9 anymore
A concrete child class of an Abstract Class must define all the abstract methods. An Abstract child class can have abstract methods. An interface extending another interface need not provide default implementation for methods inherited from the parent interface.
A child class can only extend a single class. An interface can extend multiple interfaces. A class can implement multiple interfaces.
A child class can define abstract methods with the same or less restrictive visibility, whereas class implementing an interface must define all interface methods as public.
Abstract Classes can have constructors but not interfaces.
Interfaces from Java 9 have private static methods.
In Interfaces now:
public static - supported
public abstract - supported
public default - supported
private static - supported
private abstract - compile error
private default - compile error
private - supported

Many junior developers make the mistake of thinking of interfaces, abstract and concrete classes as slight variations of the same thing, and choose one of them purely on technical grounds: Do I need multiple inheritance? Do I need some place to put common methods? Do I need to bother with something other than just a concrete class? This is wrong, and hidden in these questions is the main problem: "I". When you write code for yourself, by yourself, you rarely think of other present or future developers working on or with your code.
Interfaces and abstract classes, although apparently similar from a technical point of view, have completely different meanings and purposes.
Summary
An interface defines a contract that some implementation will fulfill for you.
An abstract class provides a default behavior that your implementation can reuse.
Alternative summary
An interface is for defining public APIs
An abstract class is for internal use, and for defining SPIs
On the importance of hiding implementation details
A concrete class does the actual work, in a very specific way. For example, an ArrayList uses a contiguous area of memory to store a list of objects in a compact manner which offers fast random access, iteration, and in-place changes, but is terrible at insertions, deletions, and occasionally even additions; meanwhile, a LinkedList uses double-linked nodes to store a list of objects, which instead offers fast iteration, in-place changes, and insertion/deletion/addition, but is terrible at random access. These two types of lists are optimized for different use cases, and it matters a lot how you're going to use them. When you're trying to squeeze performance out of a list that you're heavily interacting with, and when picking the type of list is up to you, you should carefully pick which one you're instantiating.
On the other hand, high level users of a list don't really care how it is actually implemented, and they should be insulated from these details. Let's imagine that Java didn't expose the List interface, but only had a concrete List class that's actually what LinkedList is right now. All Java developers would have tailored their code to fit the implementation details: avoid random access, add a cache to speed up access, or just reimplement ArrayList on their own, although it would be incompatible with all the other code that actually works with List only. That would be terrible... But now imagine that the Java masters actually realize that a linked list is terrible for most actual use cases, and decided to switch over to an array list for their only List class available. This would affect the performance of every Java program in the world, and people wouldn't be happy about it. And the main culprit is that implementation details were available, and the developers assumed that those details are a permanent contract that they can rely on. This is why it's important to hide implementation details, and only define an abstract contract. This is the purpose of an interface: define what kind of input a method accepts, and what kind of output is expected, without exposing all the guts that would tempt programmers to tweak their code to fit the internal details that might change with any future update.
An abstract class is in the middle between interfaces and concrete classes. It is supposed to help implementations share common or boring code. For example, AbstractCollection provides basic implementations for isEmpty based on size is 0, contains as iterate and compare, addAll as repeated add, and so on. This lets implementations focus on the crucial parts that differentiate between them: how to actually store and retrieve data.
APIs versus SPIs
Interfaces are low-cohesion gateways between different parts of code. They allow libraries to exist and evolve without breaking every library user when something changes internally. It's called Application Programming Interface, not Application Programming Classes. On a smaller scale, they also allow multiple developers to collaborate successfully on large scale projects, by separating different modules through well documented interfaces.
Abstract classes are high-cohesion helpers to be used when implementing an interface, assuming some level of implementation details. Alternatively, abstract classes are used for defining SPIs, Service Provider Interfaces.
The difference between an API and an SPI is subtle, but important: for an API, the focus is on who uses it, and for an SPI the focus is on who implements it.
Adding methods to an API is easy, all existing users of the API will still compile. Adding methods to an SPI is hard, since every service provider (concrete implementation) will have to implement the new methods. If interfaces are used to define an SPI, a provider will have to release a new version whenever the SPI contract changes. If abstract classes are used instead, new methods could either be defined in terms of existing abstract methods, or as empty throw not implemented exception stubs, which will at least allow an older version of a service implementation to still compile and run.
A note on Java 8 and default methods
Although Java 8 introduced default methods for interfaces, which makes the line between interfaces and abstract classes even blurrier, this wasn't so that implementations can reuse code, but to make it easier to change interfaces that serve both as an API and as an SPI (or are wrongly used for defining SPIs instead of abstract classes).
Which one to use?
Is the thing supposed to be publicly used by other parts of the code, or by other external code? Add an interface to it to hide the implementation details from the public abstract contract, which is the general behavior of the thing.
Is the thing something that's supposed to have multiple implementations with a lot of code in common? Make both an interface and an abstract, incomplete implementation.
Is there ever going to be only one implementation, and nobody else will use it? Just make it a concrete class.
"ever" is long time, you could play it safe and still add an interface on top of it.
A corollary: the other way around is often wrongly done: when using a thing, always try to use the most generic class/interface that you actually need. In other words, don't declare your variables as ArrayList theList = new ArrayList(), unless you actually have a very strong dependency on it being an array list, and no other type of list would cut it for you. Use List theList = new ArrayList instead, or even Collection theCollection = new ArrayList if the fact that it's a list, and not any other type of collection doesn't actually matter.

Not really the answer to the original question, but once you have the answer to the difference between them, you will enter the when-to-use-each dilemma:
When to use interfaces or abstract classes? When to use both?
I've limited knowledge of OOP, but seeing interfaces as an equivalent of an adjective in grammar has worked for me until now (correct me if this method is bogus!). For example, interface names are like attributes or capabilities you can give to a class, and a class can have many of them: ISerializable, ICountable, IList, ICacheable, IHappy, ...

You can find clear difference between interface and abstract class.
Interface
Interface only contains abstract methods.
Force users to implement all methods when implements the interface.
Contains only final and static variables.
Declare using interface keyword.
All methods of an interface must be defined as public.
An interface can extend or a class can implement multiple other
interfaces.
Abstract class
Abstract class contains abstract and non-abstract methods.
Does not force users to implement all methods when inherited the
abstract class.
Contains all kinds of variables including primitive and non-primitive
Declare using abstract keyword.
Methods and members of an abstract class can be defined with any
visibility.
A child class can only extend a single class (abstract or concrete).

I am 10 yrs late to the party but would like to attempt any way. Wrote a post about the same on medium few days back. Thought of posting it here.
tl;dr; When you see “Is A” relationship use inheritance/abstract class. when you see “has a” relationship create member variables. When you see “relies on external provider” implement (not inherit) an interface.
Interview Question: What is the difference between an interface and an abstract class? And how do you decide when to use what?
I mostly get one or all of the below answers:
Answer 1: You cannot create an object of abstract class and interfaces.
ZK (That’s my initials): You cannot create an object of either. So this is not a difference. This is a similarity between an interface and an abstract class. Counter
Question: Why can’t you create an object of abstract class or interface?
Answer 2: Abstract classes can have a function body as partial/default implementation.
ZK: Counter Question: So if I change it to a pure abstract class, marking all the virtual functions as abstract and provide no default implementation for any virtual function. Would that make abstract classes and interfaces the same? And could they be used interchangeably after that?
Answer 3: Interfaces allow multi-inheritance and abstract classes don’t.
ZK: Counter Question: Do you really inherit from an interface? or do you just implement an interface and, inherit from an abstract class? What’s the difference between implementing and inheriting?
These counter questions throw candidates off and make most scratch their heads or just pass to the next question. That makes me think people need help with these basic building blocks of Object-Oriented Programming.
The answer to the original question and all the counter questions is found in the English language and the UML.
You must know at least below to understand these two constructs better.
Common Noun: A common noun is a name given “in common” to things of the same class or kind. For e.g. fruits, animals, city, car etc.
Proper Noun: A proper noun is the name of an object, place or thing. Apple, Cat, New York, Honda Accord etc.
Car is a Common Noun. And Honda Accord is a Proper Noun, and probably a Composit Proper noun, a proper noun made using two nouns.
Coming to the UML Part. You should be familiar with below relationships:
Is A
Has A
Uses
Let’s consider the below two sentences.
- HondaAccord Is A Car?
- HondaAccord Has A Car?
Which one sounds correct? Plain English and comprehension. HondaAccord and Cars share an “Is A” relationship. Honda accord doesn’t have a car in it. It “is a” car. Honda Accord “has a” music player in it.
When two entities share the “Is A” relationship it’s a better candidate for inheritance. And Has a relationship is a better candidate for creating member variables.
With this established our code looks like this:
abstract class Car
{
string color;
int speed;
}
class HondaAccord : Car
{
MusicPlayer musicPlayer;
}
Now Honda doesn't manufacture music players. Or at least it’s not their main business.
So they reach out to other companies and sign a contract. If you receive power here and the output signal on these two wires it’ll play just fine on these speakers.
This makes Music Player a perfect candidate for an interface. You don’t care who provides support for it as long as the connections work just fine.
You can replace the MusicPlayer of LG with Sony or the other way. And it won’t change a thing in Honda Accord.
Why can’t you create an object of abstract classes?
Because you can’t walk into a showroom and say give me a car. You’ll have to provide a proper noun. What car? Probably a honda accord. And that’s when a sales agent could get you something.
Why can’t you create an object of an interface?
Because you can’t walk into a showroom and say give me a contract of music player. It won’t help. Interfaces sit between consumers and providers just to facilitate an agreement. What will you do with a copy of the agreement? It won’t play music.
Why do interfaces allow multiple inheritance?
Interfaces are not inherited. Interfaces are implemented.
The interface is a candidate for interaction with the external world.
Honda Accord has an interface for refueling. It has interfaces for inflating tires. And the same hose that is used to inflate a football. So the new code will look like below:
abstract class Car
{
string color;
int speed;
}
class HondaAccord : Car, IInflateAir, IRefueling
{
MusicPlayer musicPlayer;
}
And the English will read like this “Honda Accord is a Car that supports inflating tire and refueling”.

Key Points:
Abstract class can have property, Data fields ,Methods (complete /
incomplete) both.
If method or Properties define in abstract keyword that must override in derived class.(its work as a tightly coupled
functionality)
If define abstract keyword for method or properties in abstract class you can not define body of method and get/set value for
properties and that must override in derived class.
Abstract class does not support multiple inheritance.
Abstract class contains Constructors.
An abstract class can contain access modifiers for the subs, functions, properties.
Only Complete Member of abstract class can be Static.
An interface can inherit from another interface only and cannot inherit from an abstract class, where as an abstract class can inherit from another abstract class or another interface.
Advantage:
It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.
If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
Its allow fast execution than interface.(interface Requires more time to find the actual method in the corresponding classes.)
It can use for tight and loosely coupling.
find details here...
http://pradeepatkari.wordpress.com/2014/11/20/interface-and-abstract-class-in-c-oops/

The shortest way to sum it up is that an interface is:
Fully abstract, apart from default and static methods; while it has definitions (method signatures + implementations) for default and static methods, it only has declarations (method signatures) for other methods.
Subject to laxer rules than classes (a class can implement multiple interfaces, and an interface can inherit from multiple interfaces). All variables are implicitly constant, whether specified as public static final or not. All members are implicitly public, whether specified as such or not.
Generally used as a guarantee that the implementing class will have the specified features and/or be compatible with any other class which implements the same interface.
Meanwhile, an abstract class is:
Anywhere from fully abstract to fully implemented, with a tendency to have one or more abstract methods. Can contain both declarations and definitions, with declarations marked as abstract.
A full-fledged class, and subject to the rules that govern other classes (can only inherit from one class), on the condition that it cannot be instantiated (because there's no guarantee that it's fully implemented). Can have non-constant member variables. Can implement member access control, restricting members as protected, private, or private package (unspecified).
Generally used either to provide as much of the implementation as can be shared by multiple subclasses, or to provide as much of the implementation as the programmer is able to supply.
Or, if we want to boil it all down to a single sentence: An interface is what the implementing class has, but an abstract class is what the subclass is.

Inheritance is used for two purposes:
To allow an object to regard parent-type data members and method implementations as its own.
To allow a reference to an objects of one type to be used by code which expects a reference to supertype object.
In languages/frameworks which support generalized multiple inheritance, there is often little need to classify a type as either being an "interface" or an "abstract class". Popular languages and frameworks, however, will allow a type to regard one other type's data members or method implementations as its own even though they allow a type to be substitutable for an arbitrary number of other types.
Abstract classes may have data members and method implementations, but can only be inherited by classes which don't inherit from any other classes. Interfaces put almost no restrictions on the types which implement them, but cannot include any data members or method implementations.
There are times when it's useful for types to be substitutable for many different things; there are other times when it's useful for objects to regard parent-type data members and method implementations as their own. Making a distinction between interfaces and abstract classes allows each of those abilities to be used in cases where it is most relevant.

Differences between abstract class and interface on behalf of real implementation.
Interface: It is a keyword and it is used to define the template or blue print of an object and it forces all the sub classes would follow the same prototype,as for as implementation, all the sub classes are free to implement the functionality as per it's requirement.
Some of other use cases where we should use interface.
Communication between two external objects(Third party integration in our application) done through Interface here Interface works as Contract.
Abstract Class: Abstract,it is a keyword and when we use this keyword before any class then it becomes abstract class.It is mainly used when we need to define the template as well as some default functionality of an object that is followed by all the sub classes and this way it removes the redundant code and one more use cases where we can use abstract class, such as we want no other classes can directly instantiate an object of the class, only derived classes can use the functionality.
Example of Abstract Class:
public abstract class DesireCar
{
//It is an abstract method that defines the prototype.
public abstract void Color();
// It is a default implementation of a Wheel method as all the desire cars have the same no. of wheels.
// and hence no need to define this in all the sub classes in this way it saves the code duplicasy
public void Wheel() {
Console.WriteLine("Car has four wheel");
}
}
**Here is the sub classes:**
public class DesireCar1 : DesireCar
{
public override void Color()
{
Console.WriteLine("This is a red color Desire car");
}
}
public class DesireCar2 : DesireCar
{
public override void Color()
{
Console.WriteLine("This is a red white Desire car");
}
}
Example Of Interface:
public interface IShape
{
// Defines the prototype(template)
void Draw();
}
// All the sub classes follow the same template but implementation can be different.
public class Circle : IShape
{
public void Draw()
{
Console.WriteLine("This is a Circle");
}
}
public class Rectangle : IShape
{
public void Draw()
{
Console.WriteLine("This is a Rectangle");
}
}

I'd like to add one more difference which makes sense.
For example, you have a framework with thousands of lines of code. Now if you want to add a new feature throughout the code using a method enhanceUI(), then it's better to add that method in abstract class rather in interface. Because, if you add this method in an interface then you should implement it in all the implemented class but it's not the case if you add the method in abstract class.

To give a simple but clear answer, it helps to set the context : you use both when you do not want to provide full implementations.
The main difference then is an interface has no implementation at all (only methods without a body) while abstract classes can have members and methods with a body as well, i.e. can be partially implemented.

usually Abstract class used for core of something but interface used for appending peripheral.
when you want to create base type for vehicle you should use abstract class but if you want to add some functionality or property that is not part of base concept of vehicle you should use interface,for example you want to add "ToJSON()" function.
interface has wide range of abstraction rather than abstract class.
you can see this in passing arguments.look this example:
if you use vehicle as argument you just can use one of its derived type (bus or car-same category-just vehicle category).
but when you use IMoveable interface as argument you have more choices.

The topic of abstract classes vs interfaces is mostly about semantics.
Abstract classes act in different programming languages often as a superset of interfaces, except one thing and that is, that you can implement multiple interfaces, but inherit only one class.
An interface defines what something must be able to do; like a contract, but does not provide an implementation of it.
An abstract class defines what something is and it commonly hosts shared code between the subclasses.
For example a Formatter should be able to format() something. The common semantics to describe something like that would be to create an interface IFormatter with a declaration of format() that acts like a contract. But IFormatter does not describe what something is, but just what it should be able to to. The common semantics to describe what something actually is, is to create a class. In this case we create an abstract class... So we create an abstract class Formatter which implements the interface. That is a very descriptive code, because we now know we have a Formatter and we now know what every Formatter must be able to do.
Also one very important topic is documentation (at least for some people...). In your documentation you probably want to explain within your subclasses what a Formatter actually is. It is very convenient to have an abstract class Formatter to which documentation you can link to within your subclasses. That is very convenient and generic. On the other hand if you do not have an abstract class Formatter and only an interface IFormatter you would have to explain in each of your subclasses what a Formatter actucally is, because an interface is a contract and you would not describe what a Formatter actually is within the documentation of an interface — at least it would be not something common to do and you would break the semantics that most developers consider to be correct.
Note: It is a very common pattern to make an abstract class implement an interface.

An abstract class is a class whose object cannot be created or a class which cannot be instantiated.
An abstract method makes a class abstract.
An abstract class needs to be inherited in order to override the methods that are declared in the abstract class.
No restriction on access specifiers.
An abstract class can have constructor and other concrete(non abstarct methods ) methods in them but interface cannot have.
An interface is a blueprint/template of methods.(eg. A house on a paper is given(interface house) and different architects will use their ideas to build it(the classes of architects implementing the house interface) .
It is a collection of abstract methods , default methods , static methods , final variables and nested classes.
All members will be either final or public , protected and private access specifiers are not allowed.No object creation is allowed.
A class has to be made in order to use the implementing interface and also to override the abstract method declared in the interface. An interface is a good example of loose coupling(dynamic polymorphism/dynamic binding)
An interface implements polymorphism and abstraction.It tells what to do but how to do is defined by the implementing class.
For Eg. There's a car company and it wants that some features to be same for all the car it is manufacturing so for that the company would be making an interface vehicle which will have those features and different classes of car(like Maruti Suzkhi , Maruti 800) will override those features(functions).
Why interface when we already have abstract class?
Java supports only multilevel and hierarchal inheritance but with the help of interface we can implement multiple inheritance.

In an interface all methods must be only definitions, not single one should be implemented.
But in an abstract class there must an abstract method with only definition, but other methods can be also in the abstract class with implementation...

2 Different java classes,exact same methods but one implements an interface

Suppose, there are two java classes.
BaseA
public class BaseA extends ModuleBase{
public void doSomething{
//does something
}
}
BaseB
public class BaseB extends ModuleBase implements
SomeInterface {
public void doSomething{
//does something
}
}
SomeInterface
public interface SomeInterface {
public void doSomething();
}
so as you can see the only difference between BaseA & BaseB is that BaseB implements an interface. As far my understanding an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. It cannot be instantiated.
Questions:
it seems BaseA & BaseA would be same as the methods & code in them is same. correct?
Interface seems like a contract that spells out how software APIs interact with each other & have no effect on class functions. only purpose of interface is to enforce that BaseB has mandatorily implement doSomething, where as with BaseA, its optional & won't generate compile errors. if not, then why?
What difference implementing an interface make? I know you have to implement all methods of that particular interface but if can also you do that without the keyword implements InterfaceName as seen in BaseB Vs BaseA where we implemented exact same doSomething(). what difference having the keyword implements InterfaceName in class declaration make?

No. Classes in Java are the same when they have the same fully qualified name and when they were loaded from the same classloader. Java makes no attempt to look into methods and it doesn't compare method signatures.
Or to put it differently: Java doesn't support duck typing.
Usually, interfaces are used to make a bunch of classes easily interchangeable. So you have something that needs a certain functionality. Instead of typing this to a single class, you can use an interface. People using that service can then feed it with different classes, according to their needs, making the service much more flexible (and somewhat harder to understand).
It means you can use BaseB in any place where InterfaceName is expected. That makes BaseB (and everything derived from it) much more useful/powerful.
Example:
If you want to check passwords, you can write a PasswordService class. But that means everyone has to use this class and live with the limitations.
If you offer a PasswordService interface, then users of your code can have different implementations: They can get passwords from a file, a database, LDAP, ... or for unit tests, they can write a service that says yes or no to every password.

what difference having the keyword implements InterfaceName in class declaration make?
You can then cast to that interface.
Java is not duck-typed.
Even if your class has a method void run() like a Runnable, you still won't be able to give it to places that want a Runnable without implementing the interface.
new Thread(instanceOfMyNotRunnableClass); // won't compile

Two classes are not same by their code. The code may be same but classes are still different. Two classes with same code may behave similar but will not be same.
To understand purpose of Interface, you should understand concepts of Abstraction and Encapsulation. Interface not only provides a contract, also provides an abstraction over underlying classes. You may write an API that takes object of type Interface without bothering about actual class implementing the Interface.
You can use BaseB in place where InterfaceName but you should not. This makes your code rigid for using only BaseB, whereas you may write an utility that takes any class that has implemented the interface.

Well, I assume that SomeInterface declares "doSomething", right?
If that's the case, the benefit for you is that you can treat BaseB as SomeInterface. Let's say you have another class BaseC, which also implements SomeInterface, then this code is valid:
SomeInterface inter = new BaseB();
inter = new BaseC();
while this is not valid:
SomeInterface interr = new BaseA();
Your advantage is, that you do not have to know, if inter is BaseB() or BaseC(), because you simple work on the interface declared methods, no matter how the implementation excatly looks like.

Interface is used to make skeleton of your API. Like java.util.ArrayList and java.util.LinkedList both are classes which implement interface java.util.List.
So if you have method like below
void doSomething(java.util.List list){
}
You can pass java.util.ArrayList or java.util.LinkedList as per your requirment with no harm.You don't have to create two diff. methods where one accept java.util.ArrayList and another accept java.util.LinkedList