What is an "abstract class" in Java?
An abstract class is a class which cannot be instantiated. An abstract class is used by creating an inheriting subclass that can be instantiated. An abstract class does a few things for the inheriting subclass:
Define methods which can be used by the inheriting subclass.
Define abstract methods which the inheriting subclass must implement.
Provide a common interface which allows the subclass to be interchanged with all other subclasses.
Here's an example:
abstract public class AbstractClass
{
abstract public void abstractMethod();
public void implementedMethod() { System.out.print("implementedMethod()"); }
final public void finalMethod() { System.out.print("finalMethod()"); }
}
Notice that "abstractMethod()" doesn't have any method body. Because of this, you can't do the following:
public class ImplementingClass extends AbstractClass
{
// ERROR!
}
There's no method that implements abstractMethod()! So there's no way for the JVM to know what it's supposed to do when it gets something like new ImplementingClass().abstractMethod().
Here's a correct ImplementingClass.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
}
Notice that you don't have to define implementedMethod() or finalMethod(). They were already defined by AbstractClass.
Here's another correct ImplementingClass.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
}
In this case, you have overridden implementedMethod().
However, because of the final keyword, the following is not possible.
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
public void finalMethod() { System.out.print("ERROR!"); }
}
You can't do this because the implementation of finalMethod() in AbstractClass is marked as the final implementation of finalMethod(): no other implementations will be allowed, ever.
Now you can also implement an abstract class twice:
public class ImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("abstractMethod()"); }
public void implementedMethod() { System.out.print("Overridden!"); }
}
// In a separate file.
public class SecondImplementingClass extends AbstractClass
{
public void abstractMethod() { System.out.print("second abstractMethod()"); }
}
Now somewhere you could write another method.
public tryItOut()
{
ImplementingClass a = new ImplementingClass();
AbstractClass b = new ImplementingClass();
a.abstractMethod(); // prints "abstractMethod()"
a.implementedMethod(); // prints "Overridden!" <-- same
a.finalMethod(); // prints "finalMethod()"
b.abstractMethod(); // prints "abstractMethod()"
b.implementedMethod(); // prints "Overridden!" <-- same
b.finalMethod(); // prints "finalMethod()"
SecondImplementingClass c = new SecondImplementingClass();
AbstractClass d = new SecondImplementingClass();
c.abstractMethod(); // prints "second abstractMethod()"
c.implementedMethod(); // prints "implementedMethod()"
c.finalMethod(); // prints "finalMethod()"
d.abstractMethod(); // prints "second abstractMethod()"
d.implementedMethod(); // prints "implementedMethod()"
d.finalMethod(); // prints "finalMethod()"
}
Notice that even though we declared b an AbstractClass type, it displays "Overriden!". This is because the object we instantiated was actually an ImplementingClass, whose implementedMethod() is of course overridden. (You may have seen this referred to as polymorphism.)
If we wish to access a member specific to a particular subclass, we must cast down to that subclass first:
// Say ImplementingClass also contains uniqueMethod()
// To access it, we use a cast to tell the runtime which type the object is
AbstractClass b = new ImplementingClass();
((ImplementingClass)b).uniqueMethod();
Lastly, you cannot do the following:
public class ImplementingClass extends AbstractClass, SomeOtherAbstractClass
{
... // implementation
}
Only one class can be extended at a time. If you need to extend multiple classes, they have to be interfaces. You can do this:
public class ImplementingClass extends AbstractClass implements InterfaceA, InterfaceB
{
... // implementation
}
Here's an example interface:
interface InterfaceA
{
void interfaceMethod();
}
This is basically the same as:
abstract public class InterfaceA
{
abstract public void interfaceMethod();
}
The only difference is that the second way doesn't let the compiler know that it's actually an interface. This can be useful if you want people to only implement your interface and no others. However, as a general beginner rule of thumb, if your abstract class only has abstract methods, you should probably make it an interface.
The following is illegal:
interface InterfaceB
{
void interfaceMethod() { System.out.print("ERROR!"); }
}
You cannot implement methods in an interface. This means that if you implement two different interfaces, the different methods in those interfaces can't collide. Since all the methods in an interface are abstract, you have to implement the method, and since your method is the only implementation in the inheritance tree, the compiler knows that it has to use your method.
A Java class becomes abstract under the following conditions:
1. At least one of the methods is marked as abstract:
public abstract void myMethod()
In that case the compiler forces you to mark the whole class as abstract.
2. The class is marked as abstract:
abstract class MyClass
As already said: If you have an abstract method the compiler forces you to mark the whole class as abstract. But even if you don't have any abstract method you can still mark the class as abstract.
Common use:
A common use of abstract classes is to provide an outline of a class similar like an interface does. But unlike an interface it can already provide functionality, i.e. some parts of the class are implemented and some parts are just outlined with a method declaration. ("abstract")
An abstract class cannot be instantiated, but you can create a concrete class based on an abstract class, which then can be instantiated. To do so you have to inherit from the abstract class and override the abstract methods, i.e. implement them.
A class that is declared using the abstract keyword is known as abstract class.
Abstraction is a process of hiding the data implementation details, and showing only functionality to the user. Abstraction lets you focus on what the object does instead of how it does it.
Main things of abstract class
An abstract class may or may not contain abstract methods.There can be non abstract methods.
An abstract method is a method that is declared without an
implementation (without braces, and followed by a semicolon), like this:
ex : abstract void moveTo(double deltaX, double deltaY);
If a class has at least one abstract method then that class must be abstract
Abstract classes may not be instantiated (You are not allowed to create object of Abstract class)
To use an abstract class, you have to inherit it from another class. Provide implementations to all the abstract methods in it.
If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.
Declare abstract class
Specifying abstract keyword before the class during declaration makes it abstract. Have a look at the code below:
abstract class AbstractDemo{ }
Declare abstract method
Specifying abstract keyword before the method during declaration makes it abstract. Have a look at the code below,
abstract void moveTo();//no body
Why we need to abstract classes
In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for ex -: position, orientation, line color, fill color) and behaviors (for ex -: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for ex : fill color, position, and moveTo). Others require different implementation(for ex: resize or draw). All graphic objects must be able to draw or resize themselves, they just differ in how they do it.
This is a perfect situation for an abstract superclass. You can take advantage of the similarities, and declare all the graphic objects to inherit from the same abstract parent object (for ex : GraphicObject) as shown in the following figure.
First, you declare an abstract class, GraphicObject, to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declared abstract methods, such as draw or resize, that need to be a implemented by all subclasses but must be implemented in different ways. The GraphicObject class can look something like this:
abstract class GraphicObject {
void moveTo(int x, int y) {
// Inside this method we have to change the position of the graphic
// object according to x,y
// This is the same in every GraphicObject. Then we can implement here.
}
abstract void draw(); // But every GraphicObject drawing case is
// unique, not common. Then we have to create that
// case inside each class. Then create these
// methods as abstract
abstract void resize();
}
Usage of abstract method in sub classes
Each non abstract subclasses of GraphicObject, such as Circle and Rectangle, must provide implementations for the draw and resize methods.
class Circle extends GraphicObject {
void draw() {
//Add to some implementation here
}
void resize() {
//Add to some implementation here
}
}
class Rectangle extends GraphicObject {
void draw() {
//Add to some implementation here
}
void resize() {
//Add to some implementation here
}
}
Inside the main method you can call all methods like this:
public static void main(String args[]){
GraphicObject c = new Circle();
c.draw();
c.resize();
c.moveTo(4,5);
}
Ways to achieve abstraction in Java
There are two ways to achieve abstraction in java
Abstract class (0 to 100%)
Interface (100%)
Abstract class with constructors, data members, methods, etc
abstract class GraphicObject {
GraphicObject (){
System.out.println("GraphicObject is created");
}
void moveTo(int y, int x) {
System.out.println("Change position according to "+ x+ " and " + y);
}
abstract void draw();
}
class Circle extends GraphicObject {
void draw() {
System.out.println("Draw the Circle");
}
}
class TestAbstract {
public static void main(String args[]){
GraphicObject grObj = new Circle ();
grObj.draw();
grObj.moveTo(4,6);
}
}
Output:
GraphicObject is created
Draw the Circle
Change position according to 6 and 4
Remember two rules:
If the class has few abstract methods and few concrete methods,
declare it as an abstract class.
If the class has only abstract methods, declare it as an interface.
References:
TutorialsPoint - Java Abstraction
BeginnersBook - Java Abstract Class Method
Java Docs - Abstract Methods and Classes
JavaPoint - Abstract Class in Java
It's a class that cannot be instantiated, and forces implementing classes to, possibly, implement abstract methods that it outlines.
Simply speaking, you can think of an abstract class as like an Interface with a bit more capabilities.
You cannot instantiate an Interface, which also holds for an abstract class.
On your interface you can just define the method headers and ALL of the implementers are forced to implement all of them. On an abstract class you can also define your method headers but here - to the difference of the interface - you can also define the body (usually a default implementation) of the method. Moreover when other classes extend (note, not implement and therefore you can also have just one abstract class per child class) your abstract class, they are not forced to implement all of your methods of your abstract class, unless you specified an abstract method (in such case it works like for interfaces, you cannot define the method body).
public abstract class MyAbstractClass{
public abstract void DoSomething();
}
Otherwise for normal methods of an abstract class, the "inheriters" can either just use the default behavior or override it, as usual.
Example:
public abstract class MyAbstractClass{
public int CalculateCost(int amount){
//do some default calculations
//this can be overriden by subclasses if needed
}
//this MUST be implemented by subclasses
public abstract void DoSomething();
}
From oracle documentation
Abstract Methods and Classes:
An abstract class is a class that is declared abstract—it may or may not include abstract methods
Abstract classes cannot be instantiated, but they can be subclassed
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, then the class itself must be declared abstract, as in:
public abstract class GraphicObject {
// declare fields
// declare nonabstract methods
abstract void draw();
}
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.
Since abstract classes and interfaces are related, have a look at below SE questions:
What is the difference between an interface and abstract class?
How should I have explained the difference between an Interface and an Abstract class?
Get your answers here:
Abstract class vs Interface in Java
Can an abstract class have a final method?
BTW - those are question you asked recently. Think about a new question to build up reputation...
Edit:
Just realized, that the posters of this and the referenced questions have the same or at least similiar name but the user-id is always different. So either, there's a technical problem, that keyur has problems logging in again and finding the answers to his questions or this is a sort of game to entertain the SO community ;)
Little addition to all these posts.
Sometimes you may want to declare a
class and yet not know how to define
all of the methods that belong to that
class. For example, you may want to
declare a class called Writer and
include in it a member method called
write(). However, you don't know how to code write() because it is
different for each type of Writer
devices. Of course, you plan to handle
this by deriving subclass of Writer,
such as Printer, Disk, Network and
Console.
An abstract class can not be directly instantiated, but must be derived from to be usable. A class MUST be abstract if it contains abstract methods: either directly
abstract class Foo {
abstract void someMethod();
}
or indirectly
interface IFoo {
void someMethod();
}
abstract class Foo2 implements IFoo {
}
However, a class can be abstract without containing abstract methods. Its a way to prevent direct instantation, e.g.
abstract class Foo3 {
}
class Bar extends Foo3 {
}
Foo3 myVar = new Foo3(); // illegal! class is abstract
Foo3 myVar = new Bar(); // allowed!
The latter style of abstract classes may be used to create "interface-like" classes. Unlike interfaces an abstract class is allowed to contain non-abstract methods and instance variables. You can use this to provide some base functionality to extending classes.
Another frequent pattern is to implement the main functionality in the abstract class and define part of the algorithm in an abstract method to be implemented by an extending class. Stupid example:
abstract class Processor {
protected abstract int[] filterInput(int[] unfiltered);
public int process(int[] values) {
int[] filtered = filterInput(values);
// do something with filtered input
}
}
class EvenValues extends Processor {
protected int[] filterInput(int[] unfiltered) {
// remove odd numbers
}
}
class OddValues extends Processor {
protected int[] filterInput(int[] unfiltered) {
// remove even numbers
}
}
Solution - base class (abstract)
public abstract class Place {
String Name;
String Postcode;
String County;
String Area;
Place () {
}
public static Place make(String Incoming) {
if (Incoming.length() < 61) return (null);
String Name = (Incoming.substring(4,26)).trim();
String County = (Incoming.substring(27,48)).trim();
String Postcode = (Incoming.substring(48,61)).trim();
String Area = (Incoming.substring(61)).trim();
Place created;
if (Name.equalsIgnoreCase(Area)) {
created = new Area(Area,County,Postcode);
} else {
created = new District(Name,County,Postcode,Area);
}
return (created);
}
public String getName() {
return (Name);
}
public String getPostcode() {
return (Postcode);
}
public String getCounty() {
return (County);
}
public abstract String getArea();
}
What is Abstract class?
Ok! lets take an example you known little bit about chemistry we have an element carbon(symbol C).Carbon has some basic atomic structure which you can't change but using carbon you can make so many compounds like (CO2),Methane(CH4),Butane(C4H10).
So Here carbon is abstract class and you do not want to change its basic structure however you want their childrens(CO2,CH4 etc) to use it.But in their own way
An abstract class is a class that is declared abstract — it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
In other words, a class that is declared with abstract keyword, is known as abstract class in java. It can have abstract(method without body) and non-abstract methods (method with body).
Important Note:-
Abstract classes cannot be used to instantiate objects, they can be used to create object references, because Java's approach to run-time Polymorphism is implemented through the use of superclass references. Thus, it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object. You will see this feature in the below example
abstract class Bike{
abstract void run();
}
class Honda4 extends Bike{
void run(){
System.out.println("running safely..");
}
public static void main(String args[]){
Bike obj = new Honda4();
obj.run();
}
}
An abstract class is one that isn't fully implemented but provides something of a blueprint for subclasses. It may be partially implemented in that it contains fully-defined concrete methods, but it can also hold abstract methods. These are methods with a signature but no method body. Any subclass must define a body for each abstract method, otherwise it too must be declared abstract.
Because abstract classes cannot be instantiated, they must be extended by at least one subclass in order to be utilized. Think of the abstract class as the generic class, and the subclasses are there to fill in the missing information.
Class which can have both concrete and non-concrete methods i.e. with and without body.
Methods without implementation must contain 'abstract' keyword.
Abstract class can't be instantiated.
It do nothing, just provide a common template that will be shared for it's subclass
I am trying to understand how default methods deal with diamond problem in various scenarios. And, this is one of the scenario which I'm not able to understand.
Following is the description,
1. Interface with a default method method()
2. Abstract Class with a method method()
3. Concrete Class implementing the above interface and extending the abstract class.
interface Interface {
default void method() {
System.out.println("Interface method");
}
}
abstract class AbstractClass {
void method() {
System.out.println("Abstract class method");
}
}
// Concrete class definition first starts
public class ConcreteClass extends AbstractClass implements Interface {
#Override
public void method() {
super.method();
}
public static void main(String[] args) {
Interface test = new ConcreteClass();
test.method();
}
}
// Concrete class definition first ends
// Concrete class definition Second starts
public class ConcreteClass extends AbstractClass implements Interface {
public static void main(String[] args) {
Interface test = new ConcreteClass();
test.method();
}
}
// Concrete class definition Second ends
My queries,
1. Why does definition first always gives output as "Abstract class method" even when I use the Interface type for concrete class object?
2. Why definition second doesn't compile?
If compiler is using abstract class implementation in definition first, then it should be able to identify that it will always use Abstract class implementation in definition second.
This behavior is very confusing to me and any help is greatly appreciated.
Otherwise, the more I delve deeper into this, the more confusing it gets.
Edit 1 :
Compilation error in second definition is "The inherited method AbstractClass.method() cannot hide the public abstract method in Interface"
Default methods are just that: defaults. If there is an implementation, it will be used. If there isn't, the default will be used. There is no diamond problem here (there can be with multiple defaults, however).
1) Dynamic dispatch
2) The abstract class gives the method named method package-private access; the interface demands it be public.
I am studying for OCA and I try to find answer for every question I encounter in my head.
This is the code:
interface InterfaceA {
public default Number method(){
return 5;
}
}
abstract class AbstractB {
public Long method() {
return 3L;
}
}
public class Foo extends AbstractB implements InterfaceA {
public static void main(String[] args) {
System.out.println(new Foo().method());
}
}
}
In this situation I don't have to override in the class Foo.
I know if I implement 2 interfaces with same methods I have to explicitly override in the class Foo. In the example above I get the result from AbstractB (3). My question: is abstract class implicitly overriding or hiding the method from the interface? What is the reason that class Foo prints the result from AbstractB and not from InterfaceA?
Abstract classes implement methods from interfaces. This does not count as an override, because there is no base method to override. In other words, if you try to do this
abstract class AbstractB {
public Long method() {
return super.method(); // <<==== Does not compile!
}
}
the call to base class method() would not compile.
Hiding does not apply either, because only static methods in Java can be hidden.
why class Foo gets the result from his parent class and not from interface?
Because interface provides a default implementation, i.e. what you get when there's nothing else provided in the line of inheritance. In this case, abstract base class provides an implementation, so it "wins" over the default implementation from the interface.
in this specific case the call to println prints 3 because AbstractB implements the method method of the interface InterfaceA in which case the class Foo inherits this bevahior. However, had AbstractB not implemented the interface method the println call would delegate to the default method of InterfaceA. Lastly, to answer your question AbstractB does not hide the method of the interface rather implements it as it has the same signature as the method in InterfaceA.
I came across a piece of code where there is a call to an abstract method ? I was just curious to know what it means ? Meanwhile the class is being called from a non-abstract method.
When you implement an abstract class you will also implement a concrete class. In the concrete class all abstract methods of the abstract class need to be implemented.
When a method in the abstract class call an abstract method, it refers to the implementation of this method in the concrete class.
public abstract class A {
public String method() {
// do something
int i = abstractMethod(); // abstract method call
}
public abstract int abstractMethod();
}
public class B extends A {
public int abstractMethod() { // <-- this implementation of the method is called.
return 1;
}
}
public class C extends A {
public int abstractMethod() { // <-- this implementation of the method is called.
return 100;
}
public static void main(String[] args) {
A a = new B();
a.method(); // -> abstractMethod of class B is called
a = new C();
c.method(); // -> abstractMethod of class C is called
}
}
An abstract method is a method that belongs to an abstract class but has no implementation which means that it has to be implemented (or overridden) by any children of the abstract class.
That is the whole idea of an abstract method / class: It allows the developer to define the behavior of the class later.
Assume, your class requires to "load some resource", but you want to provide different implementations for that task. Then you can override the abstract method with either loading a resource over the network or from the file system etc.
See: http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
An abstract method is a method inside an abstract class or an interface. It has no body, and one must be defined by a subclass before it can be instantiated. This allows interface classes that define some common methods, but leave some to the implementations.
The abstract methods can be called from non-abstract ones, because:
They are non-static, so they need an instance of that class to be called.
An instance of that class may not exist if the method hasn't been implemented, as the resulting class would also be abstract.
Therefore a caller will always have an instance of a class implementing that method.