We all know abstract modifier in a class makes it
non-instantiable,
candidate for abstract methods,
Non final, non static, non private
In addition to that I would like to know exactly all the changes or side-effects it represents behind. One cannot ask what he/she does not now yet, but I'd like to know whether is something more to consider in terms of inheriting, constructors etc.
Aside from the two official points described, is there any difference or special behavior between an abstract class and a concrete one to be considered when extending, calling super.
Does the compiler assume it is a regular class and has the properties as such for everything other than disallowing instantiation?
Abstract class is, for the most part, a design concept. It does more for the readers of your code than it does for the compiler. The compiler and JVM support required for them is minimal: it boils down to setting a "do not instantiate me" flag on the class, and checking it when compiling the code and when trying to create an instance through reflection.
Benefits to human readers of your code, on the other hand, are much bigger: they know that you designed your abstract class for inheritance, and see what extension points you made for them through abstract methods. In addition, the compiler will track for them if they have provided overrides for all abstract methods.
By making private constructor, we can avoid instantiating class from anywhere outside. and by making class final, no other class can extend it. Why is it necessary for Util class to have private constructor and final class ?
This is not a mandate from a functional point of view or java complication or runtime. However, it's a coding standard accepted by the wider community. Even most static code review tools, like checkstyle, check that such classes have this convention followed.
Why this convention is followed is already explained in other answers and even OP covered that, but I'd like to explain it a little further.
Mostly utility classes are a collection of methods/functions which are independent of an object instance. Those are kind of like aggregate functions as they depend only on parameters for return values and are not associated with class variables of the utility class. So, these functions/methods are mostly kept static. As a result, utility classes are, ideally, classes with only static methods. Therefore, any programmer calling these methods doesn't need to instantiate the class. However, some robo-coders (maybe with less experience or interest) will tend to create the object as they believe they need to before calling its method. To avoid that, we have 3 options:
Keep educating people to not instantiate it. (No sane person can keep doing it.)
Mark the utility class as abstract: Now robo-coders will not create the object. However, reviewers and the wider java community will argue that marking the class as abstract means you want someone to extend it. So, this is also not a good option.
Private constructor: Not protected because it'll allow a child class to instantiate the object.
Now, if someone wants to add a new method for some functionality to the utility class, they don't need to extend it: they can add a new method as each method is independent and has no chance of breaking other functionalities. So, no need to override it. Also, you are not going to instantiate it, so no need to subclass it. Better to mark it final.
In summary, instantiating a utility class (new MyUtilityClass()) does not make sense. Hence the constructors should be private. And you never want to override or extend it, so mark it final.
It's not necessary, but it is convenient. A utility class is just a namespace holder of related functions and is not meant to be instantiated or subclassed. So preventing instantiation and extension sends a correct message to the user of the class.
There is an important distinction between the Java Language, and the Java Runtime.
When the java class is compiled to bytecode, there is no concept of access restriction, public, package, protected, private are equivalent. It is always possible via reflection or bytecode manipulation to invoke the private constructor, so the jvm cannot rely on that ability.
final on the other hand, is something that persists through to the bytecode, and the guarantees it provides can be used by javac to generate more efficient bytecode, and by the jvm to generate more efficient machine instructions.
Most of the optimisations this enabled are no longer relevant, as the jvm now applies the same optimisations to all classes that are monomorphic at runtime—and these were always the most important.
By default this kind of class normally is used to aggregate functions who do different this, in that case we didn't need to create a new object
I needed to do a java program for my class.
I did it and worked well.
In my code, I have some private methods. Do I need to build an Interface Class hide those private methods or they can be at the same class as the public methods?
At the moment all methods (public and private) are in the same class, but my coworker insists that I need to create an Interface to hide the private methods
No, you don't need to create an interface. An interface would hide them even more (from people's eyes, not from code) if you were to use the class only through the interface, but even without one, the private methods won't be available to other classes.
If you want to implement the "code to an interface" guideline fully, you can declare an interface for your public methods to implement.
What you can definitely not do is declare methods in an interface, then "implement" them as private in your class - that would reduce the visibility and not even compile.
Ultimately private methods are scoped to your class only.
It might help to repeat "what is the purpose of an Interface?"
It is: "a formal declaration of what, as far as anyone else should be concerned, this thing 'gives,' 'takes,' and 'does.'"
So ... if a client class wants to deal with "something that implements this Interface," and five other classes (none of which, say, are siblings or ancestors of one another ...), each in their own way, do so, then: any of the five would be compatible. Why? Because they supply all of the properties and/or methods that the interface requires. (That's what "implementing" actually means.)
None of this exposes any of the client's "private things." In fact, an Interface says absolutely nothing about how the class actually does what it has to do, nor what else it does. "Meet the requirements of the Interface, and you can get the job."
While reading about constant interface antipattern, i found final constant class with no instances is better than constant interface.
Please explain me how?
public interface ConstIfc {
public static final int constValue = 10;
}
public final class ConstClass {
private ConstClass{}
public static final int constValue = 10;
}
If constValue has to be used in a UtilClass without naming of Ifc/Class name we can implement/extends those. but implements supports multiple inheritance. So how extends better?
Note: I'm able to understand static import.
I think the argument is that you don't need to extend OR implement a constant class or interface, now that we have static imports. So if you're going to use static imports, having a class for your constants better fits the idea of what a class actually is, versus what an interface is. And making it a final class removes the temptation to apply the anti-pattern; that is, to extend or implement the type that defines the constants.
In practice though, I don't think it makes too much difference which pattern you use.
You should consider using Enums instead of classes/interfaces with constants. It is much more flexible, powerful and robust choice.
Interfaces are an abstraction, and to remain abstract, they should not contain implementation details (including constant variables.) Interfaces also are often used to describe a public API, in which implementation details do not belong. For this reason it makes sense to put constant data into a class, rather than an interface.
I'm not sure what you mean by "how extends better?", but I think you should avoid inheriting / extending this sort of implementation detail into multiple classes. Improperly leveraging implementation inheritance often leads to inflexible design. In your example, the final keyword on ConstClass utilizes the compiler to prevent you from doing this, which is not possible with an interface.
I am reading a book about Java and it says that you can declare the whole class as final. I cannot think of anything where I'd use this.
I am just new to programming and I am wondering if programmers actually use this on their programs. If they do, when do they use it so I can understand it better and know when to use it.
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
First of all, I recommend this article: Java: When to create a final class
If they do, when do they use it so I can understand it better and know when to use it.
A final class is simply a class that can't be extended.
(It does not mean that all references to objects of the class would act as if they were declared as final.)
When it's useful to declare a class as final is covered in the answers of this question:
Good reasons to prohibit inheritance in Java?
If Java is object oriented, and you declare a class final, doesn't it stop the idea of class having the characteristics of objects?
In some sense yes.
By marking a class as final you disable a powerful and flexible feature of the language for that part of the code. Some classes however, should not (and in certain cases can not) be designed to take subclassing into account in a good way. In these cases it makes sense to mark the class as final, even though it limits OOP. (Remember however that a final class can still extend another non-final class.)
In Java, items with the final modifier cannot be changed!
This includes final classes, final variables, and final methods:
A final class cannot be extended by any other class
A final variable cannot be reassigned another value
A final method cannot be overridden
One scenario where final is important, when you want to prevent inheritance of a class, for security reasons. This allows you to make sure that code you are running cannot be overridden by someone.
Another scenario is for optimization: I seem to remember that the Java compiler inlines some function calls from final classes. So, if you call a.x() and a is declared final, we know at compile-time what the code will be and can inline into the calling function. I have no idea whether this is actually done, but with final it is a possibility.
The best example is
public final class String
which is an immutable class and cannot be extended.
Of course, there is more than just making the class final to be immutable.
If you imagine the class hierarchy as a tree (as it is in Java), abstract classes can only be branches and final classes are those that can only be leafs. Classes that fall into neither of those categories can be both branches and leafs.
There's no violation of OO principles here, final is simply providing a nice symmetry.
In practice you want to use final if you want your objects to be immutable or if you're writing an API, to signal to the users of the API that the class is just not intended for extension.
Relevant reading: The Open-Closed Principle by Bob Martin.
Key quote:
Software Entities (Classes, Modules,
Functions, etc.) should be open for
Extension, but closed for
Modification.
The final keyword is the means to enforce this in Java, whether it's used on methods or on classes.
The keyword final itself means something is final and is not supposed to be modified in any way. If a class if marked final then it can not be extended or sub-classed. But the question is why do we mark a class final? IMO there are various reasons:
Standardization: Some classes perform standard functions and they are not meant to be modified e.g. classes performing various functions related to string manipulations or mathematical functions etc.
Security reasons: Sometimes we write classes which perform various authentication and password related functions and we do not want them to be altered by anyone else.
I have heard that marking class final improves efficiency but frankly I could not find this argument to carry much weight.
If Java is object oriented, and you declare a class final, doesn't it
stop the idea of class having the characteristics of objects?
Perhaps yes, but sometimes that is the intended purpose. Sometimes we do that to achieve bigger benefits of security etc. by sacrificing the ability of this class to be extended. But a final class can still extend one class if it needs to.
On a side note we should prefer composition over inheritance and final keyword actually helps in enforcing this principle.
final class can avoid breaking the public API when you add new methods
Suppose that on version 1 of your Base class you do:
public class Base {}
and a client does:
class Derived extends Base {
public int method() { return 1; }
}
Then if in version 2 you want to add a method method to Base:
class Base {
public String method() { return null; }
}
it would break the client code.
If we had used final class Base instead, the client wouldn't have been able to inherit, and the method addition wouldn't break the API.
A final class is a class that can't be extended. Also methods could be declared as final to indicate that cannot be overridden by subclasses.
Preventing the class from being subclassed could be particularly useful if you write APIs or libraries and want to avoid being extended to alter base behaviour.
In java final keyword uses for below occasions.
Final Variables
Final Methods
Final Classes
In java final variables can't reassign, final classes can't extends and final methods can't override.
Be careful when you make a class "final". Because if you want to write an unit test for a final class, you cannot subclass this final class in order to use the dependency-breaking technique "Subclass and Override Method" described in Michael C. Feathers' book "Working Effectively with Legacy Code". In this book, Feathers said, "Seriously, it is easy to believe that sealed and final are a wrong-headed mistake, that they should never have been added to programming languages. But the real fault lies with us. When we depend directly on libraries that are out of our control, we are just asking for trouble."
If the class is marked final, it means that the class' structure can't be modified by anything external. Where this is the most visible is when you're doing traditional polymorphic inheritance, basically class B extends A just won't work. It's basically a way to protect some parts of your code (to extent).
To clarify, marking class final doesn't mark its fields as final and as such doesn't protect the object properties but the actual class structure instead.
TO ADDRESS THE FINAL CLASS PROBLEM:
There are two ways to make a class final. The first is to use the keyword final in the class declaration:
public final class SomeClass {
// . . . Class contents
}
The second way to make a class final is to declare all of its constructors as private:
public class SomeClass {
public final static SOME_INSTANCE = new SomeClass(5);
private SomeClass(final int value) {
}
Marking it final saves you the trouble if finding out that it is actual a final, to demonstrate look at this Test class. looks public at first glance.
public class Test{
private Test(Class beanClass, Class stopClass, int flags)
throws Exception{
// . . . snip . . .
}
}
Unfortunately, since the only constructor of the class is private, it is impossible to extend this class. In the case of the Test class, there is no reason that the class should be final. The Test class is a good example of how implicit final classes can cause problems.
So you should mark it final when you implicitly make a class final by making it's constructor private.
One advantage of keeping a class as final :-
String class is kept final so that no one can override its methods and change the functionality. e.g no one can change functionality of length() method. It will always return length of a string.
Developer of this class wanted no one to change functionality of this class, so he kept it as final.
The other answers have focused on what final class tells the compiler: do not allow another class to declare it extends this class, and why that is desirable.
But the compiler is not the only reader of the phrase final class. Every programmer who reads the source code also reads that. It can aid rapid program comprehension.
In general, if a programmer sees Thing thing = that.someMethod(...); and the programmer wants to understand the subsequent behaviour of the object accessed through the thing object-reference, the programmer must consider the Thing class hierarchy: potentially many types, scattered over many packages. But if the programmer knows, or reads, final class Thing, they instantly know that they do not need to search for and study so many Java files, because there are no derived classes: they need study only Thing.java and, perhaps, it's base classes.
Yes, sometimes you may want this though, either for security or speed reasons. It's done also in C++. It may not be that applicable for programs, but moreso for frameworks.
http://www.glenmccl.com/perfj_025.htm
think of FINAL as the "End of the line" - that guy cannot produce offspring anymore. So when you see it this way, there are ton of real world scenarios that you will come across that requires you to flag an 'end of line' marker to the class. It is Domain Driven Design - if your domain demands that a given ENTITY (class) cannot create sub-classes, then mark it as FINAL.
I should note that there is nothing stopping you from inheriting a "should be tagged as final" class. But that is generally classified as "abuse of inheritance", and done because most often you would like to inherit some function from the base class in your class.
The best approach is to look at the domain and let it dictate your design decisions.
As above told, if you want no one can change the functionality of the method then you can declare it as final.
Example: Application server file path for download/upload, splitting string based on offset, such methods you can declare it Final so that these method functions will not be altered. And if you want such final methods in a separate class, then define that class as Final class. So Final class will have all final methods, where as Final method can be declared and defined in non-final class.
Let's say you have an Employee class that has a method greet. When the greet method is called it simply prints Hello everyone!. So that is the expected behavior of greet method
public class Employee {
void greet() {
System.out.println("Hello everyone!");
}
}
Now, let GrumpyEmployee subclass Employee and override greet method as shown below.
public class GrumpyEmployee extends Employee {
#Override
void greet() {
System.out.println("Get lost!");
}
}
Now in the below code have a look at the sayHello method. It takes Employee instance as a parameter and calls the greet method hoping that it would say Hello everyone! But what we get is Get lost!. This change in behavior is because of Employee grumpyEmployee = new GrumpyEmployee();
public class TestFinal {
static Employee grumpyEmployee = new GrumpyEmployee();
public static void main(String[] args) {
TestFinal testFinal = new TestFinal();
testFinal.sayHello(grumpyEmployee);
}
private void sayHello(Employee employee) {
employee.greet(); //Here you would expect a warm greeting, but what you get is "Get lost!"
}
}
This situation can be avoided if the Employee class was made final. Just imagine the amount of chaos a cheeky programmer could cause if String Class was not declared as final.
Final class cannot be extended further. If we do not need to make a class inheritable in java,we can use this approach.
If we just need to make particular methods in a class not to be overridden, we just can put final keyword in front of them. There the class is still inheritable.
Final classes cannot be extended. So if you want a class to behave a certain way and don't someone to override the methods (with possibly less efficient and more malicious code), you can declare the whole class as final or specific methods which you don't want to be changed.
Since declaring a class does not prevent a class from being instantiated, it does not mean it will stop the class from having the characteristics of an object. It's just that you will have to stick to the methods just the way they are declared in the class.
Android Looper class is a good practical example of this.
http://developer.android.com/reference/android/os/Looper.html
The Looper class provides certain functionality which is NOT intended to be overridden by any other class. Hence, no sub-class here.
I know only one actual use case: generated classes
Among the use cases of generated classes, I know one: dependency inject e.g. https://github.com/google/dagger
Object Orientation is not about inheritance, it is about encapsulation. And inheritance breaks encapsulation.
Declaring a class final makes perfect sense in a lot of cases. Any object representing a “value” like a color or an amount of money could be final. They stand on their own.
If you are writing libraries, make your classes final unless you explicitly indent them to be derived. Otherwise, people may derive your classes and override methods, breaking your assumptions / invariants. This may have security implications as well.
Joshua Bloch in “Effective Java” recommends designing explicitly for inheritance or prohibiting it and he notes that designing for inheritance is not that easy.