What's the difference between public method and abstract method in Interface?

Question

Consider the following interface :

public interface MyInterface {

    public void func1();
    public void func2();
    abstract public void func3();

}

and the class MyClass :

public class MyClass implements MyInterface{

    @Override
    public void func1() {
        // TODO Auto-generated method stub

    }

    @Override
    public void func2() {
        // TODO Auto-generated method stub

    }

    @Override
    public void func3() {
        // TODO Auto-generated method stub

    }
}

What's the difference between func2() and func3() ?

I must implement both when I want to implement MyClass , so it seems that there's no difference if I write public or abstract public in the interface .

Thanks

Answer 1

Java 7 and earlier:

There is no difference since all interface methods are public and "abstract." This is implied whether declared or not.

Java 8:

The same rules apply as in Java 7, however, it should be noted that since Java 8, only non-default methods are "abstract." Default methods , are in fact, allowed to have an implementation.

Java 9:

In Java 9, we are provided even more flexibility, and also allowed to have private methods .

Answer 2

Java Language Specification quote :

Every method declaration in the body of an interface is implicitly abstract , so its body is always represented by a semicolon, not a block.

You ask

What's the difference between func2() and func3() ?

Except for the fact that they are different methods, their modifiers are the same.

Answer 3

What's the difference between public method and abstract method in Interface?

9.1.1.1. abstract Interfaces Simply said... none. According to this, it's obsolete.

Answer 4

In a way, we are comparing apples and oranges in this question. For someone new to the Java language, some clarifications need to be made first:

Interfaces:

• Interfaces contain method stubs. These are methods without a body. These methods are implicitly "abstract" and we do not need to explicitly use a modifier to denote this.

• That being said, since Java 8 , we now have what are called " default methods ." These methods are fully implemented methods which do require an explicit declaration using the default keyword.

• That being said, the abstract keyword never comes into play in the context of an interface.

Abstract Methods:

• Abstract methods only belong in abstract classes.

• An abstract class may contain fully implemented methods along with abstract methods. However, just one abstract method in a class is enough to make it an abstract class - That is, the class must be explicitly declared as an abstract class.

Answer 5

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.