Java Interface methods

There is a rule that every member of interface is only and only public whether you define or not. So when we define the method of the interface in a class implementing the interface, we have to give it public access as child class can’t assign the weaker access to the methods.
As defined, every method present inside interface is always public and abstract whether we are declaring or not. Hence inside interface the following methods declarations are equal.

void methodOne();
public Void methodOne();
abstract Void methodOne();
public abstract Void methodOne();

public : To make this method available for every implementation class.
abstract : Implementation class is responsible to provide implementation.
Also, We can’t use the following modifiers for interface methods.

  • Private
  • protected
  • final
  • static
  • synchronized
  • native
  • strictfp
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// A Simple Java program to demonstrate that
// interface methods must be public in 
// implementing class
interface A
{
    void fun();
}
  
class B implements A
    // If we change public to anything else,
    // we get compiler error
    public void fun()
    {
        System.out.println("fun()");
    }
}
  
class C
{
    public static void main(String[] args)
    {
        B b = new B();
        b.fun();
    }
}

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Output:

fun()

If we change fun() to anything other than public in class B, we get compiler error “attempting to assign weaker access privileges; was public”

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