Clarification needed in Interface

Question

I know that interface can have only static and final values implemented inside it.. But is there any loophole by which I can change the value of a variable using interface?? The question may be absurd, but I m helpless since its my requirement. Here is the example piece of code..

public interface I {
    int val = 1;  
    int changeValue();

}

---

Class A implements I{
    int changeValue(){
        val = 2 ; 
        return 0;
    }
}

How to change the value of 'val' using interface? Can I do something similar to:

val = changeValue();

Is there anything equivalent to do this functionality in an interface?

Answer 1

You cannot do this for an interface. However, it is possible to modify a static final variable in a class:

public abstract class I {
    static final int val; 
    static
    {
        val = 1;
    }
}

---

import java.lang.reflect.*;

public class NotFinal
{
    public static void main(String args[]) throws Exception
    {
        System.out.println(I.val); // Before: 1     

        Field field = I.class.getDeclaredField("val");
        field.setAccessible(true);
        Field modifiersField = Field.class.getDeclaredField("modifiers");
        modifiersField.setAccessible(true);
        modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, 2);

        System.out.println(I.val); // After: 2
    }
}

Output:

1
2

Note that this does not work if you assign the value in the declaration, i.e.

static final int val = 1;

because the compiler treats this differently (as a constant) - see this answer.

Answer 2

You cannot. Interface variables are static and final by default.

A final variable is a variable that cannot be changed during the life of the object.

A static vairable is a class variable - it means there is only one value of it for all instances of the class (or interface in this case).

Thus - you only have one value for I.x - and this value cannot be changed.

---

What you might want to do, is define methods in your interface:

int getVal();
void setVal(int val);

And make the implementing classes implement the methods - so you will be able to use the variable with the getVal() and setVal() methods.