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?

Interface variables are by default `public static final` and methods are by default `abstract public`. You can't use the `private` or `protected`modifier within an interface anyway. The sole purpose of an interface is to achieve publicly defined protocol behaviour. — Lion, Oct 19 '12 at 23:13

amit · Accepted Answer · 2012-10-19T23:11:18.110

4

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.

edited Oct 19 '12 at 23:11

answered Oct 19 '12 at 23:04

amit

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Variables in interfaces also are public by default. – Luiggi Mendoza Oct 19 '12 at 23:07
@LuiggiMendoza: To be even more exact, everything in an `interface` is `public` – amit Oct 19 '12 at 23:08
Thank You.. My actual requirement is such that I want to reuse, "Future>" in two methods of one class.. In 1st method, the value of "Future> changes and its being used back again in 2nd method.. I must not pass the values as parameter too. That is the reason why I selected Interface.. Now that knowing, its not feasible I have to think of some other option – Vikram Oct 19 '12 at 23:13
@amit , can you kindly brief a little bit about your getVal() and setVal() way of changing the value of 'val' ?? – Vikram Oct 19 '12 at 23:22
@asvikki: It is not changing the value of `val` - it will let you define a new variable in each implementing class - but not change an interface variable - that cannot be done. – amit Oct 19 '12 at 23:27
Clarification: "let you define..." really means - will let you a single API for changing/reading a newly added variable in each implementing class - but never the field that was declared in the interface. – amit Oct 19 '12 at 23:34

score 0 · Answer 2 · edited May 23 '17 at 12:12

0

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.

edited May 23 '17 at 12:12

Community

1
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answered Oct 19 '12 at 23:22

DNA

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Looks interesting .. Thanks. I ll give a try with this – Vikram Oct 19 '12 at 23:25
I don't actually recommend this approach - I merely point it out for academic purposes ;-) There is almost certainly a better way, but you'd need to explain a little more about what you are trying to achieve, perhaps in a new question. – DNA Oct 19 '12 at 23:26
Ok, got it :) can you recommend something for my query, U can see that in the first answer comments.. – Vikram Oct 19 '12 at 23:29

Clarification needed in Interface

2 Answers2