Permanent change of a variable in static method

Question

Let's say I have a public class with static methods, one of them for example:

 public static void test(boolean b){
   b = !b;
 }

Let's say this class name is Test. From another class, where I have a variable boolean a = false, I call

 Test.test(a);

How can I make it change a permanently, and not just change it in that static methods scope?

Can you explain why you are trying to do this as it is likely there is a better way to solve your actual problem. — Peter Lawrey, Dec 27 '14 at 19:54
I know how to go around it, I just didn't know if this is possible so I felt like asking :) — Greyshack, Dec 27 '14 at 19:57
Then the answer is don't do that, do something else. Changing code on the fly is not a simple process. You should think about changing fields. — Peter Lawrey, Dec 27 '14 at 20:21
Remember, functions don't receive the variables you pass into them; they receive the values of those variables. They can change the value they stored, but they can't change the variable. See 'Is Java “pass-by-reference” or “pass-by-value”?' — Vitruvie, Dec 27 '14 at 20:34

score 2 · Answer 1 · answered Dec 27 '14 at 19:51

2

The only way to make the change permanent is to let the method have a return value and assign it to the variable :

public static boolean test(boolean b){
   return !b;
}

a = Test.test(a);

answered Dec 27 '14 at 19:51

Eran

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You sure it's the only way? How could I change two variables at once? – Greyshack Dec 27 '14 at 19:52
1

@Greyshack: You don't. If you need multiple variables to change, you should almost certainly encapsulate the combination of those variables in a class. – Jon Skeet Dec 27 '14 at 19:53
1

@Greyshack Changing two variables at once is not possible. If the variable is a reference to a mutable object, you can change the properties of the object, but you can't change the reference itself. – Eran Dec 27 '14 at 19:57

score 0 · Answer 2 · answered Dec 27 '14 at 19:53

0

Use a static field:

public static boolean flag;

public static void test(boolean b){
    flag = !b;
}

Then:

boolean a = true;
Test.test(a);

System.out.println( Test.flag); // false

answered Dec 27 '14 at 19:53

Bohemian

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void · Answer 3 · 2014-12-27T20:15:19.890

0

in your Test class you can define the boolean variable as static

public static boolean a;

and outside the class change or access it using Test.a=false; or a=Test.a

and if you need to use methods you can hide the static method with inheritance:

public class HideStatic {     
public static void main(String...args){  
    BaseA base = new ChildB();  
    base.someMethod();        
}  
}  

class BaseA {         
    public static void someMethod(){  
        System.out.println("Parent method");  
    }  
}  

class ChildB extends BaseA{  
    public void someMethod(){  
        System.out.println("Child method");  
    }  
}

edited Dec 27 '14 at 20:15

answered Dec 27 '14 at 19:54

void

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Ok but that way I have the variable in the class with static methods, and what if I want to have different variables in different classes and use that method? – Greyshack Dec 27 '14 at 19:56
so what about hiding them using a new class which inherits the static holder class? (I edited the answer) – void Dec 27 '14 at 20:05

score 0 · Answer 4 · answered Dec 27 '14 at 19:57

0

You can pass an instance to method and use setters to change more variables at once.

public static void updateData(MyClass instance) {
    instance.setX(1);
    instance.setY(2);
}

answered Dec 27 '14 at 19:57

tomasbedrich

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score 0 · Answer 5 · edited May 23 '17 at 10:26

0

I think you are asking for call by reference. You can get this in Java by using arrays:

public static void test(boolean[] b){
b[0] = !b[0];
}

boolean[] param = new boolean[] {a};
test(param);
a=param[0];
//a changed

This works, but it is ugly. If you need to return more than one value, have a look at the Pair or Tuple structures.

edited May 23 '17 at 10:26

Community

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answered Dec 27 '14 at 20:29

Cfx

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Ok, that would make sense, in c,c++ arrays are also pointers to the first element. Good to know :) – Greyshack Dec 28 '14 at 13:28

score 0 · Accepted Answer · answered Dec 27 '14 at 21:40

Sounds to me like you are looking for a Mutable Boolean, the simplest of which is an AtomicBoolean.

private void changeIt(AtomicBoolean b) {
    b.set(!b.get());
}

public void test() {
    AtomicBoolean b = new AtomicBoolean(false);
    changeIt(b);
    System.out.println(b);
}

Permanent change of a variable in static method

6 Answers6