Java changing value of a variable through a method?

Question

Hello I was wondering how I would be able to send a variable as a parameter to a method and have it be changed by the method. For example

public class Main {
    public static void main(String[] args) {
        int i = 2;
        doThis(i);
        System.out.println(i);  
    }

    public static void doThis(int i) {
        i = 3;
    }
}

I would like it to print out 3 instead of 2. Thanks.

Java use pass by value so you can't pass in a primitive and modify the primitive value. You could pass in an array on integers and then modify the integers in the array or return a different integer. — camickr, May 12 '13 at 03:39
http://stackoverflow.com/questions/5614562/how-to-pass-by-reference-in-java look at this. — Bill, May 12 '13 at 03:40
http://stackoverflow.com/questions/373419/whats-the-difference-between-passing-by-reference-vs-passing-by-value — Steve P., May 12 '13 at 03:40

score 11 · Answer 1 · answered May 12 '13 at 03:40

11

Java can't do this. However you can return the value from the method...

public static int doThis(int i) {
    return 3;
}

And reassign it...

int i = 2;
i  = doThis(i);

answered May 12 '13 at 03:40

MadProgrammer

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Yeah that's what I thought. Hmm... Been doing too much C lately. – Tommy May 12 '13 at 03:42
1

Hmm, not sure why this got an upvote since the method still returns void. – camickr May 12 '13 at 03:43
You could create a Object that allowed a method to get and set the int value (internally to the Object), this would allow you to modify the int value, you just ant modify the object itself – MadProgrammer May 12 '13 at 03:43
@camickr Nice to see someone's paying attention – MadProgrammer May 12 '13 at 03:45

score 7 · Answer 2 · answered May 12 '13 at 05:42

Java passes everything by value, so if you use the assignment operator in a class method you're not going to be changing the original object.

For example:

public class Main {
    public static void main(String[] args) {
        Integer i = new Integer(2);
        setToThree(i);
        System.out.println(i);
    }

    public static void setToThree(Integer i) {
        i = new Integer(3);
    }
}

is going to print 2.

Having said that, if the object you're passing in a reference to is mutable you can make changes to it in they way you're thinking of.

For example:

public class Main {
    public static void main(String[] args) {
        MyMutableInt i = new MyMutableInt(2);
        setToThree(i);
        System.out.println(i);
    }        

    public static void setToThree(MyMutableInt i) {
        i.set(3);
    }
}

This will print 3 (assuming MyMutableInt has a correct toString() method).

Of course, Java Integers are immutable, and so don't have the ability to be changed like that. So you have 2 choices here:

Return the value
Use something like MutableInt from commons-lang (http://commons.apache.org/proper/commons-lang/javadocs/api-3.1/org/apache/commons/lang3/mutable/MutableInt.html) (or create your own mutable object).

Note: this doesn't work with primitives of any kind. For that you're going to have to pass back by return value. If you have multiple values to mutate, you'll have to wrap them in an object to return them, so you may also use this method.

score 6 · Answer 3 · answered May 12 '13 at 03:40

6

I would like it to print out 3 instead of 2

Change method to return value

int i = 2;
i = doThis(i);


public static int doThis(int i) {
    i = 3;
    return i;
}

it copies the value of primitive from caller to argument

answered May 12 '13 at 03:40

jmj

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+1, for being the first to return a value by modifying the method signature. – camickr May 12 '13 at 03:44

score 1 · Answer 4 · edited May 23 '17 at 12:00

1

java is all about pass by values. so it wont change the way you wanted. Read here for more

however you do following to solve this problem

i = doThis(i);

public static int doThis(int i) {
    i = 3;
    return i;
}

edited May 23 '17 at 12:00

Community

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answered May 12 '13 at 03:43

Peeyush

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sorry about that, it shouldn't be a downvote. If you make an edit, I can un-downvote. – Joshua Taylor May 12 '13 at 16:22
The downvote was accidental, but when I try to remove it, it says that it's now locked in and can't be removed unless the answer is edited. If you make some trivial edit, I can remove my downvote. – Joshua Taylor May 13 '13 at 13:28
There you go—downvote removed. Sorry about that. – Joshua Taylor May 13 '13 at 14:35

score 1 · Answer 5 · answered May 12 '13 at 04:42

1

Java is pass by value for primitive data type or objects.
Reference data type parameters, such as objects, are also passed into methods by value.

answered May 12 '13 at 04:42

gifpif

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Java changing value of a variable through a method?

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