How Do You Achieve a Pass-by-Reference Effect for Primitives in Java?

Java's Equivalent to Pass by Reference for Primitives

In C , pass by reference allows functions to modify variables outside their scope. Java, however, operates on pass by value, making it challenging to achieve the same effect for primitives.

Consider the following code:

public class XYZ {
    public static void main(String[] args) {
        int toyNumber = 5;
        XYZ temp = new XYZ();
        temp.play(toyNumber);
        System.out.println("Toy number in main " + toyNumber);
    }

    void play(int toyNumber) {
        System.out.println("Toy number in play " + toyNumber);
        toyNumber++;
        System.out.println("Toy number in play after increment " + toyNumber);
    }
}

This code will result in:

Toy number in play 5
Toy number in play after increment 6
Toy number in main 5

Unlike C , changes made to toyNumber within play() do not affect its value in main(). This is because Java passes primitive values by copy, not by reference.

Alternative Approaches

1. Use an Object Wrapper:

One approach is to wrap the primitive in an object and pass the object by reference:

class ToyNumber {
    int toyNumber;
}

public static void main(String[] args) {
    ToyNumber toyNumber = new ToyNumber(5);
    XYZ temp = new XYZ();
    temp.play(toyNumber);
    System.out.println("Toy number in main " + toyNumber.toyNumber);
}

void play(ToyNumber toyNumber) {
    System.out.println("Toy number in play " + toyNumber.toyNumber);
    toyNumber.toyNumber++;
    System.out.println("Toy number in play after increment " + toyNumber.toyNumber);
}

This method allows for changes made in play() to be reflected in main(). However, it introduces additional overhead by creating an object.

2. Return the Value:

Another option is to return the modified value:

int play(int toyNumber) {
    ...
    return toyNumber;
}

public static void main(String[] args) {
    int toyNumber = 5;
    toyNumber = temp.play(toyNumber);
    System.out.println("Toy number in main " + toyNumber);
}

This approach works but requires changing the callsite to receive the return value.

3. Static or Class Variables:

If play() and main() are within the same class or class instance, consider using a static or class variable:

class XYZ {
    static int toyNumber = 5;
    ...
}

public static void main(String[] args) {
    XYZ.play();
    System.out.println("Toy number in main " + XYZ.toyNumber);
}

static void play() {
    ...
    XYZ.toyNumber++;
    ...
}

This method allows both functions to modify the same variable, but it can lead to concurrency issues and is not recommended for multi-threaded applications.

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