Pass-by-Value in Java (AP CSA)

Java is always pass-by-value. When you pass an argument to a method, the method gets a copy of that value. Changes to the parameter inside the method do not affect the original variable.

Primitives: Copy of the Value

public static void addTen(int num) {
    num = num + 10;    // changes the LOCAL copy only
    System.out.println("Inside: " + num);
}

public static void main(String[] args) {
    int x = 5;
    addTen(x);
    System.out.println("Outside: " + x);  // still 5!
}

Output: Inside: 15 then Outside: 5. The original x is unchanged.

Objects: Copy of the Reference

When you pass an object, Java copies the reference (address). The method gets its own copy of the reference, but both copies point to the same object. So you can change the object’s contents, but you can’t make the original variable point somewhere else.

public static void changeFirst(int[] arr) {
    arr[0] = 999;    // modifies the actual array object
}

public static void main(String[] args) {
    int[] nums = {1, 2, 3};
    changeFirst(nums);
    System.out.println(nums[0]);  // 999 -- the object WAS changed
}

public static void doubleIt(int x) {
    x = x * 2;
}
public static void main(String[] args) {
    int val = 7;
    doubleIt(val);
    System.out.println(val);
}