CSC300: Assignment and Parameters with Object Types [15/24]

Very Important!

Assignment, or passing a reference to an object of some type to a method with an object type parameter results in copying the address of the object to a new place in memory.

This includes boxed types, arrays, and any other object for which a class is defined. Modifying the data in the object (if the object is mutable) affects what the calling code "sees".

static class MyInteger {
    int value;
    MyInteger(int value) {
        this.value = value;
    }
    void add(int value) {
        this.value += value;
    }
    void multiply(int value) {
        this.value *= value;
    }
}

public static void main (String[] args) {
    // Try tracing the execution of this code.
    MyInteger x = new MyInteger(35);
    MyInteger y = x;
    y.add(10);
    y = f(y);
}

public static MyInteger f(MyInteger y) {
    y.multiply(10);
    return y;
}

Note: The variable y in the main program is simply an alias for the object also referenced by the variable x.

The variable y in the function f is pointing at the same object (although we wouldn't call it an alias here)!