Question

Let’s consider the following code in Java.

package obj;

final class First
{
    public int x;

    public First(int x)
    {
        this.x=x;
    }
}

final class Second
{        
    public Second(First o)
    {
        o.x=10;
    }
}

final public class Main
{
    public static void main(String[] args)
    {
        First f=new First(50);

        Second s=new Second(f);

        System.out.println("x = "+f.x);
    }
}

In the above code, we are supplying a value 50 through the statement First f=new First(50); which is being assigned to a class member x of type int in the constructor body in the class First.

---

In the class Second, the object f of the class First is being passed through the statement Second s=new Second(f); and we are modifying the value of the instance variable x held in that object to 10 which will affect the original object f because in Java objects are always passed by reference and not by value.

---

In some specific situations, it may be crucial not to allow such changes to the original objects. Is there any mechanism in Java that may allow us to prevent such modifications? (that might allow us to pass objects by value)

Answer 1

No, the object isn’t being passed at all in Second. The reference is being passed by value.

Java always uses pass-by-value, but the value of any expression is only ever a primitive type or a reference – never an object.

It sounds like you want to create a copy of an existing object, then pass a reference to that new object to the method (by value). Quite what constitutes a “copy” will depend on the data in the class. (You may be able to get away with a shallow copy, or you may need to go deeper etc.)