Recursive pass in object by reference? JAVA

Question

So I have an algorithm that forces me to pass in an object as a parameter recursively and on different depths of the recursion it sets the values of the object. The problem is Java isn't allowing me to do this because they pass in by value or something else that is just messing me up. How can you make sure that the object passed in retains the value set?

Answer 1

You never pass an object as an argument in Java. You either pass a reference or a primitive value. The argument is always passed by value. Java doesn't have pass-by-reference at all.

If you're passing in a reference to an object, and you want to make sure the code you call doesn't change the data within that object, your options are:

• Create a copy and pass in a reference to that instead
• Make your type immutable in the first place.

EDIT: Just to make it clear, if you have:

Foo f = new Foo();

then the value of f is not a Foo object. It's a reference to a Foo object. If you now call:

doSomething(f);

then the value of f is copied as the original value of the parameter within doSomething. That behaviour is basically the definition of pass-by-value. There's nothing doSomething can to do change the value of f - it will still refer to the same object afterwards. The data within that object may have changed, but it won't be a different object.

In a real-world analogy: if I give you a copy of my address, you can go and paint the front door, but you can't change where I live.

Answer 2

As Jon states, all non-primitives are passed by passing a copy of the reference. You don't have a choice. So your recursive method should be able to update the object if it is mutable. HOWEVER, you can't reassign a reference and expect it to propagate back up the call stack. And remember that Strings are not mutable.

In other words, if you do the following, you have not changed the object that was passed:

void myMethod(Object o){
     o = new Object();
}

The above changes the local reference to o but does not change the caller's reference to o. If you need to do something like this you would need to return o.