Question

Okay this is definitely a n00b question but here goes.

The way I understand it, is objects in Java are passed by value, not reference. How then do you alter a class member within a method by passing a reference to it to the method?

I know it is possible to do, because the method getRotationMatrix does it… ie if I write the following at the class level:

private float rotationMatrix[] = new float[9];

and then within a method I call:

SensorManager.getRotationMatrix(rotationMatrix, inclinationMatrix, gData, mData);

It will alter rotationMatrix. My question is how does that method change what is in the object I passed it if everything is passed by value?

The reason I ask is I am trying to write a method that does the same thing, but keep running into the pass-by-value conundrum. I’m trying to initialize a FloatBuffer by passing a reference to it to a method.

Answer 1

Yes objects are passed by value. But the Object is a reference so a copy of reference is passed. Using the reference you can modify the object but changing the reference will not have any effect on caller.

For example:

List list = new ArrayList();
modifyList(list);
noOp(list);

void modifyList(List l)
{
   l.add("A"); // A is added to list
}

void noOp(List l)
{
   l = null; // No effect to list, you are modifying local variable l.
}