passing a map into another method to be modified in java

Question

So I came across some code that I thought looked kind of strange. Wanted to see what some of your opinions are of this

public class Test {

   public static void main(String[] args) {

      HashMap m = new HashMap();
      Test2 t2 = new Test2();
      t2.fill(m);
   }
}

public class Test2 {

    public void fill(HashMap m) {
        m.put(new Integer(0), new Integer(0));
    }

}

So is this code OK or should it be done another way?

Thanks

Answer 1

This is perfectly fine since objects in java are passed by reference. If you try to assign to m directly within a method, it is wrong:

m = new HashMap();

But you can use the passed reference to modify the object passed as an argument as is the case with your sample code.

Think of it as passing the location of the object into the function. You can use this location information to fiddle with it. But since the location is just a value, assigning to the location (m) does not have an effect on m from where you call the function. That's why the article says the argument is passed by value.