Question

I am confused with one trivial thing – passing parameters to the method and changing their values… I’ll better give you some code:

public class Test {
  public static void main(String[] args) {
    Integer val = new Integer(41);
    upd(val);
    System.out.println(val);

    Man man = new Man();
    updMan(man);
    System.out.println(man.name);
  }

  static void upd(Integer val) {
    val = new Integer(42);
  }

  static void updMan(Man man) {
    man.name = "Name";
  }

  static class Man {
    String name;
  }
}

Could you explain why the Integer object I’ve passed is not updated while the Man object is? Aren’t the Integer and Man objects passed by reference (due to their non-primitive nature)?

Answer 1

Because for Integer your are creating a new Object. For Man you just change one of its value, the object man stays the same.

Consider the following:

static void updMan(Man man) {
  man = new Man();
  man.name = "Another Man";
}

This would also not change your initial man.

–EDIT

You can “simulate” the mutability of Integer by this:

static void upd(Integer val) {
    try {
        Field declaredField = val.getClass().getDeclaredField("value");
        declaredField.setAccessible(true);
        declaredField.set(val, 42);
    } catch (Exception e) {
    }
}