Java Properties and References -- I'm not getting what I expect.

Question

I'm a little embarrassed to ask this as I ought to know better, but here's what I've got.

I have an object "Pitcher" with an int property of "runsAllowed". I have an object Batter that has a property of "responsiblePitcher". I have an object Team that has a property "pitcher". When the batter reaches base:

Batter.responsiblePitcher = Team.pitcher;

All that's well and good. However, if we have a pitching change while the runner is on base, I set a new pitcher in Team.pitcher:

Team.pitcher = new Pitcher();

... and of course this changes the value of Batter.pitcher.

How should I be doing things differently such that the Batter.responsiblePitcher property continues to point to the pitcher who let him on base instead of pointing at whever is in the Team.pitcher property? Again, I feel like I ought to know this already ...

Thanks.

Answer 1

Actually your assumption is not correct. Assigning Team.pitcher to a new value does not change Batter.pitcher. Batter.pitcher will still point to the old Pitcher instance.

The reason is that you are assigning a reference to an object, and not the object itself.

Answer 2

... and of course this changes the value of Batter.pitcher.

This is not true. Your problem lies somewhere else. Maybe you're actually changing the value like this:

Team.pitcher.changeSomeProperty(newValue);

Then this will indeed get reflected in the other references since it points the same instance.

Java is a pass reference by value language. The below example proves this:

import java.util.Arrays;

public class Test {
    public static void main(String... args) {
        String[] strings = new String[] { "foo", "bar" };
        changeReference(strings);
        System.out.println(Arrays.toString(strings)); // still [foo, bar]
        changeValue(strings);
        System.out.println(Arrays.toString(strings)); // [foo, foo]
    }
    public static void changeReference(String[] strings) {
        strings = new String[] { "foo", "foo" };
    }
    public static void changeValue(String[] strings) {
        strings[1] = "foo";
    }
}