Object references pointing to the same object

Question

Let's say for instance that we have a class Board with many fields (ie a rather complex class). We instantiate a Board like so:

Board b = new Board();

Note that for the sake of this example, I am not entering any parameters into the constructor, though in a real example, those may be necessary. If we were to then instantiate a new instance of Board and set it equal to be like so:

Board c = b;

This would not actually create a new board. From what I know, c and b now point to the same area of memory, the same Board object. So, if I were to change something about b , say be incrementing an integer field, like so:

b.count++;//Assume count is an integer field in the Board class.

The value c.count should be incremented as well. However, when I do this myself, it doesn't work. b.count is incremented, yet c.count is not.
Can anyone explain to me why this happens? This effect is something I want to have happen, thus any advice on how to implement this would be very helpful (General examples are fine).

Answer 1

Wrong again:

package cruft;

/**
 * Board description here
 * @author Michael
 * @link
 * @since 11/26/12 6:46 PM
 */
public class Board {

    public int count;

    public static void main(String[] args) {
        Board b = new Board();
        Board c = b;
        System.out.println("b before: " + b);
        System.out.println("c before: " + c);

        ++b.count;

        System.out.println("b after : " + b);
        System.out.println("c after : " + c);
    }

    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder();
        sb.append("Board");
        sb.append("{count=").append(count);
        sb.append('}');
        return sb.toString();
    }
}

Answer 2

This shouldn't happen. Take a look at this example.

public class Board {

    public Integer count = 0;

    public static void main(String[] args)
    {
        Board b = new Board();
        Board c = b;

        c.count++;

        System.out.println("b.count="+b.count);
        System.out.println("c.count="+c.count);
    }

}

Answer 3

This does not happen in a single thread. if you do that c= b for a second thread, normal visibility rules apply.

Answer 4

First of all, you should try to avoid using public fields in your objects. Use a private field, then use a getFieldName() and a setFieldName(FieldType arg) method. That way you have more control over what happens, and you can add code if you want other things to happen when a certain variable changes.

That being said, you must have a bug in your code, because:

public static void main(String[] args) {
    Board b = new Board();
    Board c = b;

    System.out.println("BEFORE++: b.count = " + b.count);
    System.out.println("BEFORE++: c.count = " + c.count);

    b.count++;

    System.out.println("AFTER++: b.count = " + b.count);
    System.out.println("AFTER++: c.count = " + c.count);
}

public static class Board {
    public int count = 0;
}

Outputs:

BEFORE++: b.count = 0
BEFORE++: c.count = 0
AFTER++: b.count = 1
AFTER++: c.count = 1