Question

I am quite new to programming in Java, but I have some experience in c/c++, I am noticing some unwanted behaviour in some of my code, and I am just wondering how to fix it.

This is what I have:

List<State> list = new ArrayList<State>();
State cur = new State();
cur.x = 1;
cur.y = 1;
list.add(cur);
cur.x = 2;
cur.y = 3;
list.add(cur);
cur.x = 3;
cur.y = 4;
list.add(cur);
for (State i : list)
    System.out.println(i.x + " " + i.y);


class State
{
    public int x = 0;
    public int y = 0;
}

This gives me an output of:
3 4
3 4
3 4

Where as I want the output to be:
1 1
2 3
3 4

How do I fix this?
Thanks,
paintstripper

Answer 1

The problem you have is that you’re mutating a single object. It’s better to avoid publicly mutable member variables. If you write the State class as below you would not have been able to make the programming error.

List<State> list = new ArrayList<State>();
State cur = new State(1,1);
list.add(cur);
cur = new State(2,3);
list.add(cur);
cur = new State(3,4);
list.add(cur);
for (State i : list)
    System.out.println(i.x + " " + i.y);


class State
{
    public final int x;
    public final int y;

    public State(int x, int y) 
    {
        this.x = x;
        this.y = y;
    }

}

Note the final keyword on the x and y members, this means they cannot be assigned to after construction.