Question

I have a List of Foo Objects. If a name appears multiple times, I want to do something to the first Item with that name.

HashMap<String, Foo> map = new HashMap<String, Foo>();
for (Foo bar: this.FooList)
{
    if (!map.containsKey(bar.getName()))
    {
        map.put(bar.getName(), bar);
    }
    else
    {
        map.get(bar.getName()).doSomeThing();
    }
}

But this is not working, because every name (unique or not) gets thrown in that map. Does the HashMap check only for reference equality and not equality on key objects?

Answer 1

This is the code you need:

    HashMap<String, Foo> map = new HashMap<String, Foo>();
    for (Foo bar: this.FooList)
    {
        if (!map.containsKey(bar.getName()))
        {
            map.put(bar.getName(), bar);
        }
        else
        {
            Foo foo = map.get(bar.getName());
            if (foo != null)
                foo.doSomeThing();
            map.put(bar.getName(), null);
        }
    }

Here’s a testbed for it:

import java.util.HashMap;
import java.util.ArrayList;

public class Example
{
    public static void main(String[] args)
    {
        new Example().run();
    }

    private ArrayList<Foo> FooList = new ArrayList<Foo>();

    public void run()
    {
        FooList.add(new Foo("abc", 1));
        FooList.add(new Foo("abc", 2));
        FooList.add(new Foo("def", 3));
        FooList.add(new Foo("abc", 4));
        FooList.add(new Foo("abc", 5));
        FooList.add(new Foo("ghi", 6));
        FooList.add(new Foo("def", 7));

        HashMap<String, Foo> map = new HashMap<String, Foo>();
        for (Foo bar: this.FooList)
        {
            if (!map.containsKey(bar.getName()))
            {
                map.put(bar.getName(), bar);
            }
            else
            {
                Foo foo = map.get(bar.getName());
                if (foo != null)
                    foo.doSomeThing();
                map.put(bar.getName(), null);
            }
        }
    }

    class Foo
    {
        public Foo(String name, int i)
        {
            this.name = name;
            this.i = i;
        }

        public String getName()
        {
            return name;
        }

        public void doSomeThing()
        {
            System.out.println(getName() + " " + i);
        }

        private String name;
        private int i;
    }
}

Output is:

abc 1
def 3