Why isn't Collections.binarySearch() working with this comparable?

Question

I have this Player class which implements the Comparable interface. Then I have an ArrayList of Players. I\'m trying to use <

Answer 1

As long as you are implementing Comparable, you can make compareTo() consistent with equals() by also overriding equals() and hashCode(). This is particularly easy in this case, as you can simply delegate to String. Moreover, it's convenient if you ever need a Map containing instances of Player:

class Player implements Comparable<String> {

    private String username;
    private String password;

    // ...

    @Override
    public int compareTo(String name) {
        return username.compareTo(name);
    }

    @Override
    public boolean equals(Object obj) {
        return obj instanceof Player
            && username.equals(((Player)obj).username);
    }

    @Override
    public int hashCode() {
        return username.hashCode();
    }
}

Answer 2

Use are using generics inconsistently. Take heed of the compiler warnings. Always supply generic arguments (or never supply them).

Instead of:

class Player implements Comparable {
    [...]
    public int compareTo(Object o) {

Use

class Player implements Comparable<Player> {
    [...]
    public int compareTo(Player o) {

The rules of generics are difficult enough without the complication of rare types. So, typically the language spec gives up if you mix them up.