Which function should I override when using indexOf() function

Question

Which function should I override when using the indexOf() function in java. I have a array list, then I take in an input as the ID and create a object which contains the ID and

Answer 1

The equals() method

public boolean equals(Object o) {
  if (o instanceof MyObject) {
    //id comparison
    MyObject mo = (MyObject)o;
    return mo.id.equals(id);
  }
  return false;
}

Change MyObject to your class.

Remember to change hashCode() as well as @Hovercraft points out. equals and hashCode go together (read the javadoc for them). Else you might run into some nasty and possibly hard to find bugs.

An example:

With java 7+ you can do this:

public int hashCode() {
    return java.util.Objects.hashCode(id);
}