To summarize my requirements:

• A List (ArrayList) which can be modified as usual
• A way to find objects (contains and get) in constant time by a custom attribute (like a HashMap)
• No reliance on equals() and hashcode() because they cover more attributes (an can not be changed)

The LinkedHashMap seems to almost fit the bill but it is not a true List and therefore not a great drop in replacement.

What I’m using right now is a wrapper around a List which maintains an additional HashMap to find objects by the index criterium. The index is automatically updated whenever the list is modified. This is an example on how I use it:

// function to index Persons by their social security number
Function<Person, String> getSsnrForPerson =
  new Function<Person, String>() {
    String apply(Person person) {
      return person.getSocialSecurityNumber();
    }
  }

To create an indexed list for an empty ArrayList<Person> with an index on a person’s social security number you can then easily use the Class UniqueIndexedList:

UnqiueIndexedList<Person, String> myList =
  UnqiueIndexedList.create(getSsnrForPerson);

Of course all the usual methods of the List<Person> interface can be used as usual:

myList.add(somePerson);
myList.add(anotherPerson);
myList.get(i);
myList.iterator();

Additionaly there are  methods to access objects by index:

boolean personIsInList = myList.containsByIndex("34897634853");

Person p = myList.getByIndex("34897634853");

The code for the list implementation currently resides here:

• IndexedList.java
• UniqueIndexedList.java

It is basically a hybrid of a List and a special index-Map and works well so far. I am sure that finding objects in list for a given attribute is a very common usecase. Why hasn’t anyone solved this yet? Are there (better) alternatives that I’ve missed?

Here is another teaser screenshot of the main form with header, footer and some design improvements: