Java: What data structure to use to imitate PHP's associative arrays?

Question

I want a data structure that allows me to map keys to values, like PHP's associative arrays. Each key can only exist once, but a value can be mapped to any number of keys. What am I looking for? Something in the Google Commons Collections?

Answer 1

You're looking for a HashMap, such as a HashMap<K,V>. The former maps Objects to Objects, and the latter maps types of your choosing (like other generics).

Answer 2

The Map structure is what you want. A good implementation is the HashMap.

This data type does not allow the same value for the Key, but you can have as many duplicate values as you like.

Example usage:

Map<String, String> map = new HashMap<String, String>();
map.put("FirstName", "LastName");

System.out.println(map.get("FirstName")); // Prints 'LastName'
System.out.println(map.put("FirstName", "Foo")); // Prints 'LastName'
System.out.println(map.get("FirstName")); // Prints 'Foo'

In other words, the key can only exist once. Otherwise the value is overwritten.

Answer 3

What's wrong with regular HashMap? If your values are Strings you'll get reference counting for free and if you need to refer to a particular instance of object just don't allocate one, but use existing reference

Answer 4

HashMap is the way to go for sure. I actually view things the other way around when I first started coding in PHP. I was wondering, how do I implement a HashMap? Then I discovered associative arrays.