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

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.