Maps

A Map is an object, that maps keys to values.
A Map cannot contain duplicate keys.

Associating a Value with an Object

This example demonstrates how to associate a value with an arbitrary object.

The technique involves saving the object and the associated value as a key/value pair in an IdentityHashMap. A HashMap cannot be used for this purpose since if two objects happen to equal via the Object.equals() method, one of the objects will not be stored.

// Create the map
Map objMap = new IdentityHashMap();

// Add the object and value pair to the map
Object o1 = new Integer(123);
Object o2 = new Integer(123);
objMap.put(o1, "first");
objMap.put(o2, "second");

// Retrieve the value associated with the objects
Object v1 = objMap.get(o1); // first
Object v2 = objMap.get(o2); // second

System.out.println(v1.equals(v2)); //false