Creating a Type-Specific Map [5.0]

Generics can be used to create a map that will hold only objects of a certain type. This example creates a map whose keys are Integer objects and values are String objects.
Map<Integer, String> map = new HashMap<Integer, String>();

map.put(1, "first");
map.put(2, "second");
// map.put(1, 2);       <- Syntax error
A map declared to hold objects of a type T can also hold objects that extend from T. In this example, a map is created to hold Number objects as keys. Both Integer and Float are subclasses of Number.
Map<Number, String> numMap = new HashMap<Number, String>();

numMap.put(.5, "half");
numMap.put(1, "first");
Note that although null is not a subclass of any type, if the collection supports null values, it can be added to the type-specific collection.
map.put(null, null);
A value retrieved from a type-specific collection does not need to be casted. In this example, a URL value is retrieved and used without an explicit cast.
Map<String, URL> urlMap = new HashMap<String, URL>();
try {
    urlMap.put("java", new URL(""));
} catch (MalformedURLException e) {
String s = urlMap.get("java").getHost();