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("http://java.com")); 

} catch (MalformedURLException e) { } 

String s = urlMap.get("java").getHost();