Collections - Beginning Java 8 Language Features - page 569

// Print the details
printDetails(map);
}
public static void printDetails(Map<String,String> map) {
// Get the value for the "Donna" key
String donnaPhone = map.get("Donna");
// Print details
System.out.println("Map: " + map);
System.out.println("Map Size: " + map.size());
System.out.println("Map is empty: " + map.isEmpty());
System.out.println("Map contains Donna key: " + map.containsKey("Donna"));
System.out.println("Donna Phone: " + donnaPhone);
System.out.println("Donna key is removed: " + map.remove("Donna"));
}
}
Map: {Donna=[phone], Ken=[phone], John=[phone], Richard=[phone]}
Map Size: 4
Map is empty: false
Map contains Donna key: true
Donna Phone: [phone]
Donna key is removed: [phone]
Removed all entries from the map.
Map: {}
Map Size: 0
Map is empty: true
Map contains Donna key: false
Donna Phone: null
Donna key is removed: null
The WeakHashMap class is another implementation for the Map interface. As the name of the class implies,
it contains weak keys . When there is no reference to the key except in the map, keys are candidates for garbage
collection. If a key is garbage collected, its associated entry is removed from the Map . You use a WeakHashMap as
implementation class for a Map when you want to maintain a cache of key-value pairs and you do not mind if your
key-value pairs are removed from the Map by the garbage collector. The WeakHashMap implementation allows a null
key and multiple null values. Please refer to Chapter 11 for a complete example of using the WeakHashMap class.
Sometimes you want to iterate over keys, values, or entries of a Map . The keySet() , values() and entrySet()
methods of a map returns a Set of keys, a Collection of values, and a Set of entries, respectively. Iterating over
elements of a Set or a Collection is the same as described in the “Traversing Collections” section.
The following snippet of code shows how to print all keys of a map:
Map<String,String> map = new HashMap<>();
map.put("John", "[phone]");
map.put("Richard", "[phone]");
map.put("Donna", "[phone]");
map.put("Ken", "[phone]");