Java Collections Framework - Building Java Programs: A Back to Basics Approach - page 701

Table 11.5 Useful Methods of Maps

Method Description

Removes all keys and values from a map

clear()

Returns true if the given key maps to some value in this map

containsKey(key)

Returns true if some key maps to the given value in this map

containsValue(value)

Returns the value associated with this key, or null if not found

get(key)

Returns true if this collection contains no keys or values

isEmpty()

Returns a Set of all keys in this map

keySet()

Associates the given key with the given value

put(key, value)

Adds all key/value mappings from the given map to this map

putAll(map)

Removes the given key and its associated value from this map

remove(key)

Returns the number of key/value mappings in this map

size()

Returns a Collection of all values in this map

values()

Map Views ( keySet and values )
Unlike most collections, a map doesn't have an iterator method, because it wouldn't
be clear what you wanted to examine. The keys? The values? Both? Instead, maps
have a pair of methods called keySet and values that respectively return a Set of
all keys in the map and a Collection of all values in the map. These are sometimes
called collection views of a map because each is a collection that exists conceptually
within the map.
For example, consider a map that associates people's social security numbers with
their names. In other words, the map's keys are nine-digit social security numbers
and its values are names. We could create the map with the following code:

Map<Integer, String> ssnMap = new HashMap<Integer, String>();
ssnMap.put(867530912, "Jenny");
ssnMap.put(239876305, "Stuart Reges");
ssnMap.put(504386382, "Marty Stepp");

If we wanted to write a loop that printed the social security numbers of every
person in the map, we could then call the keySet method on the map. This method
returns a Set containing every key from the hash table—in this case, every int for
a person's number. If you store the keySet in a variable, you should declare that
variable as type Set , with the type of the map's keys between the < and > brackets:

Set<Integer> ssnSet = ssnMap.keySet();
for (int ssn : ssnSet) {
System.out.println("SSN: " + ssn);

}