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Define an empty map

Map map = new HashMap();

Define an unmodifiable empty map

Map empty = Collections.EMPTY_MAP;
SortedMap empty = MapUtils.EMPTY_SORTED_MAP;

Define an initial map

Define the map {circle:1, triangle:3, square:4}
Map shapes = new HashMap();
shapes.put("circle", 1);
shapes.put("triangle", 3);
shapes.put("square", 4);
Map shapes = new HashMap() {{ put("circle",1); put("triangle",3); put("square",4); }}

Check if a key exists in a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print "ok" if an pet exists for "mary"
if (pets.containsKey("mary")) System.out.println("ok");

Retrieve a value from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} print the pet for "joe" ("cat")
String pet = pets.get("joe");

Add an entry to a map

Given an empty pets map, add the mapping from "rob" to "dog"
pets.put("rob", "dog");

Remove an entry from a map

Given a map pets {joe:cat,mary:turtle,bill:canary} remove the mapping for "bill" and print "canary"

Create a histogram map from a list

Given the list [a,b,a,c,b,b], produce a map {a:2, b:3, c:1} which contains the count of each unique item in the list
Map map = new HashMap();
for (Iterator it = list.iterator(); it.hasNext();) {
String s = (String);
if (!map.containsKey(s)) {
map.put(s, new Integer(1));
} else {
map.put(s, new Integer(((Integer)map.get(s)).intValue() + 1));
LinkedMap histogram = new LinkedMap();

for (Object letter : list)
histogram.put(letter, !histogram.containsKey(letter) ? 1 : MapUtils.getIntValue(histogram, letter) + 1);

Categorise a list

Given the list [one, two, three, four, five] produce a map {3:[one, two], 4:[four, five], 5:[three]} which sorts elements into map entries based on their length
SortedMap<Integer, List<String> > map = new TreeMap<Integer, List<String> >(); int key; List<String> vlist;

for (String item : list)
key = item.length(); vlist = map.containsKey(key) ? map.get(key) : new ArrayList<String>();
vlist.add(item); map.put(key, vlist);
MultiValueMap map = new MultiValueMap();
for (Object item : list) map.put(((String) item).length(), item);