You can query a HashMap in Java with entrySet()
, keySet()
and values()
methods to get all keys and values mapping; get
methods to return value by the specified key; containsKey
and containsValue
methods to check existing; size
and isEmpty
methods to check the HashMap size
Get all HashMap keys and values
Set<Map.Entry<K, V>> entrySet()
method gets all key-value mappings entries contained in the map
Map<String, Integer> immutableMap = Map.of("k1", 1, "k2", 2);
immutableMap.entrySet().forEach(System.out::println);
Set<K> keySet()
method gets all keys contained in the map
Map<String, Integer> immutableMap = Map.of("k1", 1, "k2", 2);
immutableMap.keySet().forEach(System.out::println);
Collection<V> values()
method gets all values contained in the map
Map<String, Integer> immutableMap = Map.of("k1", 1, "k2", 2);
immutableMap.values().forEach(System.out::println);
Get value by the specified key
V get(Object key)
method gets the value mapped to thekey
argumentThe returned value may be
null
if thekey
is mapped tonull
or there is no mapping value to it
Map<String, Integer> map = new HashMap<>();
map.put("k1", 1);
map.put("k2", null);
map.get("k1"); // 1
map.get("k2"); // null
map.get("k3"); // null
map.get(null); // null
- Java 8+
V getOrDefault(Object key, V defaultValue)
method gets the value mapped to thekey
argument ordefaultValue
if there is no mappings for the key
Map<String, Integer> map = new HashMap<>();
map.put("k1", 1);
map.put("k2", null);
map.get("k1", 0); // 1
map.get("k3", 0); // 0
Check if key or value existing
boolean containsKey(Object key)
method checks if a map contains a mapping for the specified key
Map<String, Integer> map = new HashMap<>();
map.put("k1", 1);
map.put("k2", null);
System.out.println(map.containsKey("k1")); // true
System.out.println(map.containsKey("k3")); // false
boolean containsValue(Object value)
method checks if a map contains a mapping for the specified value
Map<String, Integer> map = new HashMap<>();
map.put("k1", 1);
map.put("k2", null);
System.out.println(map.containsValue(1)); // true
System.out.println(map.containsValue(2)); // false
Check HashMap size
int size()
gets the number of key-value mappings in a map, O(1)
Map<String, Integer> immutableMap = Map.of("k1", 1, "k2", 2);
map.size(); // 2
boolean isEmpty()
checks if a map contains no key-value mappings, O(1)
Map<String, Integer> immutableMap = Map.of("k1", 1, "k2", 2);
map.isEmpty(); // false