Java

You can modify a HashMap in Java with put methods to add key-value pairs, replace methods to update values and remove methods to remove key-value pair entries

Add key-value pairs into a HashMap

  • V put(K key, V value) method adds the key-value mapping to a map. If key is existed, old value is replaced by the specified value
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.put("k2", 2);  
System.out.println(map); // {k1=1, k2=2}
  • Java 8+ method V putIfAbsent(K key, V value) adds the key-value mapping to a map either there's no associated or the key is associated with null. If there's an associated, return the current value
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.putIfAbsent("k3", 3);  
map.putIfAbsent("k1", 4);  
map.putIfAbsent("k2", 2);  
System.out.println(map); // {k1=1, k2=2, k3=3}
  • void putAll(Map<? extends K, ? extends V> m) copies all mappings from the specified map and adds to the caller map
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

Map<String, Integer> map2 = new HashMap<>();  
map2.put("k3", 3);  
map2.putAll(map);  
System.out.println(map2); // {k1=1, k2=null, k3=3}


Update values in HashMap

  • Java 8+ V replace(K key, V value) method updates / replaces the old value to the specified value if the specified key is existed in map
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.replace("k2", 2);  
map.replace("k3", 3); // {k1=1, k2=2}
  • Java 8+ boolean replace(K key, V oldValue, V newValue) method updates / replaces the specified oldValue to the specified newValue if the specified key is currently associated with oldValue in map
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.replace("k1", 1, 10);  
map.replace("k2", 2, 20); // {k1=10, k2=null}
  • Java 8+ void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) method updates / replaces all current values to the value returned by the function argument
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.replaceAll((k, v) -> 100)  
System.out.println(map); // {k1=100, k2=100}


Remove HashMap entries

  • V remove(Object key) method removes the entry by the specified key
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.remove("k2");  
map.remove("k3");  
System.out.println(map); // {k1=1}
  • Java 8+ boolean remove(Object key, Object value) method removes the entry by the specified key and value
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.remove("k1", 10);  
map.remove("k2", null);  
System.out.println(map); // {k1=1}
  • void clear() method removes all key-value mapping entries from map
Map<String, Integer> map = new HashMap<>();  
map.put("k1", 1);  
map.put("k2", null);  
System.out.println(map); // {k1=1, k2=null}

map.clear();  
System.out.println(map); // {}
Java