Returns the number of key-value mappings in the map.

int size = map.size();

Returns true if the map contains no key-value mappings.

boolean empty = map.isEmpty();

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

V value = map.get(key);

Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.

V previousValue = map.put(key, value);

NOTE: 

Return Type: V (the previous value associated with the key, or null if there was no mapping)

Removes the mapping for a key from this map if it is present.

V removedValue = map.remove(key);

NOTE:

Return Type: V (the previous value associated with the key, or null if there was no mapping)

Removes the entry for the specified key only if it is currently mapped to the specified value.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
boolean removed = map.remove("One", 1);
System.out.println(removed); // Output: true
System.out.println(map); // Output: {}

Returns true if this map contains a mapping for the specified key.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
boolean hasKey = map.containsKey("One");
System.out.println(hasKey); // Output: true

Returns true if this map maps one or more keys to the specified value.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
boolean hasValue = map.containsValue(1);
System.out.println(hasValue); // Output: true

Copies all of the mappings from the specified map to this map.

map.putAll(anotherMap);

Removes all of the mappings from this map.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
map.clear();
System.out.println(map); // Output: {}

Returns a Set view of the keys contained in this map.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
for (String key : map.keySet()) {
    System.out.println("Key: " + key);
}

Returns a Collection view of the values contained in this map.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
for (Integer value : map.values()) {
    System.out.println("Value: " + value);
}

Returns a Set view of the mappings contained in this map. Each element in the returned set is a Map.Entry Object.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println("Key: " + entry.getKey() + ", Value: " + entry.getValue());
}

Compares the specified object with the map for equality.

Map<String, Integer> map1 = new HashMap<>();
map1.put("One", 1);
Map<String, Integer> map2 = new HashMap<>();
map2.put("One", 1);
boolean isEqual = map1.equals(map2);
System.out.println(isEqual); // Output: true

Returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
Integer value = map.getOrDefault("Two", 0);
System.out.println(value); // Output: 0

If the specified key is not already associated with a value (or is mapped to null), associates it with the given value and returns null, else returns the current value.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
map.putIfAbsent("One", 2);
map.putIfAbsent("Two", 2);
System.out.println(map); // Output: {One=1, Two=2}

Replaces the entry for the specified key only if currently mapped to the specified value.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
boolean replaced = map.replace("One", 1, 2);
System.out.println(replaced); // Output: true
System.out.println(map); // Output: {One=2}

Replaces the entry for the specified key only if it is currently mapped to some value.

Map<String, Integer> map = new HashMap<>();
map.put("One", 1);
Integer replacedValue = map.replace("One", 2);