![]() public Collection values(): Returns a Collection view of the values contained in this map.public int size(): Returns the number of key-value mappings in this map.public V remove(Object key): Removes the mapping for the specified key from this map if present.These mappings will replace any mappings that this map had for any of the keys currently in the specified map. public void putAll(Map m): Copies all of the mappings from the specified map to this map.If the map previously contained a mapping for the key, the old value is replaced. public V put(K key, V value): Associates the specified value with the specified key in this map.The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. public Set keySet(): Returns a Set view of the keys contained in this map.public boolean isEmpty(): A utility method returning true if no key-value mappings are present.public V get(Object key): Returns the value mapped to the specified key, or null if there is no mapping for the key.This set is backed by the map, so changes to the map are reflected in the set, and vice-versa. public Set> entrySet(): This method returns a Set view of the HashMap mappings.public boolean containsValue(Object value): This HashMap method returns true if the value exists otherwise false.public boolean containsKey(Object key): This method returns ‘true’ if the key exists otherwise it will return ‘false’.public void clear(): This HashMap method will remove all the mappings and HashMap will become empty.Let’s have a look at the important methods of HashMap in java. public HashMap(Map m): Creates a Map having same mappings as the specified map and with load factor 0.75īelow code snippet is showing HashMap example of using all the above constructors.In common scenarios you should avoid this because load factor 0.75 offers a good tradeoff between space and time cost. You can use this if you know the maximum number of mappings to be stored in HashMap. public HashMap(int initialCapacity, float loadFactor): This HashMap constructor will create an empty HashMap with specified initial capacity and load factor.This is useful in avoiding rehashing if you know the number of mappings to be stored in the HashMap. public HashMap(int initialCapacity): This HashMap constructor is used to specify the initial capacity and 0.75 load factor.This constructor will create an empty HashMap with default initial capacity 16 and load factor 0.75 public HashMap(): Most commonly used HashMap constructor.Java HashMap is not thread safe, for multithreaded environment you should use ConcurrentHashMap class or get synchronized map using Collections.synchronizedMap() method.This is the reason immutable classes are better suitable for keys, for example String and Interger. So HashMap key object should provide good implementation of these methods. HashMap uses hashCode() and equals() methods on keys for get and put operations.Default number of bins is 16 and it’s always power of 2. HashMap stores entries into multiple singly linked lists, called buckets or bins. ![]() HashMap uses it’s inner class Node for storing map entries.HashMap is almost similar to Hashtable except that it’s unsynchronized and allows null key and values.You can iterate over HashMap entries through keys set but they are not guaranteed to be in the order of their addition to the HashMap. Java HashMap allows null key and null values.Some of the important points about HashMap in Java are HashMap in java extends AbstractMap class that implements Map interface. Java HashMap is Hash table based implementation. ![]() Java HashMap is one of the most popular Collection classes in java.
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