what is a hashmap in java and how does it work code example

Example 1: java hashmap methods

import java.util.HashMap; // import the HashMap class

// instantiate a HashMap instance
HashMap<String, String> capitalCities = new HashMap<String, String>();

// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------
// to use any methods below add the function name to the instance name
// General Form: HashMapInstanceName.{functionName}({parameter list});
// Specific Example Using the Put Method: 
capitalCities.put("California", "Sacramento");

// SCROLL DOWN FOR LIST OF ALL METHODS --------------------------------------

1. void	clear() 
  	- Removes all of the mappings from this map.

2. Object clone() 
    - Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.

3. boolean	containsKey(Object key)
	- Returns true if this map contains a mapping for the specified key.

4. boolean	containsValue(Object value)
	- Returns true if this map maps one or more keys to the specified value.

5. V	get(Object key)
	- Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

6. V	getOrDefault(Object key, V defaultValue)
	- Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

7. boolean	isEmpty()
	- Returns true if this map contains no key-value mappings.

8. Set<K>	keySet()
	- Returns a Set view of the keys contained in this map.
      
9. V	put(K key, V value)
	- Associates the specified value with the specified key in this map.
 
10. V	remove(Object key)
	- Removes the mapping for the specified key from this map if present.
      
11. boolean	remove(Object key, Object value)
	- Removes the entry for the specified key only if it is currently mapped to the specified value.

12. V	replace(K key, V value)
	- Replaces the entry for the specified key only if it is currently mapped to some value.

13. boolean	replace(K key, V oldValue, V newValue)
	- Replaces the entry for the specified key only if currently mapped to the specified value.

14. int	size()
	- Returns the number of key-value mappings in this map.
      
15. Collection<V>	values()
	- Returns a Collection view of the values contained in this map.

Example 2: java hashmap example

//Hash map creation
Map< String,Integer> hm =  
                        new HashMap< String,Integer>(); 
//inserting elements into hashmap
       hm.put("a", new Integer(100)); 
       hm.put("b", new Integer(200)); 
       hm.put("c", new Integer(300)); 
       hm.put("d", new Integer(400));