174 Java HashMap Class

Introduction

The Java HashMap class is the Hash table based implementation of the Map interface.Following are the important points about HashMap −

• This class makes no guarantees as to the iteration order of the map; in particular, it does not guarantee that the order will remain constant over time.

• This class permits null values and the null key.

Class declaration

Following is the declaration for java.util.HashMap class −

public class HashMap<K,V>
   extends AbstractMap<K,V>
   implements Map<K,V>, Cloneable, Serializable

Parameters

Following is the parameter for java.util.HashMap class −

• K − This is the type of keys maintained by this map.

• V − This is the type of mapped values.

Class constructors

Sr.No.	Constructor & Description
1	HashMap() This constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
2	HashMap(int initialCapacity) This constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
3	HashMap(int initialCapacity, float loadFactor) This constructs an empty HashMap with the specified initial capacity and load factor.
4	HashMap(Map<? extends K,? extends V> m) This constructs a new HashMap with the same mappings as the specified Map.

Class methods

Sr.No.	Method & Description
1	void clear() This method removes all of the mappings from this map.
2	Object clone() This method returns a shallow copy of this HashMap instance, the keys and values themselves are not cloned.
3	V compute​(K key, BiFunction<? super K,​? super V,​? extends V> remappingFunction) Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
4	V computeIfAbsent​(K key, Function<? super K,​? extends V> mappingFunction) If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
5	V computeIfPresent​(K key, BiFunction<? super K,​? super V,​? extends V> remappingFunction) If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
6	boolean containsKey(Object key) This method returns true if this map contains a mapping for the specified key.
7	boolean containsValue(Object value) This method returns true if this map maps one or more keys to the specified value.
8	Set<Map.Entry<K,V>> entrySet() This method returns a Set view of the mappings contained in this map.
9	V get(Object key) This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
10	boolean isEmpty() This method returns true if this map contains no key-value mapping.
11	Set<K> keySet() This method returns a Set view of the keys contained in this map.
12	V merge​(K key, V value, BiFunction<? super V,​? super V,​? extends V> remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
13	V put(K key, V value) This method associates the specified value with the specified key in this map.
14	void putAll(Map<? extends K,? extends V> m) This method copies all of the mappings from the specified map to this map.
15	V remove(Object key) This method removes the mapping for the specified key from this map if present.
16	int size() This method returns the number of key-value mappings in this map.
17	Collection<V> values() This method returns a Collection view of the values contained in this map.

Methods inherited

This class inherits methods from the following classes −

• java.util.AbstractMap
• java.util.Object
• java.util.Map

Example

The following program illustrates several of the methods supported by HashMap collection −

import java.util.*;
public class HashMapDemo {

   public static void main(String args[]) {
   
      // Create a hash map
      HashMap hm = new HashMap();
      
      // Put elements to the map
      hm.put("Zara", new Double(3434.34));
      hm.put("Mahnaz", new Double(123.22));
      hm.put("Ayan", new Double(1378.00));
      hm.put("Daisy", new Double(99.22));
      hm.put("Qadir", new Double(-19.08));
      
      // Get a set of the entries
      Set set = hm.entrySet();
      
      // Get an iterator
      Iterator i = set.iterator();
      
      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();
      
      // Deposit 1000 into Zara's account
      double balance = ((Double)hm.get("Zara")).

doubleValue();
      hm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " + hm.get("Zara"));
   }
}

This will produce the following result −

Output

Daisy: 99.22
Ayan: 1378.0
Zara: 3434.34
Qadir: -19.08
Mahnaz: 123.22

Zara's new balance: 4434.34