Java HashMap

Java HashMap class

Java HashMap class implements the map interface by using a hash table. It inherits AbstractMap class and implements Map interface.

Points to remember

• Java HashMap class contains values based on the key.
• Java HashMap class contains only unique keys.
• Java HashMap class may have one null key and multiple null values.
• Java HashMap class is non synchronized.
• Java HashMap class maintains no order.
• The initial default capacity of Java HashMap class is 16 with a load factor of 0.75.

Hierarchy of HashMap class

As shown in the above figure, HashMap class extends AbstractMap class and implements Map interface.

HashMap class declaration

Let's see the declaration for java.util.HashMap class.

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

HashMap class Parameters

Let's see the Parameters for java.util.HashMap class.

• K: It is the type of keys maintained by this map.
• V: It is the type of mapped values.

Constructors of Java HashMap class

Constructor	Description
HashMap()	It is used to construct a default HashMap.
HashMap(Map<? extends K,? extends V> m)	It is used to initialize the hash map by using the elements of the given Map object m.
HashMap(int capacity)	It is used to initializes the capacity of the hash map to the given integer value, capacity.
HashMap(int capacity, float loadFactor)	It is used to initialize both the capacity and load factor of the hash map by using its arguments.

Methods of Java HashMap class

Method	Description
void clear()	It is used to remove all of the mappings from this map.
boolean isEmpty()	It is used to return true if this map contains no key-value mappings.
Object clone()	It is used to return a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
Set entrySet()	It is used to return a collection view of the mappings contained in this map.
Set keySet()	It is used to return a set view of the keys contained in this map.
V put(Object key, Object value)	It is used to insert an entry in the map.
void putAll(Map map)	It is used to insert the specified map in the map.
V putIfAbsent(K key, V value)	It inserts the specified value with the specified key in the map only if it is not already specified.
V remove(Object key)	It is used to delete an entry for the specified key.
boolean remove(Object key, Object value)	It removes the specified values with the associated specified keys from the map.
V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)	It is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)	It is used to compute its value using the given mapping function, if the specified key is not already associated with a value (or is mapped to null), and enters it into this map unless null.
V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)	It is used to compute a new mapping given the key and its current mapped value if the value for the specified key is present and non-null.
boolean containsValue(Object value)	This method returns true if some value equal to the value exists within the map, else return false.
boolean containsKey(Object key)	This method returns true if some key equal to the key exists within the map, else return false.
boolean equals(Object o)	It is used to compare the specified Object with the Map.
void forEach(BiConsumer<? super K,? super V> action)	It performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
V get(Object key)	This method returns the object that contains the value associated with the key.
V getOrDefault(Object key, V defaultValue)	It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
boolean isEmpty()	This method returns true if the map is empty; returns false if it contains at least one key.
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.
V replace(K key, V value)	It replaces the specified value for a specified key.
boolean replace(K key, V oldValue, V newValue)	It replaces the old value with the new value for a specified key.
void replaceAll(BiFunction<? super K,? super V,? extends V> function)	It replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
Collection values()	It returns a collection view of the values contained in the map.
int size()	This method returns the number of entries in the map.

Java HashMap example to add() elements

Here, we see different ways to insert elements.

1. import java.util.*;  
2. class HashMap1{  
3.  public static void main(String args[]){  
4.    HashMap<Integer,String> hm=new HashMap<Integer,String>();    
5.     System.out.println("Initial list of elements: "+hm);  
6.       hm.put(100,"Amit");    
7.       hm.put(101,"Vijay");    
8.       hm.put(102,"Rahul");   
9.        
10.       System.out.println("After invoking put() method ");  
11.       for(Map.Entry m:hm.entrySet()){    
12.        System.out.println(m.getKey()+" "+m.getValue());    
13.       }  
14.         
15.       hm.putIfAbsent(103, "Gaurav");  
16.       System.out.println("After invoking putIfAbsent() method ");  
17.       for(Map.Entry m:hm.entrySet()){    
18.            System.out.println(m.getKey()+" "+m.getValue());    
19.           }  
20.       HashMap<Integer,String> map=new HashMap<Integer,String>();  
21.       map.put(104,"Ravi");  
22.       map.putAll(hm);  
23.       System.out.println("After invoking putAll() method ");  
24.       for(Map.Entry m:map.entrySet()){    
25.            System.out.println(m.getKey()+" "+m.getValue());    
26.           }  
27.  }  
28. }  

Initial list of elements: {}
After invoking put() method 
100 Amit
101 Vijay
102 Rahul
After invoking putIfAbsent() method 
100 Amit
101 Vijay
102 Rahul
103 Gaurav
After invoking putAll() method 
100 Amit
101 Vijay
102 Rahul
103 Gaurav
104 Ravi

Java HashMap example to remove() elements

Here, we see different ways to remove elements.

1. import java.util.*;  
2. public class HashMap2 {  
3.    public static void main(String args[]) {  
4.     HashMap<Integer,String> map=new HashMap<Integer,String>();          
5.       map.put(100,"Amit");    
6.       map.put(101,"Vijay");    
7.       map.put(102,"Rahul");  
8.       map.put(103, "Gaurav");  
9.     System.out.println("Initial list of elements: "+map);  
10.     //key-based removal  
11.     map.remove(100);  
12.     System.out.println("Updated list of elements: "+map);  
13.     //value-based removal  
14.     map.remove(101);  
15.     System.out.println("Updated list of elements: "+map);  
16.     //key-value pair based removal  
17.     map.remove(102, "Rahul");  
18.     System.out.println("Updated list of elements: "+map);  
19.    }      
20. }  

Output:

Initial list of elements: {100=Amit, 101=Vijay, 102=Rahul, 103=Gaurav}
Updated list of elements: {101=Vijay, 102=Rahul, 103=Gaurav}
Updated list of elements: {102=Rahul, 103=Gaurav}
Updated list of elements: {103=Gaurav}

---

Java HashMap example to replace() elements

Here, we see different ways to replace elements.

1. import java.util.*;  
2. class HashMap3{  
3.  public static void main(String args[]){  
4.    HashMap<Integer,String> hm=new HashMap<Integer,String>();    
5.       hm.put(100,"Amit");    
6.       hm.put(101,"Vijay");    
7.       hm.put(102,"Rahul");   
8.       System.out.println("Initial list of elements:");  
9.      for(Map.Entry m:hm.entrySet())  
10.      {  
11.         System.out.println(m.getKey()+" "+m.getValue());   
12.      }  
13.      System.out.println("Updated list of elements:");  
14.      hm.replace(102, "Gaurav");  
15.      for(Map.Entry m:hm.entrySet())  
16.      {  
17.         System.out.println(m.getKey()+" "+m.getValue());   
18.      }  
19.      System.out.println("Updated list of elements:");  
20.      hm.replace(101, "Vijay", "Ravi");  
21.      for(Map.Entry m:hm.entrySet())  
22.      {  
23.         System.out.println(m.getKey()+" "+m.getValue());   
24.      }   
25.      System.out.println("Updated list of elements:");  
26.      hm.replaceAll((k,v) -> "Ajay");  
27.      for(Map.Entry m:hm.entrySet())  
28.      {  
29.         System.out.println(m.getKey()+" "+m.getValue());   
30.      }  
31.  }  
32. }  

Initial list of elements:
100 Amit
101 Vijay
102 Rahul
Updated list of elements:
100 Amit
101 Vijay
102 Gaurav
Updated list of elements:
100 Amit
101 Ravi
102 Gaurav
Updated list of elements:
100 Ajay
101 Ajay
102 Ajay

Difference between HashSet and HashMap

HashSet contains only values whereas HashMap contains an entry(key and value).

Java HashMap Example: Book

1. import java.util.*;    
2. class Book {    
3. int id;    
4. String name,author,publisher;    
5. int quantity;    
6. public Book(int id, String name, String author, String publisher, int quantity) {    
7.     this.id = id;    
8.     this.name = name;    
9.     this.author = author;    
10.     this.publisher = publisher;    
11.     this.quantity = quantity;    
12. }    
13. }    
14. public class MapExample {    
15. public static void main(String[] args) {    
16.     //Creating map of Books    
17.     Map<Integer,Book> map=new HashMap<Integer,Book>();    
18.     //Creating Books    
19.     Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);    
20.     Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4);    
21.     Book b3=new Book(103,"Operating System","Galvin","Wiley",6);    
22.     //Adding Books to map   
23.     map.put(1,b1);  
24.     map.put(2,b2);  
25.     map.put(3,b3);  
26.       
27.     //Traversing map  
28.     for(Map.Entry<Integer, Book> entry:map.entrySet()){    
29.         int key=entry.getKey();  
30.         Book b=entry.getValue();  
31.         System.out.println(key+" Details:");  
32.         System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);   
33.     }    
34. }    
35. }    

Output:

1 Details:
101 Let us C Yashwant Kanetkar BPB 8
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
3 Details:
103 Operating System Galvin Wiley 6