Java LinkedHashMap class

Java LinkedHashMap class is Hashtable and Linked list implementation of the Map interface, with predictable iteration order. It inherits HashMap class and implements the Map interface.

Points to remember

• Java LinkedHashMap contains values based on the key.
• Java LinkedHashMap contains unique elements.
• Java LinkedHashMap may have one null key and multiple null values.
• Java LinkedHashMap is non synchronized.
• Java LinkedHashMap maintains insertion order.
• The initial default capacity of Java HashMap class is 16 with a load factor of 0.75.

LinkedHashMap class declaration

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

1. public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>  

LinkedHashMap class Parameters

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

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

Constructors of Java LinkedHashMap class

Constructor	Description
LinkedHashMap()	It is used to construct a default LinkedHashMap.
LinkedHashMap(int capacity)	It is used to initialize a LinkedHashMap with the given capacity.
LinkedHashMap(int capacity, float loadFactor)	It is used to initialize both the capacity and the load factor.
LinkedHashMap(int capacity, float loadFactor, boolean accessOrder)	It is used to initialize both the capacity and the load factor with specified ordering mode.
LinkedHashMap(Map<? extends K,? extends V> m)	It is used to initialize the LinkedHashMap with the elements from the given Map class m.

Methods of Java LinkedHashMap class

Method	Description
V get(Object key)	It returns the value to which the specified key is mapped.
void clear()	It removes all the key-value pairs from a map.
boolean containsValue(Object value)	It returns true if the map maps one or more keys to the specified value.
Set<Map.Entry<K,V>> entrySet()	It returns a Set view of the mappings contained in 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 getOrDefault(Object key, V defaultValue)	It returns the value to which the specified key is mapped or defaultValue if this map contains no mapping for the key.
Set<K> keySet()	It returns a Set view of the keys contained in the map
protected boolean removeEldestEntry(Map.Entry<K,V> eldest)	It returns true on removing its eldest entry.
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<V> values()	It returns a Collection view of the values contained in this map.

Java LinkedHashMap Example

1. import java.util.*;  
2. class LinkedHashMap1{  
3.  public static void main(String args[]){  
4.    
5.   LinkedHashMap<Integer,String> hm=new LinkedHashMap<Integer,String>();  
6.   
7.   hm.put(100,"Amit");  
8.   hm.put(101,"Vijay");  
9.   hm.put(102,"Rahul");  
10.   
11. for(Map.Entry m:hm.entrySet()){  
12.    System.out.println(m.getKey()+" "+m.getValue());  
13.   }  
14.  }  
15. }  

Output:100 Amit
       101 Vijay
       102 Rahul

Java LinkedHashMap Example: Key-Value pair

1. import java.util.*;  
2. class LinkedHashMap2{  
3.  public static void main(String args[]){  
4.    LinkedHashMap<Integer, String> map = new LinkedHashMap<Integer, String>();           
5.       map.put(100,"Amit");    
6.      map.put(101,"Vijay");    
7.      map.put(102,"Rahul");    
8.        //Fetching key  
9.        System.out.println("Keys: "+map.keySet());  
10.        //Fetching value  
11.        System.out.println("Values: "+map.values());  
12.        //Fetching key-value pair  
13.        System.out.println("Key-Value pairs: "+map.entrySet());  
14.  }  
15. }  

Keys: [100, 101, 102]
Values: [Amit, Vijay, Rahul]
Key-Value pairs: [100=Amit, 101=Vijay, 102=Rahul]

Java LinkedHashMap Example:remove()

1. import java.util.*;  
2. public class LinkedHashMap3 {  
3.    public static void main(String args[]) {  
4.     Map<Integer,String> map=new LinkedHashMap<Integer,String>();        
5.      map.put(101,"Amit");    
6.      map.put(102,"Vijay");    
7.      map.put(103,"Rahul");    
8.      System.out.println("Before invoking remove() method: "+map);     
9.     map.remove(102);  
10.     System.out.println("After invoking remove() method: "+map);    
11.    }      
12. }  

Output:

Before invoking remove() method: {101=Amit, 102=Vijay, 103=Rahul}
After invoking remove() method: {101=Amit, 103=Rahul}

Java LinkedHashMap 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 LinkedHashMap<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(2,b2);  
24.     map.put(1,b1);  
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:

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