When to use linkedhashmap over hashmap in java?

Question

What are the practical scenario for choosing among the linkedhashmap and hashmap? I have gone through working of each and come to the conclusion that linkedhashmap maintains the order of insertion i.e elements will be retrieved in the same order as that of insertion order while hashmap won't maintain order. So can someone tell in what practical scenarios selection of one of the collection framework and why?

Answer 1

1. LinkedHashMap will iterate in the order in which the entries were put into the map.

2. null Values are allowed in LinkedHashMap.

3. The implementation is not synchronized and uses double linked buckets.

4. LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added or accessed, so the iteration order is the same as insertion order depending on construction parameters.

5. LinkedHashMap also provides a great starting point for creating a Cache object by overriding the removeEldestEntry() method. This lets you create a Cache object that can expire data using some criteria that you define.

6. Based on linked list and hashing data structures with linked list (think of indexed-SkipList) capability to store data in the way it gets inserted in the tree. Best suited to implement LRU ( least recently used ). LinkedHashMap extends HashMap.

It maintains a linked list of the entries in the map, in the order in which they were inserted. This allows insertion-order iteration over the map. That is,when iterating through a collection-view of a LinkedHashMap, the elements will be returned in the order in which they were inserted. Also if one inserts the key again into the LinkedHashMap, the original order is retained. This allows insertion-order iteration over the map. That is, when iterating a LinkedHashMap, the elements will be returned in the order in which they were inserted. You can also create a LinkedHashMap that returns its elements in the order in which they were last accessed.

LinkedHashMap constructors

LinkedHashMap( )

This constructor constructs an empty insertion-ordered LinkedHashMap instance with the default initial capacity (16) and load factor (0.75).

LinkedHashMap(int capacity)

This constructor constructs an empty LinkedHashMap with the specified initial capacity.

 LinkedHashMap(int capacity, float fillRatio)

This constructor constructs an empty LinkedHashMap with the specified initial capacity and load factor.

LinkedHashMap(Map m)

This constructor constructs a insertion-ordered Linked HashMap with the same mappings as the specified Map.

LinkedHashMap(int capacity, float fillRatio, boolean Order)

This constructor construct an empty LinkedHashMap instance with the specified initial capacity, load factor and ordering mode.

Important methods supported by LinkedHashMap

 Class clear( )

Removes all mappings from the map.

containsValue(object value )>

Returns true if this map maps one or more keys to the specified value.

 get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

removeEldestEntry(Map.Entry eldest)

Below is an example of how you can use LinkedHashMap:

Map<Integer, String> myLinkedHashMapObject = new LinkedHashMap<Integer, String>();  
myLinkedHashMapObject.put(3, "car");  
myLinkedHashMapObject.put(5, "bus");  
myLinkedHashMapObject.put(7, "nano");  
myLinkedHashMapObject.put(9, "innova");  
System.out.println("Modification Before" + myLinkedHashMapObject);  
System.out.println("Vehicle exists: " +myLinkedHashMapObject.containsKey(3));  
System.out.println("vehicle innova Exists: "+myLinkedHashMapObject.containsValue("innova"));  
System.out.println("Total number of vehicles: "+ myLinkedHashMapObject.size());  
System.out.println("Removing vehicle 9: " + myLinkedHashMapObject.remove(9));  
System.out.println("Removing vehicle 25 (does not exist): " + myLinkedHashMapObject.remove(25));  
System.out.println("LinkedHashMap After modification" + myLinkedHashMapObject);