Using Default Methods of Map from Java 8

Examples of using Default Methods introduced in Java 8 in Map interface

Using getOrDefault

Returns the value mapped to the key, or if the key is not present, returns the default value

Map<Integer, String> map = new HashMap<>();
map.put(1, "First element");
map.get(1);                                 // => First element
map.get(2);                                 // => null
map.getOrDefault(2, "Default element");     // => Default element

Using forEach

Allows to perform the operation specified in the ‘action’ on each Map Entry

Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");
map.forEach((key, value) -> System.out.println("Key: "+key+ " :: Value: "+value));

 // Key: 1 :: Value: one
 // Key: 2 :: Value: two
 // Key: 3 :: Value: three

Using replaceAll

Will replace with new-value only if key is present

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.replaceAll((key,value)->value+10);   //{john=30, paul=40, peter=50}

Using putIfAbsent

Key-Value pair is added to the map, if the key is not present or mapped to null

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.putIfAbsent("kelly", 50);     //{john=20, paul=30, peter=40, kelly=50}

Using remove

Removes the key only if its associated with the given value

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.remove("peter",40); //{john=30, paul=40}

Using replace

If the key is present then the value is replaced by new-value. 
If the key is not present, does nothing.

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.replace("peter",50); //{john=20, paul=30, peter=50}
map.replace("jack",60); //{john=20, paul=30, peter=50}

Using computeIfAbsent

This method adds an entry in the Map. the key is specified in the function and the value is the result of the application of the mapping function

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.computeIfAbsent("kelly", k->map.get("john")+10); //{john=20, paul=30, peter=40, kelly=30}
map.computeIfAbsent("peter", k->map.get("john")+10); //{john=20, paul=30, peter=40, kelly=30} //peter already present

Using computeIfPresent

This method adds an entry or modifies an existing entry in the Map. Does nothing if an entry with that key is not present

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.computeIfPresent("kelly", (k,v)->v+10); //{john=20, paul=30, peter=40} //kelly not present
map.computeIfPresent("peter", (k,v)->v+10); //{john=20, paul=30, peter=50} // peter present, so increase the value

Using compute

This method replaces the value of a key by the newly computed value

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);
map.compute("peter", (k,v)->v+50); //{john=20, paul=30, peter=90} //Increase the value

Using merge

Adds the key-value pair to the map, if key is not present or value for the key is null Replaces the value with the newly computed value, if the key is present Key is removed from the map , if new value computed is null

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("john", 20);
map.put("paul", 30);
map.put("peter", 40);

//Adds the key-value pair to the map, if key is not present or value for the key is null
map.merge("kelly", 50 , (k,v)->map.get("john")+10); // {john=20, paul=30, peter=40, kelly=50}

//Replaces the value with the newly computed value, if the key is present
map.merge("peter", 50 , (k,v)->map.get("john")+10); //{john=20, paul=30, peter=30, kelly=50}

//Key is removed from the map , if new value computed is null
map.merge("peter", 30 , (k,v)->map.get("nancy")); //{john=20, paul=30, kelly=50}

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