Tag Archives: Map interface

Java : Collection Framework Hierarchy


All the classes and interfaces of the collection framework are in java.util package. This hierarchy for the collection framework specifically mentioned the class and interface with respect to each type.

Java Collection Framework Hierarchy

Iterable Interface

The Iterable interface is the root interface for all the collection classes because the Collection interface extends the Iterable interface, therefore, all the subclasses of Collection interface also implement the Iterable interface.

The iterable interface contains only one abstract method.

  • Iterator iterator(): It returns the iterator over the elements of type T.

Iterator Interface

The iterator interface provides the facility of iterating the elements in a forward direction only.
For more detail: Java: Iterator Interface methods and Examples

Collection Interface

The Collection interface builds the foundation for the Collection framework. The collection interface is one of the interfaces which is implemented by all the Collection framework classes. It provides common methods to implement by all the subclasses of collection interfaces.
For more detail: Java: Collection Interface methods and Examples

List Interface

List interface is the subtype/child interface of the Collection interface. It stores objects/elements in list type data structure in ordered form and also allowed duplicate values.

Implementation classes for List interface are ArrayList, LinkedList, Vector, and Stack.

For Example: To instantiate List Interface with Implementation classes follow:


List  list1= new ArrayList();  
List  list2 = new LinkedList();  
List  list3 = new Vector();  
List  list4 = new Stack();

For more detail: Java: List Interface methods and Examples

ArrayList Class

The ArrayList implements the List interface. It’s having the following features:

  • ArrayList uses a dynamic array data structure to store objects and elements.
  • ArrayList allows duplicate objects and elements.
  • ArrayList maintains the insertion order.
  • ArrayList is non-synchronized.
  • ArrayList elements/objects can be accessed randomly.

For more detail: Java: ArrayList Interface methods and Examples

LinkedList Class

LinkedList implements the List interface. It’s having the following features:

  • LinkedList uses a doubly linked list data structure to store elements.
  • LinkedList allowed storing the duplicate elements.
  • LinkedList maintains the insertion order.
  • LinkedList is not synchronized.
  • LinkedList manipulation is fast because no shifting is required.

For more detail: Java: LinkedList Class methods and Examples

Vector Class

Vector Class implements List interface. It’s having the following features:

  • Vector is similar to the ArrayList class.
  • Vector class uses data structure as a dynamic array to store the data elements.
  • Vector is synchronized.
  • Vector contains many methods that are not the part of Collection Framework.

For more detail: Java: Vector Class methods and Examples

Stack Class

The Stack is the subclass of the Vector class. It’s having the following features:

  • Stack implements the Vector data structure with the (LIFO)last-in-first-out.
  • Stack contains all of the methods of the Vector class.
  • Stack also provides its methods like boolean push(), boolean peek(), boolean push(object o), which defines its features.

For more detail: Java: Stack Class methods and Examples

Queue Interface

Queue Interface extends the Collection interface. It’s having the following features:

  • Queue interface maintains the FIFO (first-in-first-out) order.
  • Queue can be defined as an ordered list that is used to hold the elements which are about to be processed.
  • Queue interface implemented by the various classes like PriorityQueue, Deque, and ArrayDeque.

Queue interface can be instantiated as:

Queue q1 = new PriorityQueue();  
Queue q2 = new ArrayDeque();  

For more detail: Java: Queue Interface methods and Examples

Here are detail about classes which implements Queue Interface.

PriorityQueue

The PriorityQueue class implements the Queue interface.

  • PriorityQueue holds the elements or objects which are to be processed by their priorities.
    PriorityQueue doesn’t allow null values to be stored in the queue.

For more detail: Java: PriorityQueue Class methods and Examples

Deque Interface

Deque stands for the double-ended queue which allows us to perform the operations at both ends.interface extends the Queue interface.

  • Deque extends the Queue interface.
  • Deque allows remove and add the elements from both the side.
Deque d = new ArrayDeque(); 

For more detail: Java: Deque Interface methods and Examples

ArrayDeque Class

ArrayDeque class implements the Deque interface.

  • ArrayDeque facilitates us to use the Deque.
  • ArrayDeque allows add or delete the elements from both the ends.
  • ArrayDeque is faster than ArrayList and has no capacity restrictions.

For more detail: Java: ArrayQueue Class methods and Examples

Set Interface

Set Interface extends Collection Interface and present in java.util package.

  • Set doesn’t allow duplicate elements or objects.
  • Set store elements in an unordered way.
  • Set allows only one null value.
  • Set is implemented by HashSet, LinkedHashSet, and TreeSet.

We can create an instantiation of Set as below:

Set s1 = new HashSet();  
Set s2 = new LinkedHashSet();  
Set s3 = new TreeSet();

For more detail: Java: Set Interface methods and Examples

HashSet

HashSet class implements Set Interface. It’s having the following features:

  • HashSet internally uses data structure like a hash table for storage.
  • HashSet uses hashing technique for storage of the elements.
  • HashSet always contains unique items.

For more detail: Java: HashSet Class methods and Examples

LinkedHashSet

LinkedHashSet class implements Set Interface. It’s having the following features:

  • LinkedHashSet store items in LinkedList.
  • LinkedHashSet store unique elements.
  • LinkedHashSet maintains the insertion order.
  • LinkedHashSet allows null elements.

For more detail: Java: LinkedHashSet Class methods and Examples

SortedSet Interface

SortedSet Interface extends Set Interface. It’s having the following features:

  • SortedSet provides a total ordering on its elements.
  • SortedSet elements are arranged in the increasing (ascending) order.
  • SortedSet provides additional methods that inhibit the natural ordering of the elements.

The SortedSet can be instantiated as:

SortedSet set = new TreeSet();  

For more detail: Java: SortedSet Interface methods and Examples

TreeSet Class

TreeSet class implements the SortedSet interface.  It’s having the following features:

  • TreeSet uses a tree data structure for storage.
  • TreeSet also contains unique elements.
  • TreeSet elements access and retrieval time is quite fast.
  • TreeSet elements stored in ascending order.

For more detail: Java: TreeSet Class methods and Examples

Map Interface

In the collection framework, a map contains values on the basis of key and value pair. This pair is known as an entry. A map having the following features:

  • Map contains unique keys.
  • Map allows duplicate values.
  • Map is useful to search, update or delete elements on the basis of a key.
  • Map is the root interface in Map hierarchy for Collection Framework.
  • Map interface is extended by SortedMap and implemented by HashMap, LinkedHashMap.
  • Map implementation classes HashMap and LinkedHashMap allow null keys and values but TreeMap doesn’t allow null key and value.
  • Map can’t be traversed, for traversing needs to convert into the set using method keySet() or entrySet().

For more detail: Java: Map Class methods and Examples

HashMap Class

HashMap class implements Map interface. It’s having following features:

  • HashMap uses data structure as a Hash Table.
  • HashMap store values based on keys.
  • HashMap contains unique keys.
  • HashMap allows duplicate values.
  • HashMap doesn’t maintain order.
  • HashMap class allows only one null key and multiple null values.
  • HashMap is not synchronized.
  • HashMap initial default capacity is 16 elements with a load factor of 0.75.

For more detail:

LinkedHashMap Class

LinkedHashMap class extends the HashMap class. It’s having the following features:

  • LinkedHashMap contains values based on the key.
  • LinkedHashMap contains unique elements.
  • LinkedHashMap may have one null key and multiple null values.
  • LinkedHashMap is not synchronized.
  • LinkedHashMap maintains the insertion order.
  • LinkedHashMap default initial capacity is 16 with a load factor of 0.75.

For more detail: Java: LinkedHashMap Class methods and Examples

TreeMap Class

TreeMap class implements the SortedMap interface. it’s having the following features:

  • TreeMap uses data structure as red-black tree.
  • TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
  • TreeMap contains only unique elements.
  • TreeMap doesn’t allow null keys and values.
  • TreeMap is not synchronized.
  • TreeMap maintains an ascending order.

For more detail: Java: TreeMap Class methods and Examples

HashTable Class

Hashtable class implements a Map interface and extends Dictionary class to store key and values as pairs. It’s having the following features:

  • HashTable store values as an array of the list where each list is known as a bucket of the node(key and value pair).
  • HashTable class is in java.util package.
  • Hashtable contains unique elements.
  • Hashtable doesn’t allow null key or value.
  • Hashtable is synchronized.
  • Hashtable initial default capacity is 11 whereas the load factor is 0.75.

For more detail: Java: HashTable Class methods and Examples

See Also:

Java: Map Interface Methods and Examples


In the collection framework, a map contains values on the basis of key and value pairs. This pair is known as an entry.

Points to Remember

  • Map contains unique keys.
  • Map allows duplicate values.
  • Map is useful to search, update or delete elements on the basis of a key.
  • Map is the root interface in the Map hierarchy for Collection Framework.
  • Map interface is extended by SortedMap and implemented by HashMap, LinkedHashMap.
  • Map implementation classes HashMap and LinkedHashMap allow null keys and values but TreeMap doesn’t allow null key and value.
  • Map can’t be traversed, for transversing needs to convert into the set using method keySet() or entrySet().

See Also:

Methods of Map Interface

Method Description
put(Object key, Object value) This method used to insert an entry on the map.
void putAll(Map map) This method inserts the specified map in the map.
V putIfAbsent(K key, V value) This method inserts the specified value with the specified key in the map only if it is not already specified.
V remove(Object key) This method used to delete an entry for the specified key.
boolean remove(Object key, Object value) This method removes the specified values with the associated specified keys from the map.
Set keySet() It returns the Set view containing all the keys.
Set<Map.Entry> entrySet() It returns the Set view containing all the keys and values.
void clear() It is used to reset the map.
V compute(K key, BiFunction remappingFunction) This method computes a mapping for the specified key and its current mapped value (or null if there is no current mapping).
V computeIfAbsent(K key, Function mappingFunction) This method computes 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 remappingFunction) This method computes 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)  if some value equal to the value exists within the map then return true, else return false.
boolean containsKey(Object key) if some key equal to the key exists within the map return true, else return false.
boolean equals(Object o) It is used to compare the specified Object values with the Map.
void forEach(BiConsumer action) Mentioned action will perform for each entry in the map until all entries have been processed or the action throws an exception.
V get(Object key) Returns the object that contains the value with respect to the key.
V getOrDefault(Object key, V defaultValue) Returns the value with respect to key is mapped, or defaultValue if the map contains no mapping for the key.
int hashCode() It returns the hash code value for the Map
boolean isEmpty() Check if the map not having any element Returns true if the map is empty or false.
V merge(K key, V value, BiFunction remappingFunction) If the given key is not mapped with a value or with null, associates it with the given non-null value.
V replace(K key, V value) It replaces the specified value with respect to the key.
boolean replace(K key, V oldValue, V newValue) It replaces the old value with the new value with respect to the key.
void replaceAll(BiFunction function) It replaces each entry’s value with the given function on that entry until all entries have been processed or the function throws an exception.
Collection values() It returns a collection of the values contained in the map.
int size() Returns the number of elements in the map.

Map.Entry Interface

Entry is the subinterface of Map. So we will be accessed by Map.Entry name. It returns a collection-view of the map, whose elements are of this class. It provides methods to get key and value.

Methods of Map.Entry interface

Method Description
K getKey() It is used to obtain a key.
V getValue() It is used to obtain value.
int hashCode() It is used to obtain hashCode.
V setValue(V value) It is used to replace the value corresponding to this entry with the specified value.
boolean equals(Object o) It is used to compare the specified object with the other existing objects.
static ,V> Comparator<Map.Entry> comparingByKey() It returns a comparator that compare the objects in natural order on key.
static Comparator<Map.Entry> comparingByKey(Comparator cmp) It returns a comparator that compare the objects by key using the given Comparator.
static <K,V extends Comparable> Comparator<Map.Entry> comparingByValue() It returns a comparator that compare the objects in natural order on value.
static Comparator<Map.Entry> comparingByValue(Comparator cmp) It returns a comparator that compare the objects by value using the given Comparator.

Map Example: Legacy Style without Generics

import java.util.*;
public class MapExample1 {
public static void main(String[] args) {
    Map map=new HashMap();
    //Adding elements to map
    map.put(1,"Anuj");
    map.put(5,"Raghav");
    map.put(2,"Jitendra");
    map.put(3,"Anuj");
    //Traversing Map Entry
	//Converting to Set so that we can traverse
    Set set=map.entrySet();
    Iterator itr=set.iterator();
    while(itr.hasNext()){
        //Type cast to Map.Entry so that we can get key and value separately
        Map.Entry entry=(Map.Entry)itr.next();
        System.out.println(entry.getKey()+" "+entry.getValue());
    }
}
}

Output :


1 Anuj
2 Jitendra
5 Raghav
3 Anuj

Map Example: With Generics

import java.util.*;
class MapExample2{
 public static void main(String args[]){
  Map map=new HashMap();
  map.put(20,"Anuj");
  map.put(21,"Virendra");
  map.put(22,"Raghav");
  //Elements can traverse in any order
  for(Map.Entry m:map.entrySet()){
   System.out.println(m.getKey()+" "+m.getValue());
  }
 }
}

Output :


22 Raghav
20 Anuj
21 Virendra

Map Example: comparingByKey() in ascending and descending order

import java.util.*;
class MapExample3{
 public static void main(String args[]){
Map map=new HashMap();
      map.put(20,"Anuj");
      map.put(21,"Virendra");
      map.put(22,"Raghav"); 

	  //ascending order
      //Returns a Set view of the mappings contained in this map
      map.entrySet()
      //Returns a sequential Stream with this collection as its source
      .stream()
      //Sorted according to the provided Comparator
      .sorted(Map.Entry.comparingByKey())
      //Performs an action for each element of this stream
      .forEach(System.out::println);  

	  //descending oder
	  //Returns a Set view of the mappings contained in this map
      map.entrySet()
      //Returns a sequential Stream with this collection as its source
      .stream()
      //Sorted according to the provided Comparator
      .sorted(Map.Entry.comparingByKey(Comparator.reverseOrder()))
      //Performs an action for each element of this stream
      .forEach(System.out::println);
 }
}

Output :


20=Anuj
21=Virendra
22=Raghav

Map Example: comparingByValue() in ascending and descending Order

import java.uti
class MapExample5{
 public static void main(String args[]){
Map map=new HashMap();
      map.put(20,"Anuj");
      map.put(21,"Virendra");
      map.put(22,"Raghav");  

	  //ascending order 

      //Returns a Set view of the mappings contained in this map
      map.entrySet()
      //Returns a sequential Stream with this collection as its source
      .stream()
      //Sorted according to the provided Comparator
      .sorted(Map.Entry.comparingByValue())
      //Performs an action for each element of this stream
      .forEach(System.out::println);  

	  //descending order

	  //Returns a Set view of the mappings contained in this map
     map.entrySet()
     //Returns a sequential Stream with this collection as its source
     .stream()
     //Sorted according to the provided Comparator
     .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
     //Performs an action for each element of this stream
     .forEach(System.out::println);
 }
}

Output :


20=Anuj
22=Raghav
21=Virendra

21=Virendra
22=Raghav
20=Anuj