How to iterate Map in Java

In Java, iteration over Map can be done in various ways. Remember that we cannot iterate over map directly using iterators, because Map interface is not the part of Collection. All maps in Java implements Map interface. There are following types of maps in Java:

HashMap
TreeMap
LinkedHashMap

A map is not a Collection but still, consider under the Collections framework. Hence, a Map is an interface which does not extend the Collections interface.

Iterator

An iterator is an interface used for iterate over a collection. It takes the place of Enumeration in Java Collections Framework. The difference between iterator and Enumeration is:

The Iterator can traverse legacy and non-legacy elements whereas Enumeration can traverse only legacy elements.
Iterator is fail-fast whereas Enumeration is not fail-fast.

Collection Views

The collection views method allows a map to be viewed as a Collection in the following ways:

keySet: It is the set of keys contained in the Map.
values: It is the collection of values contained in the Map.
entrySet: It is a set of key-value pair in the Map.

The Map interface also has a small nested interface called Map.entry. The collection view provides the only means to iterate over a map.

Using Iterator interface

Example of iteration over HashMap

  public static void main(String args[])  {  HashMap hm = new HashMap();  //implements map interface  hm.put(110,”Ravi”);  hm.put(120,”Prateek”);  hm.put(130, “Davesh”);  hm.put(140, “Kamal”);  hm.put(150, “Pawan”);  Iterator  it = hm.keySet().iterator();//keyset is a method  while(it.hasNext())  {  int key=(int)it.next();  System.out.println(“Roll no.: “+key+”     name: “+hm.get(key));  }  }  }  ,>,>

Output:

Roll no.: 130     name: Davesh  Roll no.: 150     name: Pawan  Roll no.: 120     name: Prateek  Roll no.: 140     name: Kamal  Roll no.: 110     name: Ravi

Using keyset() and value() method

keyset(): A keySet() method of HashMap class is used for iteration over the keys contained in the map. It returns the Set view of the keys.

Syntax

values(): A values() method of HashMap class is used for iteration over the values contained in the map. It returns a collection view of the values.

Syntax

Example

  import java.util.Map;   import java.util.HashMap;   class IterationExample2   {   public static void main(String[] arg)   {   Map map = new HashMap();   map.put(“Gujarat”, “Gandhi Nagar”);   map.put(“Uttar Pradesh”, “Lucknow”);   map.put(“Sikkim”, “Ganagtok”);   for (String State : map.keySet())  //using keyset() method for iteration over keySet  System.out.println(“State: ” + State);   for (String Capital : map.values())  //using values() for iteration over keys  System.out.println(“Capiatl: ” + Capital);   }   }  ,string>,string>

Output:

State: Gujarat  State: Sikkim  State: Uttar Pradesh  Capital: Gandhi Nagar  Capital: Ganagtok  Capital: Lucknow

Using Map.entry<K,V>method

Map.Entry<K,V> is an interface. It returns a collection view of the map, whose elements are of this class.

A map.entrySet() method returns a Set view of the mapping contained in the map. The changes in the map are reflected in the Set also and vice-versa. It also supports element removal, which removes the corresponding mapping from the map.

Syntax

Example

  import java.util.*;  class IterationExample3   {   public static void main(String[] arg)   {   Map<String, Float> map = new HashMap<String, Float>();   map.put(“Cookies”,  90.87f);  map.put(“Dry Fruits”, 434.23f);   map.put(“Oats”, 220.00f);   map.put(“Chocolate”, 70.89f);   for (Map.Entry<String,Float> entry : map.entrySet()) //using map.entrySet() for iteration  {  //returns keys and values respectively  System.out.println(“Item: ” + entry.getKey() + “, Price: ” + entry.getValue());   }   }  }  

Output:

Item: Oats, Price: 220.0  Item: Dry Fruits, Price: 434.23  Item: Cookies, Price: 90.87  Item: Chocolate, Price: 70.89

Iteration over keys and getting values

Example

In the following example we first iterate over the keys and then getting the values.

  import java.util.*;  class IterationExample4   {   public static void main(String[] arg)   {   Map map = new HashMap();   map.put(“Sumit”,  “Singh”);  map.put(“Devesh”, “Mishra”);   map.put(“Rahul”, “Tiwari”);   for (String name: map.keySet()) //iteration over keys  {  //returns the value to which specified key is mapped  String lastname=map.get(firstname);   System.out.println(“Key: ” + name + “, Value: ” + lastname);   }   }  }  ,>,>

Output:

Key: Rahul, Value: Tiwari  Key: Devesh, Value: Mishra  Key: Sumit, Value: Singh

Using forEach() method

The forEach() method of ArrayList is used to perform an action for each element of the Iterable until all elements have been processed.

Syntax

The method takes action (the action to be performed for each element) as a parameter. It does not return anything. It throws NullPointerException if the specified action is null.

Example

In the following example, we are using lambda expression inside the forEach() method to print each element of the map.

  import java.util.Map;   import java.util.HashMap;   class IterationExample5   {   public static void main(String[] arg)   {   Map map = new HashMap();   map.put(“TCS”, “$100 billion”);   map.put(“Wipro”, “$21.5 billion”);   //iteration over map using forEach() method  map.forEach((k,v) -> System.out.println(“Company: “+ k + “, Net worth: ” + v));   }   }  ,string>,string>

Output:

Company: Wipro, Net worth: $21.5 billion  Company: TCS, Net worth: $100 billion

Next TopicJava Tutorial

How to iterate Map in Java

How to iterate Map in Java

Iterator

Collection Views

Using Iterator interface

Using keyset() and value() method

Using Map.entry<K,V>method

Iteration over keys and getting values

Using forEach() method

How annotation works in Java

Hyperfactorial in Java

You may also like