How to Iterate Map in Java
We have known in the previous tutorial, a map in Java is a container object that stores elements in the form of key and value pairs. If the key is provided, its associated value can be retrieved easily. Keys must be unique.
Now we will learn in this tutorial how to iterate map in Java.
In Java, there are multiple convenient ways to iterate over a map. We can iterate over keys, values, or both. We can also remove an element from a map while iterating over it.
So, let’s understand how to iterate map in Java by the following methods.
Iterating Map in Java using Iterator over Map.Entry<K, V>
An iterator is an interface that contains methods to retrieve the entries of a map one by one. It provides three methods:
- boolean hasNext (): It returns true if the iterator has more elements.
- element next (): It returns the next element in the iterator.
- void remove (): It removes the last element returned by the iterator.
Note that we cannot iterate over a map directly using iterator because Map interface is not the part of the Collection interface. To iterate map using iterator, you must familiar with Map.Entry<K, V> interface.
Since all maps in Java implement Map interface, this technique can be used with any map implementations such as HashMap, TreeMap, LinkedHashMap, Hashtable, etc.
[adinserter block=”5″]
Let’s take an example program where we will iterate map in java using iterator concept. Look at the following source code.
Program code 1:
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
public class IterationTest1 {
public static void main(String[] args)
{
// Creating a map object.
Map<String, String> map = new HashMap<>();
// Adding elements into map using put() method.
map.put("V ", " Violet");
map.put("I ", " Indigo");
map.put("B ", " Blue");
map.put("G ", " Green");
map.put("Y ", " Yellow");
map.put("O ", " Orange");
map.put("R ", " Red");
// Iterating map using Iterator.
// entrySet is a method that is used to get view of entries of a map.
Iterator<Entry<String, String>> itr = map.entrySet().iterator();
System.out.println("Iterating Entries of Map");
while(itr.hasNext())
{
Object key = itr.next();
System.out.println(key);
}
// keySet is a method that is used to get view of keys of a map
Iterator<String> itr2 = map.keySet().iterator();
System.out.println("Iterating Keys of Map");
while(itr2.hasNext())
{
Object keyView = itr2.next();
System.out.println(keyView);
}
// values is a method that is used to get values of keys of a map.
Iterator<String> itr3 = map.values().iterator();
System.out.println("Iterating Values of Map");
while(itr3.hasNext())
{
Object valuesView = itr3.next();
System.out.println(valuesView);
}
}
}Output:
Iterating Entries of Map
O = Orange
I = Indigo
Y = Yellow
G = Green
V = Violet
B = Blue
R = Red
Iterating Keys of Map
O
I
Y
G
V
B
R
Iterating Values of Map
Orange
Indigo
Yellow
Green
Violet
Blue
Red
Let’s take another example program where we will remove the last entry of a map returned by the Iterator.
[adinserter block=”2″]
Program code 2:
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
public class IterationTest2 {
public static void main(String[] args)
{
Map<String, String> map = new HashMap<>();
map.put("V ", " Violet");
map.put("I ", " Indigo");
map.put("B ", " Blue");
map.put("G ", " Green");
map.put("Y ", " Yellow");
map.put("O ", " Orange");
map.put("R ", " Red");
// entrySet is a method that is used to get view of entries of a map.
Iterator<Entry<String, String>> itr = map.entrySet().iterator();
System.out.println("Iterating Entries of Map");
while(itr.hasNext())
{
Object key = itr.next();
System.out.println(key);
}
// Removing last entry returned by Iterator
itr.remove(); // This method will remove last entry of a map while iterating.
System.out.println("Entries of Map after removing: " +map.entrySet());
}
}Output:
Iterating Entries of Map
O = Orange
I = Indigo
Y = Yellow
G = Green
V = Violet
B = Blue
R = Red
Entries of Map after removing: [O = Orange, I = Indigo, Y = Yellow, G = Green, V = Violet, B = Blue]
Iterating over keys or values using keySet() and value() Methods
This technique is useful when you want to get a set view of keys or values of a map. Using keySet(), values(), and for-each loop, you can iterate over keys or values of a map.
Let’s take an example program where we will iterate over keys or values of a map using keySet() and values() methods. keySet() method returns a set view of the keys from a map and values() method returns a collection-view of values from a map.
Program code 3:
import java.util.HashMap;
import java.util.Map;
public class IterationTest3 {
public static void main(String[] args)
{
Map<Integer, String> map = new HashMap<>();
map.put(101, " John");
map.put(202, " Ricky");
map.put(303, " Deep");
map.put(404, " Mark");
map.put(505, " Maya");
// Iterating over keys of a map using keySet() method.
for (Integer rollNo : map.keySet())
System.out.println("Roll No: " + rollNo);
System.out.println();
// Iterating over values of a map using values() method.
for (String name : map.values())
System.out.println("Name: " + name);
}
}Output:
Roll No: 404
Roll No: 101
Roll No: 505
Roll No: 202
Roll No: 303
Name: Mark
Name: John
Name: Maya
Name: Ricky
Name: Deep
Iterating Map using Map.Entry<K,V> Method
Map.Entry<K,V> is an interface that is used to work on an entry in the map. It returns a collection view of the map. Each Map.Entry object contains one key/value pair.
Let’s write a Java program to iterate entry of a map using Map.Entry<K,V> method. We will use the following methods for iteration.
1. getKey(): It is used to retrieve the key for a map entry. Its return type is key.
2. getValue(): It is used to get the value for a map entry. Its return type is value.
3. entrySet(): It returns a set view of entries of a map.
Program code 4:
import java.util.HashMap;
import java.util.Map;
public class IterationTest4 {
public static void main(String[] args)
{
Map<Integer, String> map = new HashMap<>();
map.put(101, " John");
map.put(202, " Ricky");
map.put(303, " Deep");
map.put(404, " Mark");
map.put(505, " Maya");
// Iterating over entries of a map using entrySet() method.
for (Map.Entry<Integer,String> entry : map.entrySet())
{
System.out.println("Roll No: " + entry.getKey() + ", Name: " + entry.getValue());
}
}
}Output:
Roll No: 404, Name: Mark
Roll No: 101, Name: John
Roll No: 505, Name: Maya
Roll No: 202, Name: Ricky
Roll No: 303, Name: Deep
Iterate Map using forEach() Method
The forEach() method was added to the iterable interface with the release of JDK 1.8. It is a default method defined in the java.lang.Iterable interface.
This method performs the specified action for each element within Iterable interface until all the elements have been processed. The collection classes which extend the Iterable interface can use forEach() method to iterate elements of collection objects.
The forEach() method takes action to be performed for each element as a parameter. If the specified action is null, it will throw NullPointerException. It does not return anything.
Let’s take an example program based on the forEach() method. In this program, we will use lambda expression inside the forEach() method to display each entry of the map.
Program code 5:
import java.util.HashMap;
import java.util.Map;
public class KeySetDemo {
public static void main(String[] args)
{
Map<String, String> map = new HashMap<>();
map.put("India", " Delhi");
map.put("USA", " Washington, D.C.");
map.put("Australia", " Canberra");
map.put("New zealand", " Wellington");
map.put("Switzerland", " Bern");
// Iteration over map using forEach() method.
map.forEach((k,v) -> System.out.println("Country: "+ k + ", Capital: " + v));
}
}Output:
Country: USA, Capital: Washington, D.C.
Country: New zealand, Capital: Wellington
Country: Australia, Capital: Canberra
Country: Switzerland, Capital: Bern
Country: India, Capital: Delhi
In this tutorial, we have covered multiple ways to iterate a map in java with the best example programs. Hope that you will have understood all techniques to iterate a map and practiced all example programs.
Please share it on the social sites for your friends if you love this tutorial.
Thanks for reading!!!
