Java Iterate Map

Introduction to Java Iterate Map

Iterate Map defined as extracting a bunch of data from the Map collection. In most real-time cases, we have a large amount of data stored on a map. The developer wants to fetch this data; he has to iterate the entire map. A map is stored in util.Map package. In this topic, we are going to learn about Java Iterate Map. Iteration in a map can be done in 3 ways.

• forEach
• Normal for loop
• While loop with an iterator

What is a Map?

1. Map is a key and value pair structured collection. The map is an interface. So, Map implemented into

• HashMap
• LinkedHashMap
• TreeMap

2. Entry is the sub interface of Map. Therefore, it will be accessible by Entry name. It returned the collection view of the map. Map.The entry provides methods to fetch key and value pair.

Important Methods

• put(Object keyValue, Object objectValue): It is used for insert some value into a map.
• keySet(): It is used for getting a key pair set.
• entrySet(): It is used to set keys and values to the map.
• size(): It is used for getting map size.
• getKey(): It is used for getting a key value.
• getValue(): It is used for getting a key mapped value.

How does Iteration work in Map?

As we discussed above, Map can be iterated with forEach, normal for and while loop. We will understand it by Syntax.

1. forEach

Syntax:

Map<DataType, DataType>map = new HashMap<DataType, DataType>();
map.forEach((key, value) -> //logic

2. for with entrySet()

Syntax:

Map<DataType,DataType> map= new HashMap<DataType,DataType>();
for (Map.Entry<DataType,DataType> set: map.entrySet())
//logic
}

3. While loop with an iterator

Syntax:

Map<String, String> mapObj = new HashMap<>();
Iterator iterator= mapObj .entrySet().iterator();
while (iterator.hasNext()) {
//logic
}

Based on the requirement, we can iterate the map in any of the above ways.

Examples of Java Iterate Map

Here are the following examples mention below

1. Map iteration with forEach Example

Code:

import java.util.HashMap;
import java.util.Map;
public class ForEachMapIteration {
public static void main(String[] args) {
// creating a hashmap object
Map<Integer, String> names = new HashMap<>();
// adding key and values to hash map
names.put(1, "Paramesh");
names.put(2, "Amardeep");
names.put(3, "Venkatesh");
names.put(4, "Ramesh");
names.put(5, "Suresh");
names.put(3, "Krishna");
names.put(4, "Rama");
names.put(5, "Rajitha");
// iterating key and value with forEach loop
names.forEach((key, value) -> {
System.out.println("ID =>" + key + " Name => " + value);
});
}
}

Output:

Explanation:

As you can see in the above code, we have iterated the map with forEach

2. Map key and value iteration separately with forEach loop Example

Code:

package com.map;
import java.util.HashMap;
import java.util.Map;
public class ForEachKeyValueMapIteration {
public static void main(String[] args) {
// creating a hashmap object
Map<Integer, String> names = new HashMap<>();
// adding key and values to hash map
names.put(1, "Paramesh");
names.put(2, "Amardeep");
names.put(3, "Venkatesh");
names.put(4, "Ramesh");
names.put(5, "Suresh");
names.put(3, "Krishna");
names.put(4, "Rama");
names.put(5, "Rajitha");
// iterating ids with forEach loop
names.forEach((key, value) -> {
System.out.println("ID =>" + key);
});
// iterating names with forEach loop
names.forEach((key, value) -> {
System.out.println("Name => " + value);
});
}
}

Output:

Explanation:

As you can see in the above output, we can also fetch the key and values separately with the forEach loop.

3. Map Iteration with for loop Example:

Code:

 import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
public class ForMapIteration {
public static void main(String[] args) {
// creating a hashmap object
Map<Integer, String> names = new HashMap<>();
// adding key and values to hash map
names.put(1, "Paramesh");
names.put(2, "Amardeep");
names.put(3, "Venkatesh");
names.put(4, "Ramesh");
names.put(5, "Suresh");
names.put(3, "Krishna");
names.put(4, "Rama");
names.put(5, "Rajitha");
// iterating key and value with for loop
for (Entry<Integer, String> entry : names.entrySet())
System.out.println("ID => " + entry.getKey() + " Name => " + entry.getValue());
}
}

Output:

Explanation:

As you can see in the above code, we have iterated the map with for loop.

4. Map key and value iteration separately with for loop Example:

Code:

 import java.util.HashMap;
import java.util.Map;
public class ForKeyAndValueMapIteration {
public static void main(String[] args) {
// creating a hashmap object
Map<Integer, String> names = new HashMap<>();
// adding key and values to hash map
names.put(1, "Paramesh");
names.put(2, "Amardeep");
names.put(3, "Venkatesh");
names.put(4, "Ramesh");
names.put(5, "Suresh");
names.put(3, "Krishna");
names.put(4, "Rama");
names.put(5, "Rajitha");
//fetching ids
for (Integer id : names.keySet())
System.out.println("ID => " + id);
// fetching names
for (String name : names.values())
System.out.println("Name => " + name);
}
}

Output:

Explanation:

As you can see in the above output, we can also fetch the key and values separately with for loop.

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5. Map Iteration with while loop and iterator Example:

Code:

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class WhileIteratorLoopMap {
public static void main(String[] args) {
// creating a hashmap object
Map<Integer, String> names = new HashMap<>();
// adding key and values to hash map
names.put(1, "Paramesh");
names.put(2, "Amardeep");
names.put(3, "Venkatesh");
names.put(4, "Ramesh");
names.put(5, "Suresh");
names.put(3, "Krishna");
names.put(4, "Rama");
names.put(5, "Rajitha");
//get entry set from map
Set set = names.entrySet();
//get iterator from set
Iterator iterator = set.iterator();
//fetching id and names with while loop
while (iterator.hasNext()) {
Map.Entry entry = (Map.Entry) iterator.next();
System.out.println("ID =>"+entry.getKey() + " Name => " + entry.getValue());
}
}
}

Output:

Explanation:

As you can see in the above code, we have iterated the map with iterator and while loop.

Conclusion – Java Iterate Map

A map can be iterated by for, forEach and while loop from the Entry interface. We can also iterate keys and values separately without any error.

Recommended Articles

This is a guide to Java Iterate Map. Here we discuss How does iteration works in Map along with the methods and examples. You may also have a look at the following articles to learn more –

1. HashMap in Java
2. Stack Class in Java
3. Final Class in Java
4. Nested Class in Java