How to loop Map in Java

By mkyong | Updated: March 13, 2024

Viewed: 34,536 (+128 pv/w)

Tags: java.util java8 loop map map map filter

This article show few ways of loop Map in Java.

• 1. Using entrySet()
• 2. Using forEach (Java 8 and later)
• 3. Using stream() (Java 8 and later)
• 4. Using iterator()
• 5. Using keySet() and get()
• 6. References

1. Using `entrySet()`

Before Java 8, this is the most common method to loop a Map in Java. We use the map’s entry set entrySet() to loop and access to each key-value pair.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class LoopMap1 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            System.out.println("ID: " + entry.getKey() + ", Name: " + entry.getValue());
        }

    }

}

output

ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar

2. Using `forEach` (Java 8 and later)

Java 8 introduced the forEach method, which provides a more simple way to iterate over maps using lambda expressions.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class LoopMap2 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        // map.forEach((key, value) -> System.out.println("ID: " + key + ", Name: " + value));

        map.forEach((id, name) -> {
            System.out.println("ID: " + id + ", Name: " + name);
        });


    }

}

output

ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar

3. Using `stream()` (Java 8 and later)

We can use stream() to loop, filtering and collect in Java 8.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class LoopMap3 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        map.entrySet().stream()
                .forEach(entry -> System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()));

    }

}

output

ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar

In below example, we use stream() to get the map’s entry if it’s value is equals to Jan.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class LoopMap3 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        map.entrySet().stream()
            .filter(entry -> "Jan".equals(entry.getValue()))
            .forEach(entry -> System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()));

        // Map stream, filter and collect to a Map. 
        /*
        Map<Integer, String> result = map.entrySet().stream()
                .filter(entry -> "Jan".equals(entry.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));*/


    }

}

output

ID: 1, Name: Jan

4. Using `iterator()`

The iterator did the jobs but not a common way to loop Map in Java.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class LoopMap4 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        // Map -> Set -> Iterator -> Map.Entry
        Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<Integer, String> entry = iterator.next();
            System.out.println("Key : " + entry.getKey() + " Value :" + entry.getValue());
        }

    }

}

5. Using `keySet()` and `get()`

This method involves iterating over the key set and then using each key to get its corresponding value. It’s slightly less efficient than using entrySet() because of the additional get() call. It might be useful for conditionally fetch values.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class LoopMap5 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "Jan");
        map.put(2, "Feb");
        map.put(3, "Mar");

        for (Integer key : map.keySet()) {
            System.out.println("Key : " + key + " Value : " + map.get(key));
        }

    }

}

6. References

• java.util.Map JavaDoc
• Java 8 filter a Map examples
• Java 8 forEach examples
• Java 8 Streams map() examples