Five Best Ways to Iterate a Java HashMap

In this article we discuss five best ways to iterate a Java HashMap.

Using Iterator to traverse HashMap EntrySet

Using Iterator to traverse HashMap KeySet

Using For‑each loop to iterate HashMap

Using Lambda expression to traverse HashMap

Using Stream API to traverse HashMap

1. Using Iterator to traverse HashMap EntrySet

package com.java.tutorials.iterations;

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

/**
 * 在 Java 中遍历 HashMap 的5种最佳方法
 * @author Ramesh Fadatare
 */
public class IterateHashMapExample {
    public static void main(String[] args) {
        // 1. 使用 Iterator 遍历 HashMap EntrySet
        Map<Integer, String> coursesMap = new HashMap<Integer, String>();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "Spring Framework");
        coursesMap.put(5, "Hibernate ORM framework");

        Iterator<Entry<Integer, String>> iterator = coursesMap.entrySet().iterator();
        while (iterator.hasNext()) {
            Entry<Integer, String> entry = iterator.next();
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }
    }
}

Output:

1
C
2
C++
3
Java
4
Spring Framework
5
Hibernate ORM framework

2. Using Iterator to traverse HashMap KeySet

package com.java.tutorials.iterations;

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

/**
 * 在 Java 中遍历 HashMap 的5种最佳方法
 * @author Ramesh Fadatare
 */
public class IterateHashMapExample {
    public static void main(String[] args) {
        Map<Integer, String> coursesMap = new HashMap<Integer, String>();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "Spring Framework");
        coursesMap.put(5, "Hibernate ORM framework");

        // 2. 使用 Iterator 遍历 HashMap KeySet
        Iterator<Integer> iterator = coursesMap.keySet().iterator();
        while (iterator.hasNext()) {
            Integer key = iterator.next();
            System.out.println(key);
            System.out.println(coursesMap.get(key));
        }
    }
}

Output:

1
C
2
C++
3
Java
4
Spring Framework
5
Hibernate ORM framework

3. Using For‑each loop to iterate HashMap

package com.java.tutorials.iterations;

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

/**
 * 在 Java 中遍历 HashMap 的5种最佳方法
 * @author Ramesh Fadatare
 */
public class IterateHashMapExample {
    public static void main(String[] args) {
        Map<Integer, String> coursesMap = new HashMap<Integer, String>();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "Spring Framework");
        coursesMap.put(5, "Hibernate ORM framework");

        // 3. 使用 For-each 循环遍历 HashMap
        for (Map.Entry<Integer, String> entry : coursesMap.entrySet()) {
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }
    }
}

Output:

1
C
2
C++
3
Java
4
Spring Framework
5
Hibernate ORM framework

4. Using Lambda expression to traverse HashMap

package com.java.tutorials.iterations;

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

/**
 * 在 Java 中遍历 HashMap 的5种最佳方法
 * @author Ramesh Fadatare
 */
public class IterateHashMapExample {
    public static void main(String[] args) {
        Map<Integer, String> coursesMap = new HashMap<Integer, String>();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "Spring Framework");
        coursesMap.put(5, "Hibernate ORM framework");

        // 4. 使用 Lambda 表达式遍历 HashMap
        coursesMap.forEach((key, value) -> {
            System.out.println(key);
            System.out.println(value);
        });
    }
}

Output:

1
C
2
C++
3
Java
4
Spring Framework
5
Hibernate ORM framework

5. Using Stream API to traverse HashMap

package com.java.tutorials.iterations;

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

/**
 * 在 Java 中遍历 HashMap 的5种最佳方法
 * @author Ramesh Fadatare
 */
public class IterateHashMapExample {
    public static void main(String[] args) {
        Map<Integer, String> coursesMap = new HashMap<Integer, String>();
        coursesMap.put(1, "C");
        coursesMap.put(2, "C++");
        coursesMap.put(3, "Java");
        coursesMap.put(4, "Spring Framework");
        coursesMap.put(5, "Hibernate ORM framework");

        // 5. 使用 Stream API 遍历 HashMap
        coursesMap.entrySet().stream().forEach((entry) -> {
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        });
    }
}

Output:

1
C
2
C++
3
Java
4
Spring Framework
5
Hibernate ORM framework