How To Iterate TreeMap in older Java and new Java 8? Or How to iterate over each entry in a Java Map?

1. Overview

In this tutorial, We will learn how to iterate the map in older java versions and examples in java 8.

And alos let us explore how to iterate the keys of TreeMap sorted that means in the ascending order.

TreeMap can be used where we want the map sorted based on the key by default in the ascending or descending order use cases.

2. Example to Iterate the TreeMap before JDK 8

In the below code we have created the TreeMap object and added few key value pairs. Here key is the type String and Value is type of Integer.

package com.javaprogramto.collections.treemap;

import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class TreeMapIterate {

	public static void main(String[] args) {
		
		// Creating the TreeMap object
		Map<String, Integer> studentsCountMap = new TreeMap<>();
		
		// Adding students class and no of students in the class
		studentsCountMap.put("2nd class", 200);
		studentsCountMap.put("1nd class", 100);
		studentsCountMap.put("4nd class", 400);
		studentsCountMap.put("5nd class", 500);
		studentsCountMap.put("3nd class", 300);
		
		// Getting the Set object using keySet() method
		Set<String> keysSet = studentsCountMap.keySet();
		
		// Getting the iterator object
		Iterator<String > it = keysSet.iterator();
		
		// Iterating the map using regular method of iterator.
		while (it.hasNext()) {
			String key = it.next();
			System.out.println("Key - "+key+", Value - "+studentsCountMap.get(key));
		}

	}

}

Output:

Key - 1nd class, Value - 100
Key - 2nd class, Value - 200
Key - 3nd class, Value - 300
Key - 4nd class, Value - 400
Key - 5nd class, Value - 500

In the above program, we have first got the all the keys of treemap using keySet() method and next used the iterator() method to get the Iterator instance.

Finally, used the traditional iterate method workflow to get the each key from iterator and passed the key to treemap to get the value of the corresponding key.

3. Java 8 Lamdba Foreach TreeMap

Next, Look at the below program to get the keys and values of TreeMap using Java 8 Lambda expressions with forEach() method.

package com.javaprogramto.collections.treemap;

import java.util.Map;
import java.util.TreeMap;

public class TreeMapIterateJava8 {

	public static void main(String[] args) {

		// Creating the TreeMap object
		Map<String, Integer> studentsCountMap = new TreeMap<>();

		// Adding students class and no of students in the class
		studentsCountMap.put("2nd class", 200);
		studentsCountMap.put("1nd class", 100);
		studentsCountMap.put("4nd class", 400);
		studentsCountMap.put("5nd class", 500);
		studentsCountMap.put("3nd class", 300);

		// Java 8 lambda foreach
		studentsCountMap.forEach((key, value) -> {
			System.out.println("Key - " + key + ", Value - " + value);
		});
	}
}

This ways also produces the same output as seen in the above section. This is the simplified version in java 8 and it internally uses the BiConsumer functional interface.

4. Java 8 Stream Foreach TreeMap EntrySet

Finally, explore the java 8 stream construct to get the Entry object from stream and pass it to forEach method.

package com.javaprogramto.collections.treemap;

import java.util.Map;
import java.util.TreeMap;

public class TreeMapIterateJava8 {

	public static void main(String[] args) {

		// Creating the TreeMap object
		Map<String, Integer> studentsCountMap = new TreeMap<>();

		// Adding students class and no of students in the class
		studentsCountMap.put("2nd class", 200);
		studentsCountMap.put("1nd class", 100);
		studentsCountMap.put("4nd class", 400);
		studentsCountMap.put("5nd class", 500);
		studentsCountMap.put("3nd class", 300);

		// java 8 Stream entry forEach
		studentsCountMap.entrySet().stream().forEach(
				entry -> System.out.println("entry key - " + entry.getKey() + ", entry value - " + entry.getValue()));
	}
}

Output:

entry key - 1nd class, entry value - 100
entry key - 2nd class, entry value - 200
entry key - 3nd class, entry value - 300
entry key - 4nd class, entry value - 400
entry key - 5nd class, entry value - 500

5. Conclusion

In this article, We have seen how to iterate the TreeMap in older and new JDK versions with examples.

GitHub

How to sort the TreeMap based on the Values rather than Keys?

Ref

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