Java LinkedHashMap keySet - Get All Keys Example

This example shows how to get all keys from the Java LinkedHashMap using the keySet method. This example also shows how to iterate over and remove the keys from LinkedHashMap.

How to get all keys of the LinkedHashMap using the keySet method?

The keySet method of the LinkedHashMap class returns all keys of the map object.

1	public Set<K> keySet()

The keySet method returns a Set view of all the keys contained in the LinkedHashMap object.

import java.util.LinkedHashMap;

import java.util.Set;

public class LinkedHashMapKeySetExample {

public static void main(String[] args) {

LinkedHashMap<Integer, String> lhmap = new LinkedHashMap<Integer, String>();

lhmap.put(1, "One");

lhmap.put(2, "Two");

lhmap.put(3, "Three");

lhmap.put(4, "Four");

lhmap.put(5, "Five");

* To get all the keys of the LinkedHashMap, use

* the keySet method

Set<Integer> setKeys = lhmap.keySet();

System.out.println(setKeys);

}

Output

1	[1, 2, 3, 4, 5]

The Set returned by the keySet method is backed by the original LinkedHashMap object. That means any changes you make to the Set will also be reflected in the LinkedHashMap object, and vice-versa.

import java.util.LinkedHashMap;

import java.util.Set;

public class LinkedHashMapKeySetExample {

public static void main(String[] args) {

LinkedHashMap<Integer, String> lhmap = new LinkedHashMap<Integer, String>();

lhmap.put(1, "One");

lhmap.put(2, "Two");

lhmap.put(3, "Three");

lhmap.put(4, "Four");

lhmap.put(5, "Five");

Set<Integer> setKeys = lhmap.keySet();

* remove a key from the key set. The

* change will be reflected in the LinkedHashMap too

setKeys.remove(2);

System.out.println("Key set contains: " + setKeys);

System.out.println("LinkedHashMap contains: " + lhmap);

* Add a mapping to LinkedHashMap, the change

* will be reflected in the key set too.

lhmap.put(6, "Six");

System.out.println("Key set contains: " + setKeys);

System.out.println("LinkedHashMap contains: " + lhmap);

}

Output

Key set contains: [1, 3, 4, 5]

LinkedHashMap contains: {1=One, 3=Three, 4=Four, 5=Five}

Key set contains: [1, 3, 4, 5, 6]

LinkedHashMap contains: {1=One, 3=Three, 4=Four, 5=Five, 6=Six}

How to iterate all keys using the keySet method?

Use the keySet method to get all the keys from the LinkedHashMap as given above. Once you get the key Set, get an iterator for it using the iterator method and then use the hasNext and the next methods to iterate over it as given below.

LinkedHashMap<Integer, String> lhmap = new LinkedHashMap<Integer, String>();

lhmap.put(1, "One");

lhmap.put(2, "Two");

lhmap.put(3, "Three");

lhmap.put(4, "Four");

lhmap.put(5, "Five");

Set<Integer> setKeys = lhmap.keySet();

//get an iterator

Iterator<Integer> iterator = setKeys.iterator();

//use hasNext and next methods

while( iterator.hasNext() ){

System.out.println( iterator.next() );

}

Output

Note: The key set obtained using the keySet method of the LinkedHashMap class does not support add and addAll operations.

This example is a part of the LinkedHashMap in Java Tutorial.

Please let me know your views in the comments section below.

References:
Java 8 LinkedHashMap

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Rahim

I have a master's degree in computer science and over 18 years of experience designing and developing Java applications. I have worked with many fortune 500 companies as an eCommerce Architect. Follow me on LinkedIn and Facebook.