Usage and example of Java HashMap keySet()

Java HashMap Methods

The Java HashMap keySet() method returns a view of the set of all keys contained in the hash table entries.

The syntax of the keySet() method is: }}

hashmap.keySet()

keySet() parameters

The keySet() method does not take any parameters.

The return value of keySet()

• 　Returns the set view of all keys of the hash map

Note: The collection view only displays all keys of the hash map as a collection. It does not contain the actual keys.

Example1: Java HashMap keySet()

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　//　Create a HashMap
　　　　HashMap<String, Integer> prices = new HashMap<>();
　　　　//　Insert entries into the HashMap
　　　　prices.put("Shoes",　200);
　　　　prices.put("Bag",　300);
　　　　prices.put("Pant",　150);
　　　　System.out.println("HashMap: ");　+　prices);
　　　　//　Returns a set view of all keys
　　　　System.out.println("Keys: ");　+　prices.keySet());
　　}
}

Output Result

HashMap: {Pant =150, Bag =300, Shoes =200}
Keys: [Pant, Bag, Shoes]

In the above example, we created a hash map named prices. Note the expression

prices.keySet()

Here, the keySet() method returns a set view of all keys existing in the hash map.

The keySet() method can also be used withfor-The each loopused together to iterate over each key in the hash map.

Example2: for-The keySet() method in the each loop

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　//　Create a HashMap
　　　　HashMap<String, Integer> numbers = new HashMap<>();
　　　　numbers.put("One",　1);
　　　　numbers.put("Two",　2);
　　　　numbers.put("Three",　3);
　　　　System.out.println("HashMap: ");　+　numbers);
　　　　//Access all keys of the HashMap
　　　　System.out.print("Keys: ");
　　　　//keySet() returns a set view of all keys
　　　　//for-The loop iterates over each key in the view
　　　　for (String key : numbers.keySet()) {
　　　　　　//　Print each key
　　　　　　System.out.print(key　+　", ");
　　　　}
　　}
}

Output Result

HashMap: {One=1, Two=2, Three=3}
Keys: One, Two, Three,

In the above example, we created a hash map named numbers. Note this line,

String key: numbers.keySet()

Here, the keySet() method returnsall keysofSet collection view. The variable key accesses each key from the view.

Note: The keys of HashMap are of String type. Therefore, we use a String variable to access the keys.

Related Reading

• HashMap entrySet() - Return a set view of all mappings (entries)

• HashMap values() - Return a set view of all values

Java HashMap Methods