Java HashMap keySet().stream() Method

The HashMap.keySet().stream() method in Java is used to create a Stream of the keys contained in the HashMap. This guide will cover the method's usage, explain how it works, and provide examples to demonstrate its functionality.

Table of Contents

1. Introduction
2. keySet().stream() Method Syntax
3. Examples
   • Using keySet().stream() to Iterate Over Keys
   • Real-World Use Case: Filtering Keys Based on a Condition
4. Conclusion

Introduction

The HashMap.keySet().stream() method is a combination of HashMap.keySet() and Set.stream(). It provides a Stream of the keys contained in the HashMap. A Stream is a sequence of elements supporting sequential and parallel aggregate operations. This method can be useful for performing complex operations on the keys of a HashMap.

keySet().stream() Method Syntax

The syntax for obtaining a Stream of keys from a HashMap is as follows:

hashMap.keySet().stream()

• The method does not take any parameters.
• The method returns a Stream of the keys in the HashMap.

Examples

Using keySet().stream() to Iterate Over Keys

The keySet().stream() method can be used to create a Stream for iterating over the keys in a HashMap.

Example

import java.util.HashMap;
import java.util.stream.Stream;

public class KeyStreamExample {
    public static void main(String[] args) {
        // Creating a HashMap with String keys and Integer values
        HashMap<String, Integer> people = new HashMap<>();

        // Adding entries to the HashMap
        people.put("Ravi", 25);
        people.put("Priya", 30);
        people.put("Vijay", 35);

        // Getting the key stream
        Stream<String> keyStream = people.keySet().stream();

        // Using the key stream to iterate over the keys
        keyStream.forEach(key -> System.out.println("Key: " + key));
    }
}

Output:

Key: Ravi
Key: Priya
Key: Vijay

Real-World Use Case: Filtering Keys Based on a Condition

In a real-world scenario, you might use the keySet().stream() method to filter keys based on a certain condition, such as finding keys that start with a specific letter.

Example

import java.util.HashMap;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FilterKeysExample {
    public static void main(String[] args) {
        // Creating a HashMap with String keys and Integer values
        HashMap<String, Integer> people = new HashMap<>();

        // Adding entries to the HashMap
        people.put("Ravi", 25);
        people.put("Priya", 30);
        people.put("Vijay", 35);
        people.put("Rohit", 40);

        // Getting the key stream
        Stream<String> keyStream = people.keySet().stream();

        // Filtering keys that start with "R"
        Stream<String> filteredKeys = keyStream.filter(key -> key.startsWith("R"));

        // Collecting the filtered keys into a list and printing them
        System.out.println("Keys that start with 'R': " + filteredKeys.collect(Collectors.toList()));
    }
}

Output:

Keys that start with 'R': [Ravi, Rohit]

Conclusion

The HashMap.keySet().stream() method in Java provides a way to create a Stream of the keys contained in the HashMap. By understanding how to use this method, you can efficiently perform complex operations on the keys, such as filtering, mapping, and reducing. This method is useful in various scenarios, such as iterating over keys, filtering keys based on conditions, and performing aggregate operations on keys in a HashMap.