Java 8 – Stream.forEach()

Introduction

Java 8 introduced the Stream API, providing a more declarative and functional approach to processing collections of data. Among the various methods available in the Stream API, forEach() is one of the most commonly used. The forEach() method allows you to iterate over each element in a stream and perform a specific action, making it used for scenarios where you need to apply a function to every element of a collection.

In this guide, we’ll explore how to use the Stream.forEach() method effectively, with examples that demonstrate its versatility in handling different types of data.

Table of Contents

• Problem Statement
• Solution Steps
• Java Program
  • Using forEach() with a List of Integers
  • Using forEach() with a List of Strings
  • Using forEach() with a Stream of Custom Objects
• Advanced Considerations
• Conclusion

Problem Statement

The task is to create a Java program that:

• Demonstrates how to use the forEach() method to perform operations on each element in a stream.
• Applies forEach() to different types of data, including integers, strings, and custom objects.
• Outputs the results of the forEach() operations.

Example 1:

• Input: List of integers [1, 2, 3, 4, 5]
• Output: Print each integer.

Example 2:

• Input: List of strings ["apple", "banana", "cherry"]
• Output: Print each string in uppercase.

Solution Steps

1. Create a Stream: Start with a stream of elements (e.g., integers, strings, custom objects).
2. Apply the forEach() Method: Use the forEach() method to perform an action on each element in the stream.
3. Display the Result: Print or process each element as required.

Java Program

Using forEach() with a List of Integers

The forEach() method can be used to iterate over a list of integers and perform a specific action, such as printing each number.

import java.util.Arrays;
import java.util.List;

/**
 * Java 8 - Using forEach() with a List of Integers
 * Author: https://www.rameshfadatare.com/
 */
public class ForEachIntegerList {

    public static void main(String[] args) {
        // Step 1: Create a list of integers
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

        // Step 2: Use forEach() to print each number
        numbers.stream()
            .forEach(System.out::println);
    }
}

Output

1
2
3
4
5

Explanation

• The numbers.stream() method creates a stream from the list of integers.
• The forEach(System.out::println) method prints each number in the stream.

Using forEach() with a List of Strings

The forEach() method is also effective when working with strings. For example, you can convert each string to uppercase and then print it.

import java.util.Arrays;
import java.util.List;

/**
 * Java 8 - Using forEach() with a List of Strings
 * Author: https://www.rameshfadatare.com/
 */
public class ForEachStringList {

    public static void main(String[] args) {
        // Step 1: Create a list of strings
        List<String> fruits = Arrays.asList("apple", "banana", "cherry");

        // Step 2: Use forEach() to print each string in uppercase
        fruits.stream()
            .map(String::toUpperCase)
            .forEach(System.out::println);
    }
}

Output

APPLE
BANANA
CHERRY

Explanation

• The fruits.stream() method creates a stream from the list of strings.
• The map(String::toUpperCase) method converts each string to uppercase.
• The forEach(System.out::println) method prints each uppercase string.

Using forEach() with a Stream of Custom Objects

You can use the forEach() method with custom objects, allowing you to perform actions on each object in a stream. For instance, you might want to print the name and price of each product.

import java.util.Arrays;
import java.util.List;

/**
 * Java 8 - Using forEach() with a Stream of Custom Objects
 * Author: https://www.rameshfadatare.com/
 */
public class ForEachCustomObjects {

    public static void main(String[] args) {
        // Step 1: Create a list of products
        List<Product> products = Arrays.asList(
            new Product("Laptop", 1500),
            new Product("Phone", 800),
            new Product("Tablet", 600)
        );

        // Step 2: Use forEach() to print each product's details
        products.stream()
            .forEach(product -> 
                System.out.println("Product: " + product.getName() + ", Price: " + product.getPrice()));
    }
}

// Custom class Product
class Product {
    private String name;
    private double price;

    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public double getPrice() {
        return price;
    }
}

Output

Product: Laptop, Price: 1500.0
Product: Phone, Price: 800.0
Product: Tablet, Price: 600.0

Explanation

• The products.stream() method creates a stream from the list of Product objects.
• The forEach() method is used to iterate over each product and print its name and price.

Advanced Considerations

• Parallel Streams: The forEach() method can be used in parallel streams. However, be cautious as the order of execution is not guaranteed in parallel streams. Use forEachOrdered() if you need to maintain the order.

• Side Effects: The forEach() method is typically used for performing side effects, such as printing or modifying external state. Avoid using forEach() for operations that should be purely functional (i.e., without side effects).

• Stream Finality: forEach() is a terminal operation, meaning that after calling forEach(), the stream is considered consumed and cannot be reused.

Conclusion

This guide provides methods for using the Stream.forEach() method in Java 8, covering different types of data including integers, strings, and custom objects. The forEach() method is a powerful feature of the Stream API that allows you to apply an action to each element in a stream, making your code more concise and expressive. By understanding how to use forEach() effectively, you can simplify your data processing tasks and improve the readability of your Java applications.