Daily Hack #day47 - Java Streams: Summarizing Your Data

The reduce operation accumulates elements within a stream into a single result.

This is useful for calculating various statistics or summarizing your data. Here’s an example of finding the total price of all products:

int totalPrice = products.stream()
  .mapToInt(product -> product.getPrice()) // Map to int stream of prices
  .reduce(0, Integer::sum); // Reduce using sum function

This code first maps the product stream to an IntStream containing just the prices.

Then, reduce with the Integer::sum method efficiently calculates the sum of all prices, providing the total cost.

Here is a code example you can copy/modify at your own pace to see the above concept in action:

import java.util.ArrayList;
import java.util.List;

class Product {
    private String name;
    private int id;
    private double price;

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

    public double getPrice() {
        return price;
    }
}

public class ProductSum {
    public static void main(String[] args) {
        // Create a list of products
        List<Product> products = new ArrayList<>();
        products.add(new Product("Product 1", 1, 10.0));
        products.add(new Product("Product 2", 2, 20.0));
        products.add(new Product("Product 3", 3, 30.0));

        // Calculate the sum of all product prices using Streams API
        double totalPrice = products.stream()
                                    .mapToDouble(Product::getPrice)
                                    .reduce(0, Double::sum);

        System.out.println("Total price of all products: " + totalPrice);
    }
}

In this program:

• We define a Product class with three fields: name, id, and price.
• We create a list of Product objects and populate it with some sample data.
• We use the stream() method on the products list to obtain a stream of Product objects.
• We use the mapToDouble() method to map each Product object to its price (double value).
• We use the sum() terminal operation to calculate the sum of all the prices in the stream.
• Finally, we print the total price of all products.