# 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.