Java 8 FAQ: Top Questions

The reduce() method is a terminal operation in Java 8 that combines all elements of a stream into a single result using a binary operation. It’s commonly used for summing numbers, concatenating strings, or computing aggregates.

🗺️ Step-by-Step Instructions:

1. Define a binary operation using a lambda or method reference.
2. Apply reduce() with or without an identity value.
3. With an identity: returns a concrete value. Without identity: returns an Optional.

📥 Example Input:

List numbers = Arrays.asList(1, 2, 3, 4, 5);

🏆 Using reduce() to sum values:

int sum = numbers.stream()
                 .reduce(0, (a, b) -> a + b);

✅ Java 8 Solution:

import java.util.*;
import java.util.stream.*;

public class StreamReduceExample {
  public static void main(String[] args) {
    List numbers = Arrays.asList(1, 2, 3, 4, 5);

    int sum = numbers.stream()
                     .reduce(0, Integer::sum);

    System.out.println("Sum: " + sum);
  }
}

📘 Detailed Explanation:

• reduce(identity, accumulator): Applies a binary operation starting with a base value.
• reduce(accumulator): Returns an Optional in case the stream is empty.
• Works best when converting streams into a single result like sum, product, max, or string join.

🛠️ Use Cases:

• Calculating total, product, or max values from a list.
• Concatenating string values from a stream of words.
• Implementing custom aggregation logic over data.