Lambda Expressions & Streams

A Lambda Expression is just a shorthand way to write a tiny method in one line. It looks like this: (input) -> expression.

The Stream Pipeline

• filter: Keeps only the items that match a condition.

• map: Transforms each item (e.g., turning text to Uppercase).

• sorted: Reorders the items.

• collect: Gathers the final results back into a List.

The Code Example:

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

public class ModernJavaDemo {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "Amy", "David");

        System.out.println("Original List: " + names);

        // --- The Modern Pipeline ---
        // Goal: Find names starting with 'A', make them Uppercase, and Sort them.
        List<String> result = names.stream()
            .filter(name -> name.startsWith("A"))    // Lambda: Keep only 'A' names
            .map(name -> name.toUpperCase())         // Lambda: Convert to Uppercase
            .sorted()                                // Sort alphabetically
            .collect(Collectors.toList());           // Turn back into a List

        System.out.println("Processed List: " + result);

        // --- Quick Math with Streams ---
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        int sumOfEvens = numbers.stream()
            .filter(n -> n % 2 == 0)                 // Keep only even numbers
            .mapToInt(n -> n * n)                    // Square them: (2->4, 4->16)
            .sum();                                  // Add them up (4 + 16)

        System.out.println("Sum of squares of even numbers: " + sumOfEvens);
    }
}

Expected Output:

Original List: [Alice, Bob, Charlie, Amy, David]
Processed List: [ALICE, AMY]
Sum of squares of even numbers: 20