Java Streams and Lambda Expressions

// Traditional anonymous class
Runnable r1 = new Runnable() {
    @Override
    public void run() { System.out.println("Hello!"); }
};

// Lambda expression
Runnable r2 = () -> System.out.println("Hello!");

// With parameters
Comparator<String> comp = (a, b) -> a.compareTo(b);

import java.util.function.*;

// Predicate — takes input, returns boolean
Predicate<Integer> isEven = n -> n % 2 == 0;
isEven.test(4); // true

// Function — takes input, returns output
Function<String, Integer> length = s -> s.length();
length.apply("Hello"); // 5

// Consumer — takes input, returns nothing
Consumer<String> printer = s -> System.out.println(s);
printer.accept("Hi!"); // prints "Hi!"

// Supplier — takes nothing, returns output
Supplier<Double> random = () -> Math.random();
random.get(); // 0.73...

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

List<String> names = List.of("Alice", "Bob", "Charlie", "David", "Eve");

// Filter: keep names starting with vowels
names.stream()
     .filter(name -> name.startsWith("A") || name.startsWith("E"))
     .forEach(System.out::println); // Alice, Eve

List<Integer> nums = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
List<Integer> evens = nums.stream().filter(n -> n % 2 == 0).collect(Collectors.toList());
// [2, 4, 6, 8, 10]

List<String> upper = names.stream().map(String::toUpperCase).collect(Collectors.toList());
// [ALICE, BOB, CHARLIE, DAVID, EVE]

int sum = nums.stream().reduce(0, Integer::sum); // 55

names.stream().sorted().forEach(System.out::println);

List.of(1, 2, 2, 3, 3, 3).stream().distinct().forEach(System.out::print); // 123

long count = names.stream().filter(n -> n.length() > 3).count(); // 3

names.forEach(System.out::println);    // equivalent to: n -> System.out.println(n)
names.stream().map(String::toUpperCase); // equivalent to: s -> s.toUpperCase()

// Join strings
String joined = names.stream().collect(Collectors.joining(", ")); // "Alice, Bob, ..."

// Group by length
Map<Integer, List<String>> byLength = names.stream()
    .collect(Collectors.groupingBy(String::length));

// Partition (split into two groups)
Map<Boolean, List<Integer>> partitioned = nums.stream()
    .collect(Collectors.partitioningBy(n -> n > 5));