/**

* Day 21 - Generics: Wildcards (PECS)

*

* <p>Demonstrates:

* <ul>

* <li>{@code List<?>} (unknown type) is effectively read-only for adding</li>

* <li>{@code List<? extends Number>} is a producer (read numbers)</li>

* <li>{@code List<? super Integer>} is a consumer (add integers)</li>

* <li>PECS: Producer Extends, Consumer Super</li>

* </ul>

*/

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

public class WildcardsExample {

static void printAnyList(List<?> list) {

System.out.println("List<?> contents: " + list);

Object first = list.isEmpty() ? null : list.get(0);

System.out.println("First element as Object: " + first);

// list.add(...) is not allowed because the compiler does not know the element type.

// list.add("x"); // does not compile

}

static double sumNumbers(List<? extends Number> numbers) {

double sum = 0.0;

for (Number n : numbers) {

sum += n.doubleValue();

}

// numbers.add(1); // does not compile (can't safely add to ? extends)

return sum;

}

static void addSomeIntegers(List<? super Integer> sink) {

// We can add Integer values safely.

sink.add(10);

sink.add(20);

sink.add(30);

// Reading back only guarantees Object.

Object first = sink.get(0);

System.out.println("After adding integers, first element as Object: " + first);

}

public static void main(String[] args) {

System.out.println("=== Wildcards (PECS) ===\n");

List<String> names = Arrays.asList("Alice", "Bob");

printAnyList(names);

System.out.println();

List<Integer> ints = Arrays.asList(1, 2, 3);

List<Double> doubles = Arrays.asList(1.5, 2.5);

System.out.println("Sum of ints: " + sumNumbers(ints));

System.out.println("Sum of doubles: " + sumNumbers(doubles));

System.out.println();

List<Number> numberSink = new ArrayList<>();

addSomeIntegers(numberSink);

System.out.println("Number sink contents: " + numberSink);

System.out.println("\nKey Points:");

System.out.println("1. List<?>: unknown type (can read as Object)");

System.out.println("2. ? extends T: producer (read T), cannot add safely");

System.out.println("3. ? super T: consumer (add T), reads as Object");

System.out.println("4. PECS: Producer Extends, Consumer Super");

}

}