Java Generics

// Generic class — T is a type parameter (placeholder)
class Box<T> {
    private T value;

    public Box(T value) { this.value = value; }
    public T getValue()  { return value; }
    public void setValue(T value) { this.value = value; }

    @Override
    public String toString() { return "Box[" + value + "]"; }
}

// Generic class with two type parameters
class Pair<K, V> {
    private K key;
    private V value;

    public Pair(K key, V value) { this.key = key; this.value = value; }
    public K getKey()   { return key; }
    public V getValue() { return value; }

    @Override
    public String toString() { return "(" + key + ", " + value + ")"; }
}

public class Generics {

    // Generic method
    public static <T> void printArray(T[] arr) {
        for (T item : arr) System.out.print(item + " ");
        System.out.println();
    }

    public static void main(String[] args) {
        Box<Integer> intBox = new Box<>(42);
        Box<String>  strBox = new Box<>("Hello");
        System.out.println(intBox);  // Box[42]
        System.out.println(strBox);  // Box[Hello]

        Pair<String, Integer> p = new Pair<>("age", 30);
        System.out.println(p);  // (age, 30)

        Integer[] nums = {1, 2, 3, 4, 5};
        String[]  strs = {"a", "b", "c"};
        printArray(nums);  // 1 2 3 4 5
        printArray(strs);  // a b c
    }
}

import java.util.*;

public class BoundedGenerics {

    // Upper bound: T must be Number or a subclass
    public static <T extends Number> double sum(List<T> list) {
        double total = 0;
        for (T item : list) total += item.doubleValue();
        return total;
    }

    // Wildcard ? — unknown type
    // ? extends Number — upper bounded wildcard (read-only)
    public static void printNumbers(List<? extends Number> list) {
        for (Number n : list) System.out.print(n + " ");
        System.out.println();
    }

    // ? super Integer — lower bounded wildcard (write-friendly)
    public static void addIntegers(List<? super Integer> list) {
        list.add(1);
        list.add(2);
        list.add(3);
    }

    public static void main(String[] args) {
        List<Integer> ints    = Arrays.asList(1, 2, 3, 4, 5);
        List<Double>  doubles = Arrays.asList(1.1, 2.2, 3.3);

        System.out.println(sum(ints));     // 15.0
        System.out.println(sum(doubles));  // 6.6

        printNumbers(ints);     // 1 2 3 4 5
        printNumbers(doubles);  // 1.1 2.2 3.3

        List<Number> numbers = new ArrayList<>();
        addIntegers(numbers);
        System.out.println(numbers);  // [1, 2, 3]
    }
}