Working with Generics

public class Box<T> {
    private T value;
    public void set(T value) { this.value = value; }
    public T get() { return value; }
}
Box<String> b = new Box<>();   // diamond operator

public static <T> List<T> repeat(T item, int n) {
    List<T> list = new ArrayList<>();
    for (int i = 0; i < n; i++) list.add(item);
    return list;
}
List<String> xs = repeat("x", 3);
List<Integer> ones = ListUtil.<Integer>repeat(1, 5);   // explicit type witness

double sum(List<? extends Number> list) {
    double s = 0;
    for (Number n : list) s += n.doubleValue();
    return s;
}
sum(List.of(1, 2, 3));        // List<Integer> OK
sum(List.of(1.5, 2.5));       // List<Double> OK

void addNumbers(List<? super Integer> list) {
    for (int i = 0; i < 5; i++) list.add(i);
}
addNumbers(new ArrayList<Number>());   // OK
addNumbers(new ArrayList<Object>());   // OK

public record Pair<K, V>(K key, V value) {}
Pair<String, Integer> p = new Pair<>("age", 30);