什么是泛型（参数多态）？

// With generics: type is known and checked at compile time
List<String> list = new ArrayList<>();
list.add("hi");
String s = list.get(0);            // no cast; list.add(42) won't compile

class Box<T> {                     // T is a type parameter
    private T value;
    Box(T value) { this.value = value; }
    T get() { return value; }
}

Box<Integer> b = new Box<>(42);    // T = Integer, checked at compile time
int n = b.get();                   // no cast needed

<T extends Comparable<T>>          // bounded: T must be comparable
List<? extends Number>             // covariance: accept List<Integer>, List<Double>