Java 泛型详解之类型参数的力量(最新推荐)

List list = new ArrayList();
list.add("Hello");
list.add(123); // 不安全
String s = (String) list.get(0); // 强制类型转换

List<String> list = new ArrayList<>();
list.add("Hello");
// list.add(123); // 编译错误
String s = list.get(0); // 无需强转

class Box<T> {          // 定义泛型类
    private T value;
    public void set(T value) { this.value = value; }
    public T get() { return value; }
}
Box<Integer> box = new Box<>();
box.set(100);
Integer data = box.get();

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; }
}

interface Converter<F, T> {
    T convert(F from);
}
class StringToInteger implements Converter<String, Integer> {
    public Integer convert(String from) {
        return Integer.valueOf(from);
    }
}

public static <T> void printArray(T[] array) {
    for (T element : array) {
        System.out.println(element);
    }
}
String[] arr = {"A", "B", "C"};
printArray(arr); // 自动推断 T = String

public static void show(List<? extends Number> list) {
    for (Number n : list) {
        System.out.println(n);
    }
}

class Calculator<T extends Number & Comparable<T>> {
    public boolean isGreater(T a, T b) {
        return a.compareTo(b) > 0;
    }
}

List<String> list = List.of("A", "B");
var map = Map.of(1, "A", 2, "B"); // Java 10+

class Box<T> {
    // static T value; // 错误！
}

if (obj instanceof Box<String>) // 错误：类型擦除导致编译不通过