Java

Question

为什么 doSomething2() 和 doSomething3() 不能在 Java 中编译？

(Toyota extends Car 和 Car implements ICar)

 public static void main(String[] args) {
    Map<String, Toyota> toyotas = new HashMap<>();
    doSomething0(toyotas);
    doSomething1(toyotas);
    doSomething2(toyotas);
    doSomething3(toyotas);
}

private static void doSomething0(Map<String, ? extends Car> cars) {}
private static void doSomething1(Map<String, ? extends ICar> cars) {}
private static void doSomething2(Map<String, ICar> cars) {}
private static void doSomething3(Map<String, Car> cars) {}

Answer 1

因为：

public static void main(String[] args) {
    Map<String, Toyota> toyotas = new HashMap<>();
    doSomething4(toyotas);

    Toyota t = toyotas.get("hello"); // boom
}

private static void doSomething4(Map<String, Car> cars) {
    cars.put("Hello", new Honda());
}

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