一篇文章带你了解Java泛型的super和extends

/**
 * 生物
 */
static class Biological{
}
/**
 * 动物
 */
static class Animal extends Biological{
}
/**
 * 植物
 */
static class Plant extends Biological{
}

static class Dog extends Animal{
}
static class Cat extends Animal{
}
static class Flower extends Plant{
}
static class Tree extends Plant{
}

public static void main(String[] args) {
    List<? super Animal> listA = new ArrayList<>();
    List<? extends Plant> listB = new ArrayList<>();
    //listA.add(new Biological());  
    listA.add(new Animal());
    listA.add(new Dog());
    //listA.add(new Flower());
    Object object = listA.get(0);
    //listB.add(new Plant());
    //listB.add(new Flower());
    listB.add(null);
    Plant plant = listB.get(0);
}

List<? super Animal> listA = new ArrayList<>();

listA.add(new Biological());  

listA.add(new Animal());
listA.add(new Dog());

listA.add(new Flower());

Object object = listA.get(0);

List<? extends Plant> listB = new ArrayList<>();

listB.add(new Plant());

listB.add(new Flower());

listB.add(null);

Plant plant = listB.get(0);