泛型高级之通配符

class Animal {
}

class Dog extends Animal {
}

class Cat extends Animal {
}

//泛型如果明确的写的时候，前后必须一致
Colletion<Object> c1 = new ArrayList<Object>();
Colletion<Object> c2 = new ArrayList<Animal>();    //错误
Colletion<Object> c3 = new ArrayList<Dog>();       //错误
Colletion<Object> c4 = new ArrayList<Cat>();       //错误

Colletion<?> c5 = new ArrayList<Object>();
Colletion<?> c6 = new ArrayList<Animal>();
Colletion<?> c7 = new ArrayList<Dog>();
Colletion<?> c8 = new ArrayList<Cat>();

Colletion<? extends Animal> c5 = new ArrayList<Object>();    //错误
Colletion<? extends Animal> c6 = new ArrayList<Animal>();
Colletion<? extends Animal> c7 = new ArrayList<Dog>();
Colletion<? extends Animal> c8 = new ArrayList<Cat>();

Colletion<? super Animal> c5 = new ArrayList<Object>();
Colletion<? super Animal> c6 = new ArrayList<Animal>();
Colletion<? super Animal> c7 = new ArrayList<Dog>();        //错误
Colletion<? super Animal> c8 = new ArrayList<Cat>();        //错误