public class Box<T> {
private T data;
public void setData(T data) {
this.data=data;
}
public T getData() {
return data;
}
}
public class Test {
public static void main(String[] args) {
Box<Integer> box_integer = new Box<>();
box_integer.setData(5);
Box<BigDecimal> box_bigDecimal = new Box<>();
box_bigDecimal.setData(new BigDecimal(5));
Box<? extends Number> box=box_integer;
box=box_bigDecimal;
// box.setData(6); //error
// box.setData(new BigDecimal(6));//error
Number number=box.getData();
}
}
? extends Number表示可接收的实参类型为Number和Number的子类,有上界
例如:Box<? extends Number>可接收Box<Integer>和Box<BigDecimal>,具体哪个子类未知,所以不能调用set方法给泛型变量赋值
public class Test {
public static void main(String[] args) {
Box<Number> box_number = new Box<>();
box_number.setData(5);
Box<Object> box_Object = new Box<>();
box_Object.setData(new Object());
Box<? super Integer> box=box_number;
box=box_Object;
box.setData(6);
box.setData(new Object());//error
}
}
? super Number表示可接收的实参类型为Number和Number的父类,有下界例如:Box<? super Integer>可接收Box<Number>和Box<Object>,
具体哪个父类未知,所以可以调用set方法给泛型变量赋值,实参只能是Integer和Integer的子类,不能调用get方法(除非用Object接收)