对象转型(casting)
- 一个基类的引用类型变量可以“指向”其子类的对象。
- 一个基类的引用不可以访问其子类对象新增加的成员(属性和方法)。
- 可以使用引用变量instanceof类名来判断该引用型变量所“指向”的对象是否属于该类或该子类的子类。
- 子类的对象可以当做基类的对象来使用称作向上转型(upcasting),反之称为向下转型(downcasting)
class Animal {
public String name;
Animal(String name) {
this.name = name;
}
}
class Cat extends Animal {
public String eyesColor;
Cat(String n, String c) {
super(n);
eyesColor = c;
}
}
class Dog extends Animal {
public String furColor;
Dog(String n, String c) {
super(n);
furColor = c;
}
}
public class Test {
public static void main(String args[]){
Animal a = new Animal("name");
Cat c = new Cat("catname", "blue");
Dog d = new Dog("dogname", "black");
System.out.println(a instanceof Animal);//true
System.out.println(c instanceof Animal);//true
System.out.println(d instanceof Animal);//true
System.out.println(a instanceof Cat);//false
a = new Dog("bigyellow", "yellow");
System.out.println(a.name);//bigyellow
System.out.println(a.furnmae);//error
System.out.println(a instanceof Animal);//true
System.out.println(a instanceof Dog);//true
Dog d1 = (Dog)a;
System.out.println(d1.furColor);//yellow
}
}
父类引用指向子类对象,只能访问父类所拥有的属性和方法。a指向的是dog对象,但是只能访问其父类(animal)对象的属性和方法。
执行a = new Dog("bigyellow", "yellow");
和Dog d1 = (Dog)a;
后的内存图。
public class Test {
public static void main(String args[]) {
Test test = new Test();
Animal a = new Animal("name");
Cat c = new Cat("catname", "blue");
Dog d = new Dog("dogname", "black");
test.f(a);
test.f(c);
test.f(d);
}
public void f(Animal a) {
System.out.println("name: " + a.name);
if(a instanceof Cat) {
Cat cat = (Cat)a;
System.out.println(" " + cat.eyesColor + " eyes");
} else if(a instanceof Dog) {
Dog dog = (Dog)a;
System.out.println(" " + dog.furColor + " fur");
}
}
}