对象的转型

向上转型

将子类型对象转换成父类型，称之为向上转型。

java">class People {}
class Teacher extends People {}public class Main {public static void main(String args[]) {People teacher = new Teacher();System.out.println("0");}
}

这里将子类(Teacher)转换成了一个父类(People)，将一个具体类转换成了一个抽象类。需要说明的是，向上转型是安全的。也就是说父类可以引用子类。

java">class People {int c = 4;
}
class Teacher extends People {public int a = 3;
}public class Main {public static void main(String args[]) {People teacher = new Teacher();
//		 teacher.a = 3;	// 编译报错teacher.c = 7;System.out.println(teacher.c);}
}

可以看出，转成父类后，只能调用父类拥有的方法和变量。

向下转型

向下转型是不安全的，举个例子，人不一定是老师，老师一定是人。如果将随便一个人随便任命为老师，很可能会出错。

java">class People {
}
class Teacher extends People {
}public class Main {public static void main(String args[]) {Teacher tom = new People();}
}

显而易见的得到一个报错提示。但其实可以使用强制转型来向下转型，相当于告诉编译器：这个人就是个老师！

java">class Father {
}
class Child extends Father {
}public class Main {public static void main(String args[]) {var father = new Father();Child chlid = (Child) father;}
}

编译器不会有错误提示，但是运行后会得到报错。

Exception in thread "main" java.lang.ClassCastException: class Father cannot be cast to class Child (Father and Child are in unnamed module of loader 'app')at Main.main(Main.java:10)

注意：没有继承关系的对象之间不可以进行转型操作。