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7.1 向上转型：多态
7.2 向下转型：
7.2.1 为什么使用向下转型：
有了对象的多态性以后，内存中实际上是加载了子类特有的属性和方法的，但是由于变量声明为父类类型，导致编译时，只能调用父类中声明的属性和方法。子类特有的属性和方法不能调用。如何才能
调用子类特有的属性和方法？使用向下转型。
7.2.2 如何实现向下转型：
使用强制类型转换符：()
7.2.3 使用时的注意点：
① 使用强转时，可能出现ClassCastException的异常。
②
为了避免在向下转型时出现ClassCastException的异常，我们在向下转型之前，先进行instanceof的判断，一旦返回true，就进行向下转型。如果返回false，不进行向下转型。
向下转型前提：1.子父类关系，父类向子类转。2. 父类是子类的实例。（父类的引用指向子类的对象）
//类man和Woman都是Person的子类 Person p3 = new Woman();//向上转型 p3是Woman的实例 Man m3 = (
Man)p3;//运行时ClassCastException的异常 Woman m3 = (Woman)p3;//向下转型通过，因为p3是Woman的实例
理解：虽然new Woman()在堆中加载了Woman类和Person类的成员，但是p3是Person类型，只能用来调用Person类的成员，若想要Woman
特有的成员，就要向下转型成Woman类型。 （在引用变量里：第三行代码出错也可以记为先有向上转型才能再向下转型。）
7.2.4 instanceof的使用：
① a instanceof A:判断对象a是否是类A的实例。如果是，返回true；如果不是，返回false。
② 如果类B是类A的父类 且a instanceof A返回true, 则 a instanceof B也返回true。
③ 要求a所属的类与类A必须是父类和子类的关系，否则编译错误。
