Java基础 5.27

多态注意事项和细节讨论

• 属性没有重写之说 属性的值看编译类型

package com.logic.poly_.detail_;public class PolyDetail02 {public static void main(String[] args) {Base base = new Sub();System.out.println(base.count);Sub sub = new Sub();System.out.println(sub.count);}
}class Base {int count = 10;
}class Sub extends Base {int count = 20;
}

• instanceOf比较操作符 用于判断对象的运行类型是否为XX类型或XX类型的子类型 

package com.logic.poly_.detail_;public class PolyDetail03 {public static void main(String[] args) {//编译类型是BB 运行类型是BBBB bb = new BB();System.out.println(bb instanceof BB);System.out.println(bb instanceof AA);//编译类型是AA 运行类型是BBAA aa = new BB();System.out.println(aa instanceof AA);System.out.println(aa instanceof  BB);Object obj = new Object();System.out.println(obj instanceof AA);String str = "logic";System.out.println(str instanceof Object);}
}class AA {} //父类
class BB extends AA{} //子类

多态练习

package com.logic.poly_.detail_;public class PolyPractice {public static void main(String[] args) {double d = 13.4;//oklong i = (long)d;//okSystem.out.println(i);//13int in = 5;//okboolean b = (boolean)in;//不对 boolean -> intObject obj = "hello";//ok 向上转型String objStr = (String) obj;//ok 向下转型System.out.println(objStr);//helloObject objPri = new Integer(5);//ok 向上转型String str = (String) objPri;//不对 ClassCastException//指向Integer的父类引用 转成StringInteger str1 = (Integer) objPri;//ok 向下转型}
}

package com.logic.poly_.detail_;public class PolyPractice02 {public static void main(String[] args) {B b = new B();System.out.println(b.count);//20b.display();//20A a = b;System.out.println(a == b);//trueSystem.out.println(a.count);//10a.display();//20}
}
class A {public int count = 10;public void display() {System.out.println(this.count);}
}
class B extends A {public int count = 20;public void display() {System.out.println(this.count);}
}