[Java知识库]Java学习之运算符instanceof使用场景

Java学习之运算符instanceof使用场景

1.意义

instanceof是二元运算符，用于判断对象是否为特定类的实例。

2.用法

boolean result = object instanceof class

3.举例分析

class Uncle{}
class Pare{}
class Pare1 extends Pare{}
class Pare2 extends Pare1{}
class Pare3 {
  public static void main(String[] args){
   Uncle  u  = new Uncle();
   Pare   p  = new Pare();
   Pare1  p1 = new Pare1();
   Pare2  p2 = new Pare2();

//本类对象是本类的实例
if ( p instanceof Pare)
 {System.out.println("p instanceof Pare");}

//子类对象是父类的实例
 if (!( p1 instanceof Pare))
   {System.out.println("p1 not instanceof Pare");}
else
   {System.out.println("p1  instanceof Pare");}
 
//父类对象不是子类的实例
 if (p1 instanceof Pare2)
   {System.out.println("p1 instanceof Pare2");}
 else
   {System.out.println("p1 not instanceof Pare2");}

 /*if (p instanceof Uncle)
 {System.out.println("p instanceof Uncle");}
 else
 {System.out.println("p not instanceof Uncle");}*/
  if (null instanceof String)
   {System.out.println("null instanceof String");}  else
   {System.out.println("null not instanceof String");}

Shape s = new Rect(10,30);//父类声明引用子类实例，向上转型
s = new Triangle(5,5,8);
s = new Circle(12.5);

 java.lang.ClassCastException: Circle cannot be cast to Triangle
//Triangle triangle =(Triangle)s  //向下转型抛出运行时异常
//triangle.drawTri();
        
   if(s instanceof Rect){     //避免向下转型抛出运行时异常
        Rect rectangle = (Rect)s;
        rectangle.drawRect();
   } else if (s instanceof Triangle){
        Triangle triangle =(Triangle)s;
        triangle.drawTri();
   } else if (s instanceof Circle){
        Circle circle = (Circle) s;
        circle.drawCir();
   }