多态02 练习题

多态-练习题

1、判断正误

	double d = 13.4; 
	longl = (long)d; 
	System.out.println(l); 
	
	int in = 5;
	boolean b =(boolean)in; 
	
	Object obj="Hello";
	String objStr = (String)obj;
	System.out.println(objstr);
	
	Object objPri = new Integer(5);
	String str = (String)objPri;
	lnteger str1 = (lnteger)objPri;

double d = 13.4; //ok
longl = (long)d; //ok
System.out.println(l); //13

int in = 5;//ok
boolean b =(boolean)in; //不对，boolean类型不参与强制类型转换

Object obj=“Hello”;//可以，向上转型
String objStr = (String)obj;//可以，向下转型
System.out.println(objstr); //hello

Object objPri = new Integer(5);//可以，向上转型
String str = (String)objPri;//错误ClassCastException，指向Integer的父类引用，转成String
lnteger str1 = (lnteger)objPri;//可以，向下转型

2、分析输出

	class Base {
		int count = 10;
		public void display(){
			System.out.println(this.count);
		}
	}
	class Sub extends Base {
	    int count = 20;
		public void display(){
			System.out.println(this.count);
		}
	}

	public class Test02 {
		public static void main(String[] args) {
			Sub s = new Sub();
			System.out.println(s.count);
			s.display();
			Base b = s;
			System.out.println(b == s);
			System.out.println(b.count);
			b.display();
		}
	}

我的分析out:
20
20
false? × 这里是true
10
20
属性看编译类型，方法看运行类型