对象转型

public class Exe1{public static void main(String[] args){    Person p=new Person(1);    Student s1=new Student(12,"john");    p=new Student(12,"nihao");    Student s2=(Student)p;//想调用student新增的属性，强制转换}}class Person{    private int age;    private String name;    Person(int age)    {         this.age=age;         name="merry";     }     Person(int age,String name)     {          this.age=age;          this.name=name;     }}class Student extends Person{      private String school;      Student(int a,String n,String school)      {          super(a,n);          this.school=school;      }      Student(int a,String n)     {          this(a,n,"qinghua");      }}```![这里写图片描述](http://img.blog.csdn.net/20170913084232387?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvcXhobHk=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast)<div class="se-preview-section-delimiter"></div>

public class Exe1{

public static void main(String[] args)
{
Animal a=new Animal(“smallwhite”);
Cat cat=new Cat(“smallwhite”,”red”);
System.out.print(a instanceof Animal);
a=new Cat(“smallblack”,”blue”);
// System.out.println(a.eyeColor);err
System.out.println(a.name);
Cat c=(Cat)a;
System.out.println(c.eyeColor);
Exe1 exe=new Exe1();
exe.f(a);
exe.f(cat);//基类对象指向子类对象

}
public void f(Animal a)
{
System.out.println(“name: “+a.name);
}

}

class Animal{

public String name;
Animal(String name)
{
this.name=name;
}
}

class Cat extends Animal{

public String eyeColor;
Cat(String n,String e)
{
super(n);
eyeColor=e;
}
}
“`

一个基类的引用类型变量可以“指向”其子类的对象如： p=new Student(12,”nihao”);

一个基类的引用不可以访问其子类对象新增加的成员（属性和方法）

子类的对象可以当做基类的对象来使用称为向上转型（upcasting）,反之称为向下转型

public class Exe1{ public static void main(String[] args) {     Animal a=new Animal("smallwhite");     Cat cat=new Cat("smallwhite","red");     System.out.print(a instanceof Animal);     a=new Cat("smallblack","blue");    // System.out.println(a.eyeColor);err      System.out.println(a.name);       Cat c=(Cat)a;      System.out.println(c.eyeColor);      Exe1 exe=new Exe1();      exe.f(a);      exe.f(cat);//基类对象指向子类对象 }    public void f(Animal a)    {         System.out.println("name: "+a.name);      }}class Animal{  public String name;  Animal(String name)  {    this.name=name;  }}class Cat extends Animal{   public String eyeColor;   Cat(String n,String e)   {      super(n);      eyeColor=e;   }}

一个基类的引用类型变量可以“指向”其子类的对象如： p=new Student(12,”nihao”);

一个基类的引用不可以访问其子类对象新增加的成员（属性和方法）,也即其指向的是内存中子类中的父类

子类的对象可以当做基类的对象来使用称为向上转型（upcasting）,反之称为向下转型