下塑造型

关于下塑造型，以前一直没有弄明白，因为老是出抛出“Exception in thread "main" java.lang.ClassCastException:”。然后看书发现，原来用下逆告型之前需要用instanceof进行检测父类对像类型是否为子类类型。如果是，才可以进行强制转换。如：

//super Class
public class Person {
public void print() {
out.println("this is the person");
}

}


public class Student extends Person {

public void print() {
out.println("this is the student");
}

public void printHello() {

out.println("say hello from student.");
}
}

然后在main 方法里面这样用：
public static void main(String args[]) {  Student s = new Student();//上朔造型    Student sss=(Student)(new Person());    sss.print();            }

这样肯定会报错，那么我们要如下进行编程：
public static void main(String args[]) {
Person p = new Student();//上朔造型  
if(p instance of Student){
   Student sss=(Student)(new Person());//下塑造型  
   sss.print();
}   


  只能上塑造型这样才会打印出“this is the student”想一想，用到下塑造型，必须先上塑造型才能下塑造型，那样又有什么意义呢？显然下塑造型不是用在这种地方。那么用在哪里呢？在一声明了一个父类类型数组，且里面存放着父类的不同子类对像的时候，就需要用到下塑造型了。如下例子:

//super Class
public class Person {
public void print() {
out.println("this is the person");
}

}


public class Student extends Person {

public void print() {
out.println("this is the student");
}

public void printHello() {

out.println("say hello from student.");
}
}


public class Farmer extends Person {

public void print() {

  System.out.println("the is the farmer.");
}

public void printHello() {

  out.println("say hello from farmer.");
}
}


public class Test {
public static void main(String args[]) {
  Student s = new Student();
  Farmer f = new Farmer();
  Teacher t = new Teacher();

  Person persons[] = new Person[3];

  persons[0] = s;
  persons[1] = f;
  persons[2] = t;

  Student sss = (Student) (new Person());

  for (int i = 0; i < 3; i++) {

   Person p = persons[i];
   if (p instanceof Student) {
    Student s1 = (Student) p;
    s1.print();
    s1.printHello();
   } else if (p instanceof Teacher) {
    Teacher s1 = (Teacher) p;
    s1.print();
    s1.printHello();
   } else if (p instanceof Farmer) {
    Farmer s1 = (Farmer) p;
    s1.print();
    s1.printHello();
   }

  }

}
}



这样就可以一一分流出父类数组中的一个个子类对像,并且调用只属天它们自己的方法"printHello()"，从而达到了下塑造型的目的