java对象转换的有关问题

package test;import java.io.*;public class SerializableTest {    public static void main(String[] args){                /*         *          * 序列化         */        FileOutputStream fos  = null;        ObjectOutputStream oos = null;        Person person = new Person("小明",32);        person.setSpouse("春花");                 try {                        fos = new FileOutputStream("d://text.person");            oos = new ObjectOutputStream(fos);            oos.writeObject(person);            oos.close();            fos.close();        } catch (FileNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }finally{                    }                /*         *          * 反序列化         */         Person p = null;          try {            FileInputStream fis = new FileInputStream("d://test.person");            ObjectInputStream ois = new ObjectInputStream(fis);            Object obj = ois.readObject();                        if(obj instanceof Person)                /*                 *  //这里的变量p为什么要在上面Person p = null定义，                 *  直接在这里Person p = (Person)obj要报错，是什么原因啊???                 */                 p  =  (Person)obj;                      } catch (FileNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (ClassNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }            }} /* * 可序列化的类 */class Person implements java.io.Serializable{            /**     *      */    private static final long serialVersionUID = 1L;        private String name;     private int age;    private String spouse;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }    public String getSpouse() {        return spouse;    }    public void setSpouse(String spouse) {        this.spouse = spouse;    }            public Person(){}        public Person(String name,int age){}}

import java.io.*;public class SerializableTest {    public static void main(String[] args){        /*        *        * 序列化        */        FileOutputStream fos  = null;        ObjectOutputStream oos = null;        Person person = new Person("小明",32);        person.setSpouse("春花");        try {            fos = new FileOutputStream("d://test.person");            oos = new ObjectOutputStream(fos);            oos.writeObject(person);            oos.close();            fos.close();        } catch (FileNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }finally{        }        /*        *        * 反序列化        */        Person p = null;        try {            FileInputStream fis = new FileInputStream("d://test.person");            ObjectInputStream ois = new ObjectInputStream(fis);            Object obj = ois.readObject();            if(obj instanceof Person)                /*                 *  //这里的变量p为什么要在上面Person p = null定义，                 *   Person p = null根本无所谓                 *  直接在这里Person p = (Person)obj要报错，是什么原因啊???                 *   直接这样转换也不会报错                 */                p  =  (Person)obj;                System.out.println(p);        } catch (FileNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        } catch (ClassNotFoundException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}/** 可序列化的类*/class Person implements java.io.Serializable{    /**     *     */    private static final long serialVersionUID = 1L;    private String name;    private int age;    private String spouse;    @Override    public String toString() {        return "Person{" +                "name='" + name + '\'' +                ", age=" + age +                ", spouse='" + spouse + '\'' +                '}';    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getAge() {        return age;    }    public void setAge(int age) {        this.age = age;    }    public String getSpouse() {        return spouse;    }    public void setSpouse(String spouse) {        this.spouse = spouse;    }    public Person(){}    public Person(String name,int age){        this.name=name;        this.age=age;    }}