反射中创建类的实例. 打印输出，为什么在Person类中定义的toString()方法就会报错？？

1. <p><p>public class Person {
   
2. private String name;
   
3. private int age;
   
4. public Person(){
   
5.   System.out.println("Person run");
   
6. }
   
7. public Person(String name,int age){
   
8.   this.name=name;
   
9.   this.age=age;
   
10. }</p><p>public String toString() {
    
11.   return "name:"+name.toString()+",age:"+age;
    
12. }
    
13. 
    
14. public void show(String name,int age){
    
15.   System.out.println("show run..name="+name+",age="+age);
    
16. }
    
17. 
    
18. public static void staticShow(){
    
19.   System.out.println("static show run");
    
20. }
    
21. }
    
22. </p></p>

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1. <p>
   
2. public class Reflect_GetClassDemo {
   
3. public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
   
4.   getClass_3();
   
5. }
   
6. public static void getClass_3() throws ClassNotFoundException, InstantiationException, IllegalAccessException {
   
7.   String className="review11Reflect.Person";
   
8.   Class clazz=Class.forName(className);
   
9.   System.out.println(clazz);  
   
10.   Object obj=clazz.newInstance();
    
11.   System.out.println(obj);  
    
12.   
    
13. }
    
14. 
    
15. 
    
16. 
    
17. }
    
18. 报错信息：</p><p>Exception in thread "main" java.lang.NullPointerException
    
19. at review11Reflect.Person.toString(Person.java:15)
    
20. at java.lang.String.valueOf(Unknown Source)
    
21. at java.io.PrintStream.println(Unknown Source)
    
22. at review11Reflect.Reflect_GetClassDemo.getClass_3(Reflect_GetClassDemo.java:28)
    
23. at review11Reflect.Reflect_GetClassDemo.main(Reflect_GetClassDemo.java:20)</p><p> </p>

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1. return "name:"+name.toString()+",age:"+age;

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你的Person类中，name并没有初始化，默认的是null，你覆写的toString中，调用的name的toString方法，所以会报空指针。

hejinzhong 发表于 2014-8-23 08:04
你的Person类中，name并没有初始化，默认的是null，你覆写的toString中，调用的name的toString方法，所以 ...

明白了，非常感谢