Jiris Mock 1, Q1

[Cathy Song]

  
    class Test001
   {
      int i;
   
       public Test001(int i) { this.i = i; }
   
       public String toString()
      {
         if(i == 0) 
            return null;
         else 
            return "" + i;
      }
   
       public static void main(String[ ] args)
      {
         Test001 t1 = new Test001(0);
         Test001 t2 = new Test001(2); 
         System.out.println(t1);
         System.out.println(t2);
      }
   }

Why does this code throw an exception at runtime?

[Thomas Paul]

Originally posted by Cathy Song:
Why does this code throw an exception at runtime?

Ultimately this null String gets passed into the write method of Writer which assumes that the String is not null. It runs the getChars method of the String which is where the exception occurs. Again, this is another case where reading through the source and following the stack trace can help you understand the API better.

[Mona Gadkari]

Originally posted by Cathy Song:

  
    class Test001
   {
      int i;
   
       public Test001(int i) { this.i = i; }
   
       public String toString()
      {
         if(i == 0) 
            return null;
         else 
            return "" + i;
      }
   
       public static void main(String[ ] args)
      {
         Test001 t1 = new Test001(0);
         Test001 t2 = new Test001(2); 
         System.out.println(t1);
         System.out.println(t2);
      }
   }

Why does this code throw an exception at runtime?

hello,
I suppose that the return type String is expected and null is returned if we return "null" it works - no runtime exception is thrown.
just make few changes in toString method and it works....
public String toString(){
if(i==0)
return "null";
else
return ""+i;
}
pls correct me am wrong....

[Lakshmi Saradha]

The following code did not throw any exception for me and the output was 2 and null.
I compiled with jdk1.4
class Class2
{
int i;
public Class2(int i) { this.i = i; }
public String toString()
{
if(i == 0) return null;
else return ""+i;
}
public static void main(String[ ] args)
{
Class2 t1 = new Class2(0);
Class2 t2 = new Class2(2);
System.out.println(t2);
System.out.println(""+t1);
}
}
Any thoughts?

[Vad Fogel]

This is a known JVM bug ID=4339222 (Synopsis: System.out.println throws NullPointerException when toString() returns null).
Workaround: Subclass and override toString, avoid to System.out.println.
Login to Bug Database for a full description.

[PJ Pillai]

Hi,
Following expression works as well.
public String toString()
{
if(i == 0)
return "" + null;
else
return "" + i;
}

Thanks,
PJ

[dennis zined]

class Test001   {
    int i;
    public Test001(int i) {
        this.i = i;
    }
    public String toString(){
        if(i == 0)
            return null;
        else
            return "" + i;
    }
    public static void main(String[ ] args){
        Test001 t1 = new Test001(0);
        Test001 t2 = new Test001(2);
        System.out.println(t1.toString());
        System.out.println(t2);
    }
}

But explicitly calling "toString()" method works (jdk1.4). Wierd.