Null Pointer Exception in Java

[Thomas Markl]

Why does this code throw a Nullpointer Exception?
Source: http://www.geocities.com/sun_guoqiao/scjp/mockexam1.html

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);  //1
          System.out.println(t2);  //2
       }
    }

: It will throw the following exception at runtime at //1.
java.lang.NullPointerException
at java.io.Writer.write(Writer.java:129)
at java.io.PrintStream.write(PrintStream.java:267)
at java.io.PrintStream.print(PrintStream.java:426)
at java.io.PrintStream.println(PrintStream.java:563)
at Test001.main(Test001.java:18)
But it is strange that
System.out.println(t1.toString());
will run properly with output null.
Thomas

[This message has been edited by Cindy Glass (edited December 18, 2001).]

[BJ Grau]

You are passing null to println(Object), so somewhere along the chain of method calls, a method is being called on null, which throws the NullPointerException.

[jason adam]

The error seems to deal with the length() method in String. When Writer.write(String) is called, it calls an overloaded write(str, 0, str.length()); method. If you write test code like this:

<font size= +0>public class TestNull
{
public static void main( String[] args )
{
String s = null ;
int i = s.length() ;
System.out.println( s );
System.out.println( i );
}
}

You'll get the same type of error. Apparently, null Strings can not have a set length, so when a method calls for s.length(), you get this exception.
Jason
[This message has been edited by jason adam (edited September 04, 2001).]

[BJ Grau]

Yes, you are right. Also don't forget the part where you can't call a method on null. Thats what your doing when you try s.length() when String s = null.

[John Dale]

According to Java Class Libraries, Second Edition, Volume 1, toString() must return a non-null string. (page 1224) Perhaps the compiler is generating code that calls toString(), knowing that toString cannot return null. (Checking that out would take a bit more work than I'm willing to do tonight.)

[Jose Botella]

Hey you are right.
This program shows the difference

class Test001 {
public String toString() { return null;}
public static void main(String[ ] args) {
Test001 t1 = new Test001();
System.out.println(t1.toString());//1
System.out.println(t1);//NullPointerException
}
}

This the output from javap for the line //1
15 invokevirtual #6 (Method void println(java.lang.String))
and this for the other
22 invokevirtual #7 (Method void println(java.lang.Object))
println(String) was written for returning "null" whenever the argument was null.
But println(Object) was not written for providing a "null" String to the write method when the method toString() of the Object argument returned null.

[R K Singh]

Originally posted by Jose Botella:

But println(Object) was not written for providing a "null" String to the write method when the method toString() of the Object argument returned null.

Hi
I am pasting source code for println(Object obj):
from System.java :
public final static PrintStream out = nullPrintStream();
--------------------------------------------------------------
from PrintStream.java :
public void println(Object x) {
synchronized (this) {
print(x);
newLine();
}
public void print(Object obj) {
write(String.valueOf(obj));
}
-----------------------------------------------------------------
from String.java :
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}

Now will any one please tell me why NullPointerException should be thrown?
Thanks in adv

------------------
Regards
Ravish