Does the toString() method cannot return null?

[Kelvin Mak]

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
    }
}

compile ok, but it will throw a runtime exception. Does the toString() method cannot return null?

[Neil Laurance]

Try instead:

    System.out.println(""+t1); //1
    System.out.println(""+t2); //2

(removed incorrect interpretation of stack trace)
Cheers, Neil
[ November 14, 2002: Message edited by: Neil Laurance ]

[Ron Newman]

What do you mean by the "wrong" version of println() ?

[david eberhardt]

class Test001 {
    int i;
    public Test001(int i) { this.i = i; }
    public String toString() {
        [B]if(i == 0)
            return null;[/B]
        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
    }
}

compile ok, but it will throw a runtime exception. Does the toString() method cannot return null
.................................................
how about making one small change to your return of the null(where i==0), which would be consistent with how you wrote the return where i==2, in the IF decision block within your toString definition, as follows:

class Test001 {
      int i;
      public Test001(int i) {
        this.i = i;
      }
      public String toString() {
 if(i == 0)
            return "" + null; // the change
         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
}
  }  

[ November 13, 2002: Message edited by: david eberhardt ]

[Arpana Rai]

Kelvin,
String method can return null but the exception is thrown at line 1 where its trying to print null as print() method invoke toString() method of object class on its argument and invoking toString() method on null value gives NullPointerException.You can make the code run fine by making slight changes suggested by Neil and David.
Plz correct me if i am wrong.
regds
Arpana

[david eberhardt]

Originally posted by Arpana Rai:
Kelvin,
String method can return null but the exception is thrown at line 1 where its trying to print null as print() method invoke toString() method of object class on its argument and invoking toString() method on null value gives NullPointerException.You can make the code run fine by making slight changes suggested by Neil and David.
Plz correct me if i am wrong.
regds
Arpana

Yes, excellent description of what is happening there !!

[david eberhardt]

here's another idea on this topic:
I added another call to System.out.println at the end of the original code (which originally compiled but failed at runtime due to nullPointerException)

class Test002 {
int i;
public Test002(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);  // null
System.out.println(t2);  // 2
// what if we try
System.out.println(null);
}
}
/* result of running the above code

C:\BIN>javac Test002.java
Test002.java:22: reference to println is ambiguous, both
method println(char[])in java.io.PrintStream and method
println(java.lang.String) in java.io.PrintStream match
System.out.println(null);
                          ^
1 error
C:\BIN>
*/
  

This time it did not even compile!
It has no idea what needs to be printed because there is ... nothing to print.
[ November 13, 2002: Message edited by: david eberhardt ]

[david eberhardt]

take a look at PrintStream class :
public void print(String s)
Print a string. If the argument is null then the string "null" is printed. Otherwise, the string's characters are converted into bytes according to the
platform's default character encoding, and these bytes are written in exactly the manner of the write(int) method.
Parameters:
s - The String to be printed
public void print(Object obj)
Print an object. The string produced by the String.valueOf(Object) method is translated into bytes according to the platform's default
character encoding, and these bytes are written in exactly the manner of the write(int) method.
Parameters:
obj - The Object to be printed
See Also:
Object.toString()
.....

you might be able to accomplish your goal, IF you had been trying to print the value of i or null, instead of invoking the print (Object obj) method ...
... by passing t2 into the println method, you invoke the print(Object obj) method signature.

[Kelvin Mak]

I am clearly know this question, and appricated all the reply!!!

[Arpana Rai]

You are right David.There is one more thing.If u call print/println() method with no arguments i.e
System.out.print() it will give error at the time of compliling b'coz no-argument print() method is not defined in PrintStream class.
regds
Arpana

[david eberhardt]

 
public class Test003 {
int i;
String string;
public Test003(int i) {
this.i = i;
this.string = null;
}
public String toString() {
if(i == 0)
return "" + null;
else
return "" + i;
}
public static void main(String[ ] args) {
Test003 t1 = new Test003(0);
Test003 t2 = new Test003(2);
System.out.println(t1);  // null
System.out.println(t2);  // 2
// what if we try
System.out.println(); // this is ok, it causes a blank line to print
System.out.println(t1.string);
}
}
/* result of running the above code
C:\BIN>java Test003
null
2

null
C:\BIN>

*/

[ November 13, 2002: Message edited by: david eberhardt ]

[Arpana Rai]

you are right David, Thanx for the correction
regds
Arpana

[david eberhardt]

Originally posted by Arpana Rai:
you are right David, Thanx for the correction
regds
Arpana

Thanks Arpana ... this was a very stimulating topic as I looked thru alot of docs and tried compiling and running many variations.
Thanks to all!