Null Values..

[Andres Gonzalez]

Consider the following lines of code...
System.out.println(null + true) // 1
System.out.println(true + null) // 2
System.out.println(null + null) // 3
Which of the following statements are correct?
a) None of the 3 lines will compile
b) All the 3 line will compile and print nulltrue, truenull and nullnull respectively
c) line 1 and 2 won't compile but line 3 will print nullnull
d) line 3 won't compile but line 1 and 2 will print nulltrue and truenull respectively
e) none of the above.
And the answer is.... hmmm... don't know!
The answer said option "A" was correct, but I run this code and get option "B".
the explanation for this answer (option A) is:

... None of the parameters is a String so conversion to String will not happen. It will try to convert everything to an int and you will get:
Incompatible type for +. Can't convert null to int.

But my compiler didn't complain at all!

P.S. Taken from JQPlus.
[ July 07, 2003: Message edited by: Andres Gonzalez ]
[ July 07, 2003: Message edited by: Andres Gonzalez ]

[Vicky Nag]

I compiled the above using jdk1.2.2 on solaris and got the following error
T1.java:5: Reference to println is ambiguous. It is defined in void println(java.lang.String) and void println(char[]).
System.out.println(null);
^
T1.java:6: Incompatible type for +. Can't convert null to int.
System.out.println(null + true);
.....
..
Which compiler are u using??
Regards
Vicky

[Andres Gonzalez]

ooppss... Sorry Vicky, I have edited the post, changing line 2. Now it compiles.

[Andres Gonzalez]

One more thing:

public class B
{
   public static void main(String[] args)
   {
     System.out.println(null + true);
     System.out.println(true + null);
     System.out.println(null + null);
   }
}

This code compiles ok. But this one

public class B
{
   public static void main(String[] args)
   {
     System.out.println(null + true);
     System.out.println(true + true); //Compiler error!!
     System.out.println(null + null);
   }
}

hmmm...
[ July 07, 2003: Message edited by: Andres Gonzalez ]

[Paul Anilprem]

Check this out

[Andres Gonzalez]

+ operator is overloaded only for Strings and at least one operands of + should be a String. But expressions like true+null, null+true etc. compile and run fine.

hmm... should be a String? or should be able to implicitly promote to a String.
none of the operands in true+null are strings, but I guess the compiler promotes null to a String, concatenating these 2 values.
Is that the reason "true + true" does not compile?

[Darren McLeod]

My compiler(JDK 1.1.3 on SunOS 5.6) doesn't accept any of them:

public class mytest                         
{                                           
  public static void main(String args[])    
  {                                         
    System.out.println(null + true);        
    System.out.println(true + null);        
    System.out.println(null + null);        
  }                                         
}                                           
javac mytest.java   
mytest.java:5: Incompatible type for +. Can't convert null to int.
    System.out.println(null + true);
                            ^ 
mytest.java:5: Incompatible type for +. Can't convert boolean to int.
    System.out.println(null + true);
                            ^               
mytest.java:6: Incompatible type for +. Can't convert boolean to int.
    System.out.println(true + null);
                            ^               
mytest.java:6: Incompatible type for +. Can't convert null to int.
    System.out.println(true + null);
                            ^                 
mytest.java:7: Incompatible type for +. Can't convert null to int.
    System.out.println(null + null);
                            ^               
5 errors

[Andres Gonzalez]

I think you're getting that because you're using an old version of JDK.