In the first case the arithmetic addition is done, and a corresponding Wrapper class is created on the fly and its toString() method is called,and the return String of this toString() method is printed.

In the second case the non-String object's toString() output and the String object are concatenated according to their places in the + operator and the resulted String is printed.

Also note that there is a String conversion for every object type in java including the famous ( ) true,false,null literals.

quote:

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Originally posted by raja pratap:
Which of the following are valid statements and why?
Can anyone help me out??

[1]System.out.println(true);
[2]System.out.println(null+null);
[3]System.out.println(true+null);
[4]System.out.println(true+8);

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I don't really consider this a very good question. I can say that because I'm the author of the question . I was orginally trying to test whether or not you knew that System.out.println(null) would work. The reason that I now consider this a poor question is because different versions of the JDK give different results.

When I originally tested this in VisualAge for Java (where I wrote my mock) the answers are as I indicated (the correct answers in VaJava were 1,2,3.) I tried to also test everything outside VaJava but apparently I missed this one.

I get interesting results depending on the version of the JDK I use (all of these on WinNT):

J D K 1.1.8 [correct answers would be 1]
NullTest.java:4: Reference to println is ambiguous. It is defined in void println (java.lang.String) and void println(char[]).
System.out.println(null);
^
NullTest.java:5: Incompatible type for +. Can't convert boolean to int.
System.out.println(true+null);
^
NullTest.java:5: Incompatible type for +. Can't convert null to int.
System.out.println(true+null);
^
NullTest.java:6: Incompatible type for +. Can't convert boolean to int.
System.out.println(true+8);

J D K 1.3 (build 1.3beta-O) [correct answers would be 1, 3]
NullTest.java:4: reference to println is ambiguous, both method println(char[])
in java.io.PrintStream and method println(java.lang.String) in java.io.PrintStre
am match
System.out.println(null);
^
NullTest.java:6: cannot resolve symbol
symbol : method + (boolean,int)
location: class NullTest
System.out.println(true+8);

J D K 1.2 ((Classic VM (build JDK-1.2-V, native threads)) [correct answers would be 1]
NullTest.java:4: Reference to println is ambiguous. It is defined in void printl
n(java.lang.String) and void println(char[]).
System.out.println(null);
^
NullTest.java:5: Incompatible type for +. Can't convert boolean to int.
System.out.println(true+null);
^
NullTest.java:5: Incompatible type for +. Can't convert null to int.
System.out.println(true+null);
^
NullTest.java:6: Incompatible type for +. Can't convert boolean to int.
System.out.println(true+8);

The above type of convertion to String for all objects takes place when they are used in the overloaded '+' operator. As I mentioned in previous post, when one of the operand of + operator is a String object, the other undergoes a string conversion and then added as stated in JLS. So

System.out.println("maha"+null); //prints mahanull
System.out.println("maha"+true); //prints mahatrue
System.out.println("maha"+true); //prints mahafalse

When you see the PrintStream class, there are so many overloaded println(..) methods. (Note that System.out is a PrintStream). Whenever you invoke a method on a object/class the more specific method is always called. Here there are 2 overloaded println(..) methods which take null as an argument and none of them are more specific than the other. They are

System.out.println(Char[] x);
and
System.out.println(String s);
There is another println(Object o) which also can take 'null' as an argument but this is elminated because the println(char[] c) and println(String s) are more specific than this.

When you call System.out.println(null) the compiler does not know which version of println(..) to call. So it says it is ambiguous.

If you would have been called as System.out.println((Object)null); //prints null
System.out.println((String)null); //prints null

Note: One funny thing is
System.out.println((char[])null); //prints NullPointerException
Because the authors who wrote this function forgot to handle the 'null' argument. So the JVM throws Runtime NullPointerException