why System.out.println(null+null) gives nullnull ?

[ananda s. roy]

while using

System.out.println(null);  

gives compile time error complaining about method ambiguity but

System.out.println(null+null)  

outputs nullnull.
I know that + operator converts the operands to string if any one of the operand is a string, but i am in the above case.
Thanks in advance.

[Swamy Nathan]

Both ur statements wont compile but this will

System.out.println(""+null+null);

I suspect it has something to do with StringBuffers and that appending a null gives a appending of a "null" text literal

[ananda s. roy]

i have compiled the

 System.out.println(nul+null) ; 

with jdk 1.3._01 version without error giving nullnull as output. :roll:

[Jeroen Wenting]

null objects are treated as a String "null" by output mechanisms producing Strings.
null + null will therefore generate "null" + "null" = "nullnull".

[Sanyev Babu]

well for me null+null is giving compile time error.

The operator + is undefined for the argument type(s) null, null

[Swamy Nathan]

I dont have JDK1.3 but I checked up with 1.4. It does not compile.
If there is indeed a discrepancy the importrant question is what is the correct behaviour from certification point of view?
Ananda did you type in nul+null or null+null?

Jeroen Wenting's theory does not feel right. Any output from System.out should only happen after null+null (which does not compile). But one never knows. Did it compile for anyone else?
[ May 20, 2004: Message edited by: Swamy Nathan ]

[ananda s. roy]

 System.out.println(null+null); 

compiles & run without error or warning both on jdk 1.3.x and 1.4.x. .
still

[Ashok Mash]

Yup, System.out.println(null+null) prints "nullnull" in jdk 1.3.1_06 for me. I don't know why though! Must be one of those things they fixed later!

[Barry Gaunt]

The following code prints nullnull:

String s1 = null;
String s2 = s1 + s1;
System.out.println(s2);

If you read the two snippets of code from the j2sdk1.4.2_04 source files, then you will get a better idea what happens. What will be executed is something very like:

String s2 = (new StringBuffer()).append(s1).append(s1).toString();

From String.java:

public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}

From StringBuffer.java:

    public synchronized StringBuffer append(String str) {
if (str == null) {
    str = String.valueOf(str);
}
 
int len = str.length();
int newcount = count + len;
if (newcount > value.length)
    expandCapacity(newcount);
str.getChars(0, len, value, count);
count = newcount;
return this;
    }
 

[Anonymous]

hello, whit a string that work but whit a primitive doesnt,if we try this, we get a complier error:
System.out.println(null+ null+1);


Congratulation Barry in you post number 2500!

[Barry Gaunt]

The "overloaded" string concatentation operator is defined to work when one operand is a reference to a String and the other is either a primitive, a reference to a String, or a reference to an object. The object reference can be null, in which case the string "null" will be used.

So "" + null, and null + "" are both OK, and result in a String "null". null + null should not work because not one of the operands is a reference to a String. The compiler can now detect this case and gives a compilation error.