when toString() is implicitly called

[narendra bhattacharya]

Sir , i want to know when toString() method of String class is implicitly called.....

    

class Str
{
public static void main(String[ ] args)
{
String S1=null;
System.out.println(null+S1);
System.out.println(S1+null);

}
}

this gives output nullnullnullnull

[Afzal Rehman]

You override the toString method when you are more conern to read about your object of your class.
When you pass the Object reference in System.out.println then toString method is called and gives you appropriate result.
checkout toString method.

[Ankit Garg]

narendra were you expecting a NullPointerException?? Using + operator on null references produce "null" as output. From JLS

The string concatenation operator + (§15.18.1), which, when given a String operand and a reference, will convert the reference to a String by invoking the toString method of the referenced object (using "null" if either the reference or the result of toString is a null reference),...

[narendra bhattacharya]

but how like this null.toString();

[narendra bhattacharya]

narendra wrote:how it is possible to call null.toString() method.......

[Henry Wong]

narendra bhattacharya wrote:how it is possible to call null.toString() method.......

It not... what happens is that when you do object concatenation, it uses a string builder (or prior to java 5, a string buffer). The append() methods of the string builder will check to see if the object is null. And if it is, it will be replaced with a string with the value "null".

Henry

[narendra bhattacharya]

narendra wrote: thank you sir i am very greatful to you...

[Abimaran Kugathasan]

Henry Wong wrote:

narendra bhattacharya wrote:how it is possible to call null.toString() method.......

It not... what happens is that when you do object concatenation, it uses a string builder (or prior to java 5, a string buffer). The append() methods of the string builder will check to see if the object is null. And if it is, it will be replaced with a string with the value "null".

Henry

In this case we use String, not StringBuilder or StringBuffer! Then how the method append() will be invoked on 'null'?

Thanks in Advanced!

[Henry Wong]

Abimaran Kugathasan wrote:
In this case we use String, not StringBuilder or StringBuffer! Then how the method append() will be invoked on 'null'?

That is how it works. The compiler uses StringBuilder (or StringBuffer) to do string concats. Meaning this...

String result = string1 + string2 + string3;

will be replaced with...

String result = new StringBuilder().append(string1).append(string2).append(string3).toString();


Henry

[Prithvi Sehgal]

Great insight Henry. Didn't know it before.

Best Regards,

[narendra bhattacharya]

Narendra wrote: thank you sir you have cleared my doubt you are absolutely right ......sir java has many things to learn how you are able to memorise it....[ /quote]

[narendra bhattacharya]

thanks to Henry Wong sir he has certainly great knowledge in java ...

[Abimaran Kugathasan]

Thanks Henry Wong, for your information!