What is the difference "" and null for string?

[Darren]

What should I put in the fllowing program?

"" or null

public static void main(String args[]) throws IOException{
BufferedReader stdin = new BufferedReader(
new InputStreamReader(System.in));
String line = stdin.readLine();
while(!(line.equals(""))){
System.out.println(line);
line = stdin.readLine();
}
}

Thanks in advance!

 

[Patricia Shanahan]

What should I put in the fllowing program?

"" or null

public static void main(String args[]) throws IOException{
BufferedReader stdin = new BufferedReader(
new InputStreamReader(System.in));
String line = stdin.readLine();
while(!(line.equals(""))){
System.out.println(line);
line = stdin.readLine();
}
}

Thanks in advance!

You need to test for null, because BufferedReader's readLine
returns "A String containing the contents of the line, not
including any line-termination characters, or null if the
end of the stream has been reached".

You may also need to make the println conditional on line
not being equal to "", if you don't want to print empty lines.

In general, line==null asks whether there is a line at all.
line.equals("") assumes there is a line, and asks whether it
contains any characters.

Patricia

 

[R.F. Pels]

What should I put in the fllowing program?

"" or null

The first is a string containing zero characters, and the second is a string
(or - more general, an object) that does not exist. The convention here is
that a function that returns a string either returns a string or - if there
is nothing available - null. The latter is equivalent to: sorry, caller,
there is nothing to give you.

So basically, this:

String foo = null;

is equivalent to saying:

foo does not exist.

while:

String bar = "";

means:

bar contains zero characters.