while loop with a readLine()

java while-loop bufferedreader readline inputstreamreader
Seba picture Seba · Mar 24, 2016 · Viewed 12.3k times · Source

Here is my question: normal while loop looks like this -

while(condition){statements}

and statements execute until condition becomes false and the loop is over. I'm interested in logic of this kind of statement: 

while((a=b.readLine) != null) { ... }

It's used in client - server communication in my case. The condition is sometimes true, sometimes isn't, but loop looks like it's testing the condition forever and when true, statements in {} execute. It looks to me that loop waits for the condition to be true and then runs statements. Is it somehow linked to the way BufferedReader and InputStreamReader work or what? Also, it seems this loop is never over, it just waits for a condition to be true and then runs statements, and then again waits for a condition to be true, etc. I would be thankful for any clarification.

Answer

Andrew Tobilko picture Andrew Tobilko · Mar 24, 2016

The following code snippet

String a;
while((a = b.readLine()) != null) { // a = b.readLine() -> a -> a != null
    ... 
}

is equivalent to 

String a = b.readLine();
while(a != null) {
    ...
    a = b.readLine();
}

but the first way is more simple, readable, and Java allows using assignment operators here because they return the value of a renewed variable.

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