flush()

When exactly should I use the .flush() function?
I read about this function, but do I also need it when you read data and found a hit and you write away that line to an output file?

The flush function of what class? What does the API say?

This is from the doc:
Flushes this output stream and forces any buffered output bytes to be written out.

So I guess it is better for the computer then to work faster, and to use less memory?

I'd recommend a google search of something like "java flush". There are many explanations that are better than any one person can explain to you. Here's the first result: When to use flush() in Java? (Java in General forum at JavaRanch)

For bonus points, report back here with what you find.