What is the purpose of flush() in Java streams?

From the docs of the flush method:

Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.

The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.

When you write data to a stream, it is not written immediately, and it is buffered. So use flush() when you need to be sure that all your data from buffer is written.

We need to be sure that all the writes are completed before we close the stream, and that is why flush() is called in file/buffered writer's close().

But if you have a requirement that all your writes be saved anytime before you close the stream, use flush().

When we give any command, the streams of that command are stored in the memory location called buffer(a temporary memory location) in our computer. When all the temporary memory location is full then we use flush(), which flushes all the streams of data and executes them completely and gives a new space to new streams in buffer temporary location. -Hope you will understand