Why Doesn't System.out.println() and System.err.println() Output in the Order They Are Called?

System.out.println and System.err.println Output Order

When utilizing System.out.println() and System.err.println(), it's crucial to note that they are separate output streams that may behave unpredictably. As you pointed out, output from these streams may not be printed in the sequence invoked.

To understand why this occurs, consider that these streams employ caching mechanisms. When writing to these streams, data is temporarily stored in a buffer. Flushing the buffer triggers the actual output to the console. However, the timing of flushes differs between System.out and System.err.

Typically, output streams are flushed periodically or after a period of inactivity. This behavior can lead to the observed output order where System.out statements appear consecutively, followed by System.err statements in a similar manner.

To address this issue, you can explicitly flush the output buffers using System.out.flush() and System.err.flush() within the loop. This ensures that each write triggers an immediate flush, yielding the desired alternating output of "out" and "err" as intended.

Comprehending stream caching and flushing is essential to manage output efficiently in Java programs, avoiding unexpected behaviors and ensuring reliable console output.

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