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

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

Your observation that System.out.println() and System.err.println() calls are not printing to the console in the order you make them is a common issue.

In Java, System.out and System.err are two different streams that are used to write output to the console. System.out is used for standard output, while System.err is used for error messages. By default, these streams are flushed differently.

When you call System.out.println(), the output is added to the standard output buffer. However, when you call System.err.println(), the output is added to both the standard output buffer and the error output buffer. This is because System.err is intended for error messages, which should always be displayed, even if standard output is being redirected.

The contents of these buffers are flushed at different times. The standard output buffer is flushed whenever a line break is encountered, while the error output buffer is flushed when an error occurs or when the program exits.

As a result, if you alternate calls to System.out.println() and System.err.println() within a loop, the output will not appear in the order you expect. Instead, all the System.out.println() calls will be flushed together, followed by all the System.err.println() calls.

Solution

To fix this issue, you can force the buffers to be flushed immediately after each call to System.out.println() and System.err.println(). This can be done using the flush() method on the corresponding streams.

for (int i = 0; i < 5; i++) {
    System.out.println("out");
    System.out.flush(); // Flush the standard output buffer
    System.err.println("err");
    System.err.flush(); // Flush the error output buffer
}

By adding the flush() calls, you ensure that the output from each stream is sent to the console immediately, so it will appear in the correct order.

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