https://bugs.openjdk.org/browse/JDK-8025883 reproducer

AutoFlushShouldNotFlushTooOften.java

package org.basilevs.example.flush;

import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Objects;

/// Demonstrates that PrintStream does not check for new line character despite documentation mentioning it
public class AutoFlushShouldNotFlushTooOften {
	private static final Charset CHARSET = StandardCharsets.UTF_8;
	private final static String TEST_DATA = "Text without EOL. It is not supposed to flush";
	private final static int BUFFER_SIZE = 10_000; // large enough to to fit test data
	
	public static void main(String[] args) throws IOException, InterruptedException {
		ByteArrayOutputStream bytes = new ByteArrayOutputStream();
		try (  PrintStream output = new PrintStream(
				new BufferedOutputStream(bytes, BUFFER_SIZE), true,  CHARSET)) {
			/// Documentation for autoFlush = true says:
			/// > The output buffer will be flushed whenever a byte array is written, one of the println methods is invoked, or a newline character or byte ('\n') is written
			/// This implies that newline character is significant.
			/// However, the flush happens always, even if data has no line breaks.
			/// This leads to a minor performance loss, as flush happens on every character, not on every line
			/// Arguably, autoFlush should be false for performance, but documentation is still misleading.
			/// # Entrypoints
			/// - java.io.PrintStream.implWrite(String) - does unconditional charOut.flushBuffer() before content check is done
			/// - it leads to java.io.PrintStream.implWrite(byte[], int, int) which has no content check
			/// - java.io.PrintStream.implWrite(String) then checks content for new line character, but flush is already done, so the check is redundant
			output.print(TEST_DATA); // PrintStream.print() is not expected to flush until a new line character
			Thread.sleep(1000);
			assertContent("", bytes); 
			
			
		}
		assertContent(TEST_DATA, bytes); // PrintStream.close() is expected to flush
	}

	static void assertContent(String expectedContent, ByteArrayOutputStream flushable) throws IOException {
		String flushedContent = new String(flushable.toByteArray(), CHARSET);
		if (!Objects.equals(expectedContent, flushedContent)) {
			throw new AssertionError("No content is supposed to be flushed, but received: " + flushedContent);
		}
	}
}