Specification for the method
java.util.Base64.getEncoder(int lineLength, byte[] lineSeparator)

says:

Parameters:
    lineLength - the length of each output line (rounded down to nearest multiple of 4). If lineLength <= 0 the output will not be separated in lines


However if a zero line length is specified encoding methods wrap() and encode(ByteBuffer src, ByteBuffer dst, int bytesOut) return encoded string which starts from the given line separator.

Please see the following example:


import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Base64;

import static java.nio.charset.StandardCharsets.*;

public class NoLineSeparators {

    public static void main(String[] args) throws IOException {

        final Base64.Encoder encoder = Base64.getEncoder(0, "$$$".getBytes(US_ASCII));

        final byte[] bytesIn = "fo".getBytes(US_ASCII);

        ByteArrayOutputStream encodingStream = new ByteArrayOutputStream();
        OutputStream encoding = encoder.wrap(encodingStream);
        encoding.write(bytesIn);
        encoding.close();

        final byte[] encodedBytes = encodingStream.toByteArray();

        System.err.println("result = " + new String(encodedBytes, US_ASCII));
    }

}

for b75 the output will be

result = $$$Zm8=


The following testcase will fail due to this issue:
api/java_util/Base64/index.html#GetEncoderMimeCustom[noLineSeparatorInEncodedString]