b64-cheatsheet

/**
     * @param finPath  where to read binary data from
     * @param foutPath where to write encoded data. if null, please create and use temporary file.
     * @return file to read encoded data from
     * @throws IOException is case of input/output errors
     */
    @NotNull
    public File encodeFile(@NotNull String finPath, @Nullable String foutPath) throws IOException {
        final File fin = new File(finPath);
        final File fout;

        if (foutPath != null) {
            fout = new File(foutPath);
        } else {
            fout = File.createTempFile("based_file_", ".txt");
            fout.deleteOnExit();
        }

        try (
                final FileInputStream fis = new FileInputStream(fin);
                final FileOutputStream fos = new FileOutputStream(fout);
        ) {
            while(true) {
                int b1 = fis.read();
                int b2 = fis.read();
                int b3 = fis.read();

                if (b1 == -1) {
                    break;
                }
                fos.write(toBase64[b1 >> 2]);
                if (b2 != -1) {
                    fos.write(toBase64[((b1 & 3) << 4) | (b2 >> 4)]);
                } else {
                    fos.write(toBase64[((b1 & 3) << 4)]);
                    fos.write("==".getBytes());
                    break;
                }
                if (b3 != -1) {
                    fos.write(toBase64[((b2 & 15) << 2) | (b3 >> 6)]);
                } else {
                    fos.write(toBase64[((b2 & 15) << 2)]);
                    fos.write('=');
                    break;
                }
                fos.write(toBase64[b3 & 63]);
            }
        }

        return fout;
    }