Make SubField instance variable private and final

- Fix spelling in Javadoc
- Add missing Javadoc
- Remove unused and untested APIs
- Use String.valueOf instead of throw away String
- Change new method names closer to standard Java API names
- Use JUnit 5, not 4 APIs
- Use compact array notation
- Use final
- Use Objects.requireNonNull()
- More precise exception testing
- Remove extra parentheses
- Remove dead comment
- Remove extra whitespace
- Sentences start with a capital

src/main/java/org/apache/commons/compress/compressors/gzip/GzipCompressorInputStream.java

@@ -248,9 +248,9 @@ private boolean init(final boolean isFirstMember) throws IOException {
         if ((flg & FEXTRA) != 0) {
             int xlen = inData.readUnsignedByte();
             xlen |= inData.readUnsignedByte() << 8;
-            byte[] extra = new byte[xlen];
+            final byte[] extra = new byte[xlen];
             inData.readFully(extra);
-            parameters.setExtra(Extra.fromBytes(extra));
+            parameters.setExtra(HeaderExtraField.fromBytes(extra));
         }
 
         // Original file name

src/main/java/org/apache/commons/compress/compressors/gzip/GzipCompressorOutputStream.java

@@ -173,7 +173,7 @@ private void write(final String value, final Charset charset) throws IOException
     private void writeHeader(final GzipParameters parameters) throws IOException {
         final String fileName = parameters.getFileName();
         final String comment = parameters.getComment();
-        final byte[] extra = parameters.getExtra() != null ? parameters.getExtra().toBytes() : null;
+        final byte[] extra = parameters.getExtra() != null ? parameters.getExtra().toByteArray() : null;
         final ByteBuffer buffer = ByteBuffer.allocate(10);
         buffer.order(ByteOrder.LITTLE_ENDIAN);
         buffer.putShort((short) GZIPInputStream.GZIP_MAGIC);
@@ -193,7 +193,7 @@ private void writeHeader(final GzipParameters parameters) throws IOException {
         out.write(buffer.array());
         if (extra != null) {
             out.write(extra.length & 0xff); // little endian
-            out.write((extra.length >>> 8) & 0xff);
+            out.write(extra.length >>> 8 & 0xff);
             out.write(extra);
         }
         write(fileName, parameters.getFileNameCharset());

src/main/java/org/apache/commons/compress/compressors/gzip/GzipParameters.java

@@ -290,7 +290,7 @@ public int type() {
      * </p>
      */
     private Instant modificationTime = Instant.EPOCH;
-    private Extra extra;
+    private HeaderExtraField extra;
     private String fileName;
     private Charset fileNameCharset = GzipUtils.GZIP_ENCODING;
     private String comment;
@@ -344,12 +344,12 @@ public int getDeflateStrategy() {
     }
 
     /**
-     * Gets the Extra.
+     * Gets the Extra subfields from the header.
      *
-     * @return the extra.
+     * @return the extra subfields from the header.
      * @since 1.28.0
      */
-    public Extra getExtra() {
+    public HeaderExtraField getExtra() {
         return extra;
     }
 
@@ -481,14 +481,13 @@ public void setDeflateStrategy(final int deflateStrategy) {
     }
 
     /**
-     * Sets the Extra subfields. Note that a non-null Extra will appear in the gzip
-     * header regardless of the presence of subfields, while a null Extra will not
+     * Sets the extra subfields. Note that a non-null extra will appear in the gzip header regardless of the presence of subfields, while a null extra will not
      * appear at all.
      *
-     * @param extra the collections of extra sub fields.
+     * @param extra the series of extra sub fields.
      * @since 1.28.0
      */
-    public void setExtra(Extra extra) {
+    public void setExtra(final HeaderExtraField extra) {
         this.extra = extra;
     }