An encoder encodes the integers that are passed to {@link org.apache.lucene.util.encoding.IntEncoder#encode(int) encode} into a set output stream (see {@link org.apache.lucene.util.encoding.IntEncoder#reInit(OutputStream) reInit}). One should always call {@link org.apache.lucene.util.encoding.IntEncoder#close() close} when all integers have been encoded, to ensure proper finish by the encoder. Some encoders buffer values in-memory and encode in batches in order to optimize the encoding, and not closing them may result in loss of information or corrupt stream.

A proper and typical usage of an encoder looks like this:

int[] data = <the values to encode>
IntEncoder encoder = new VInt8IntEncoder();
OutputStream out = new ByteArrayOutputStream();
encoder.reInit(out);
for (int val : data) {
  encoder.encode(val);
}
encoder.close();

// Print the bytes in binary
byte[] bytes = out.toByteArray();
for (byte b : bytes) {
  System.out.println(Integer.toBinaryString(b));
}

IntDecoder d = e.createMatchingDecoder();
d.reInit(new ByteArrayInputStream(bytes));
long val;
while ((val = d.decode()) != IntDecoder.EOS) {
  System.out.println(val);
}

Some encoders don't perform any encoding at all, or do not include an encoding logic. Those are called {@link org.apache.lucene.util.encoding.IntEncoderFilter}s. A filter is an encoder which delegates the encoding task to a given encoder, however performs additional logic before the values are sent for encoding. An example is {@link org.apache.lucene.util.encoding.DGapIntEncoder} which encodes the gaps between values rather than the values themselves. Another example is {@link org.apache.lucene.util.encoding.SortingIntEncoder} which sorts all the values in ascending order before they are sent for encoding. This encoder aggregates the values in its {@link org.apache.lucene.util.encoding.IntEncoder#encode(int) encode} implementation and decoding only happens upon calling {@link org.apache.lucene.util.encoding.IntEncoder#close() close}.

Extending IntEncoder

public class TaggingIntEncoder extends IntEncoderFilter {
  
  public TaggingIntEncoder(IntEncoder encoder) {
    super(encoder);
  }
  
  @Override
  public void encode(int value) throws IOException {
    encoder.encode(value);
  }

  @Override
  public IntDecoder createMatchingDecoder() {
    return new TaggingIntDecoder();
  }
	
  @Override
  public void reInit(OutputStream out) {
    super.reInit(os);
    // Assumes the application has a static EncodersMap class which is able to 
    // return a unique ID for a given encoder.
    int encoderID = EncodersMap.getID(encoder);
    this.out.write(encoderID);
  }

  @Override
  public String toString() {
    return "Tagging (" + encoder.toString() + ")";
  }

}

public class TaggingIntDecoder extends IntDecoder {
  
  // Will be initialized upon calling reInit.
  private IntDecoder decoder;
  
  @Override
  public void reInit(InputStream in) {
    super.reInit(in);
    
    // Read the ID of the encoder that tagged this stream.
    int encoderID = in.read();
    
    // Assumes EncodersMap can return the proper IntEncoder given the ID.
    decoder = EncodersMap.getEncoder(encoderID).createMatchingDecoder();
  }
	
  @Override
  public long decode() throws IOException {
    return decoder.decode();
  }

  @Override
  public String toString() {
    return "Tagging (" + decoder == null ? "none" : decoder.toString() + ")";
  }

}