How to establish the codepoint of encoded characters?

Given a stream of bytes (that represent characters) and the encoding of the stream, how would I obtain the code points of the characters?

InputStreamReader r = new InputStreamReader(bla, Charset.forName("UTF-8"));
int whatIsThis = r.read();

What is returned by read() in the above snippet? Is it the unicode codepoint?

Solution

Reader.read() returns a value that can be cast to char or -1 if no more data is available.

A char is (implicitly) a 16-bit code unit in the UTF-16BE encoding. This encoding can represent basic multilingual plane characters with a single char. The supplementary range is represented using two-char sequences.

The Character type contains methods for translating UTF-16 code units to Unicode code points:

A code point that requires two chars will satisfy the isHighSurrogate and isLowSurrogate when you pass in two sequential values from a sequence. The codePointAt methods can be used to extract code points from code unit sequences. There are similar methods for working from code points to UTF-16 code units.

A sample implementation of a code point stream reader:

import java.io.*;
public class CodePointReader implements Closeable {
  private final Reader charSource;
  private int codeUnit;

  public CodePointReader(Reader charSource) throws IOException {
    this.charSource = charSource;
    codeUnit = charSource.read();
  }

  public boolean hasNext() { return codeUnit != -1; }

  public int nextCodePoint() throws IOException {
    try {
      char high = (char) codeUnit;
      if (Character.isHighSurrogate(high)) {
        int next = charSource.read();
        if (next == -1) { throw new IOException("malformed character"); }
        char low = (char) next;
        if(!Character.isLowSurrogate(low)) {
          throw new IOException("malformed sequence");
        }
        return Character.toCodePoint(high, low);
      } else {
        return codeUnit;
      }
    } finally {
      codeUnit = charSource.read();
    }
  }

  public void close() throws IOException { charSource.close(); }
}