Decoding binary string message

Question

I am trying to learn some Java on my own, and I am tackling some "programming challenge" problems to practice the little I have learnt so far.

Could anybody constructively critique this beginner's effort (source on Bitbucket)?

public class BinaryCode {
  public String[] decode(String message) {
    int messageLength = message.length();
    return new String[] { decodeMessage(message, messageLength, 0),
        decodeMessage(message, messageLength, 1) };
  }
  private String decodeMessage(String encodedMessage, int messageLength,
      int assumedFirstDigit) {
    StringBuilder decodedMessage = new StringBuilder(messageLength);
    int nextDecodedDigit = assumedFirstDigit;
    for (int i = 0; i < messageLength; i++) {
      if (nextDecodedDigit != 0 && nextDecodedDigit != 1) {
        return "NONE";
      }
      decodedMessage.append(nextDecodedDigit);
      int encodedDigit = encodedMessage.charAt(i) - '0';
      int decodedDigit = decodedMessage.charAt(i) - '0';
      nextDecodedDigit = encodedDigit - decodedDigit;
      if (i > 0) {
        nextDecodedDigit -= decodedMessage.charAt(i - 1) - '0';
      }
    }
    int checkValue = decodedMessage.charAt(messageLength - 1) - '0';
    if (messageLength > 1) {
      checkValue += decodedMessage.charAt(messageLength - 2) - '0';
    }
    if (checkValue == encodedMessage.charAt(messageLength - 1) - '0') {
      return decodedMessage.toString();
    }
    return "NONE";
  }
}

Answer 1

These critiques are mostly style and syntax related, and do not address your algorithm (you could probably find the simplest possible algorithm with Google anyway).

• make decode and decodeMessage static

• explicitly hide the constructor for BinaryCode, e.g.

  // hide constructor
  private BinaryCode() {}
  

• remove unneseccary - 0 from encodedDigit and decodedDigit declarations
• let decodeMessage calculate messageLength (fewer parameters are nearly always better)
• avoid using magic numbers and string literals (-1, -2, "NONE", etc...)

• create method lastChar and call it instead of messageLength - 1 e.g. int checkValue = lastChar(decodedMessage) - '0';

  private static char lastChar(CharSequence sequence) {
      int lastIndex = sequence.length() - 1;
      return sequence.charAt(lastIndex);
  }
  

• replace decodedMessage.charAt(messageLength - 2) and decodedMessage.charAt(i - 1) with previouslyDecodedChar e.g.

  char previouslyDecodedChar = '\0';
  for (int i = 0; i < messageLength; i++) {
      // ...
      if (i > 0) {
          nextDecodedDigit -= previouslyDecodedChar - '0';
      }
      previouslyDecodedChar = decodedMessage.charAt(i);
  }
  int checkValue = lastChar(decodedMessage) - '0';
  if (messageLength > 1) {
      checkValue += previouslyDecodedChar - '0';
  }