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円';
  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';
  }
  

• Robottinosino: Thanks very much for your comments. I really appreciated them. - '0' is not redundant. The method signatures were mandated by the exercise, and I "think" the instantiability is too, so I could not hide the ctor.