Question

I’ve created this simple compression class for a client server TCP data connection and it all looks fine to me with no build errors however I am getting a run time error that I cannot correct. The error I am getting is Exception in thread “main” java.lang.StringIndexOutOfBoundsException: String index out of range: -1.

Code:

import java.io.Serializable;
import java.util.ArrayList;

public class CompressedMessage implements Serializable
{   // this instance variable will store the original, compressed and decompressed message
    private String message;

    public CompressedMessage(String message)
    {   
        // begin by coding this method first - initialise instance variable message with the original message
        this.message = message;
    }

    public String getMessage()
    {   
        return this.message;

    }

    private boolean punctuationChar(String str)
    {   
        // Hint: check if the last character in the string is a punctuation
        int length = str.length();
        str = str.substring(length -2,length-1);

        if(str.equals(",") || str.equals("!") || str.equals(".") || str.equals("?"))
        {
            return true;
        }
        else
        {
            return false;
        }

    }

    private String getWord(String str)
    {   // Hint: if last character in string is punctuation then remove 

    if(punctuationChar(str)== true)
    {
        //remove punctuation of last char
        str = str.substring(0,str.length()-1);
    }

        return str;
    }


    public void compress()
    {   /* read through section 3 of the practical 5 document 
           to get you started. This is called by the server, 
           have a look at the server code where it is called */ 
        ArrayList<String> newMessage = new ArrayList<String>();


        String[] words = message.split(" ");  

        for (String word : words)  
        {  
            getWord(word);
            //if word has already appeared replace with position of previous word
            if(newMessage.contains(word))
            {
                String str = Integer.toString(newMessage.indexOf(word));
                str = str + " ";
                newMessage.add(str);
            }
            else
            {
                word = word + "";
                newMessage.add(word);
            }

         //if word had a punctuation at the end add it back in
         //System.out.println(word);  
        }  

            this.message = newMessage.toString();
            System.out.println("****************COMPRESSING*****************");
            System.out.println(newMessage);

    }

    public void decompress()
    {   /* read through section 3 of the practical 5 document 
           to get you started. This is called by the client,
           have a look at the client code where it is called */
           ArrayList<String> decompMessage = new ArrayList<String>();

           String[] words = message.split(" ");


           for (String word : words)  
        {  
            getWord(word);

            if(word.substring(0,1).matches("[0-9]"))
            {
              int num = Integer.parseInt(word);
              decompMessage.add(decompMessage.get(num));

            }
            else
            {
                decompMessage.add(word);
            }
        }

        this.message = decompMessage.toString();
        System.out.println("****************DECOMPRESSING*****************");
            System.out.println(decompMessage);  

    }
}

Error:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
    at java.lang.String.substring(String.java:1952)
    at CompressedMessage.punctuationChar(CompressedMessage.java:24)
    at CompressedMessage.getWord(CompressedMessage.java:40)
    at CompressedMessage.compress(CompressedMessage.java:61)
    at P5_Server.waitForData(P5_Server.java:72)
    at P5_Server.main(P5_Server.java:159)

I have tried changing the way a calculated the strings based on the length() but it didn’t reduce the errors.

Can anyone see what I’m doing wrong?

Answer 1

What if you str is length 0 (empty string ) or length 1? In that case str = str.substring(length -2,length-1); will result into exception.

You need to put a length check before performing the substring:

    if(length > 1){
        str = str.substring(length-2,length-1);
    }

Since you are trying to get only one character, I think you can simply do as:

    if(length > 1){
        str = String.valueOf(str.charAt(length-2))
    }

Please make sure that str is not null otherwise put a null handling as well.