This Program Is About String Compression in Java

This program is about String Compression in java

Java always evaluates left-to-right. So this:

s.charAt(i)!=s.charAt(i+1) || i+1>=s.length()

Evaluates s.charAt(i+1) before it has checked i+1>=s.length(). When i+1 >= s.length(), s.charAt(i+1) will fail, because it is beyond the end of the string.

Reverse the order of the operands:

i+1>=s.length() || s.charAt(i)!=s.charAt(i+1)

Because || is a short-circuiting operator, this only evaluates s.charAt(i)!=s.charAt(i+1) if i+1>=s.length() is false, so it doesn't fail with an exception in that case.

String compression algorithm in Java

You don't need two for loops for this and can do it in one go like so

    String str = "aaabbbbccccca";
    char[] chars = str.toCharArray();
    char currentChar = str.length() > 0 ? chars[0] : ' ';
    char prevChar = ' ';
    int count = 1;
    StringBuilder finalString = new StringBuilder();

    if(str.length() > 0)
    for(int i = 1; i < chars.length; i++)
    {
        if(currentChar == chars[i])
        {
            count++;
        }else{
            finalString.append(currentChar + "" + count);
            prevChar = currentChar;
            currentChar = chars[i];
            count = 1;
        }
    }

    if(str.length() > 0 && prevChar != currentChar)
        finalString.append(currentChar + "" + count);

    System.out.println(finalString.toString());

Output is: a3b4c5a1 for aaabbbbccccca

Compressing a string in Java

I would start at zero, and look forward:

public static void main(String[] args){
    System.out.println("Enter a string: ");
    String str = IO.readString();
    int count = 0;
    String result = "";

    for (int i=0; i < str.length(); i++) {
        char a = str.charAt(i);
        count = 1;

        while (i + 1 < str.length() && str.charAt(i) == str.charAt(i+1)) {
            count++;
            i++;
        }

        if (count == 1) {
            result = result.concat(Character.toString(a));
        } else {
            result = result.concat(Integer.toString(count).concat(Character.toString(a)));
        }
    }

    IO.outputStringAnswer(result);
}

Some outputs:

qwwwwwwwwweeeeerrtyyyyyqqqqwEErTTT => q9w5e2rt5y4qw2Er3T
qqwwwwwwwweeeeerrtyyyyyqqqqwEErTTT => 2q8w5e2rt5y4qw2Er3T
qqwwwwwwwweeeeerrtyyyyyqqqqwEErTXZ => 2q8w5e2rt5y4qw2ErTXZ
aaa => 3a
abc => abc
a => a

Java - Compressed String

public static String compressedString(String message) {
    StringBuilder compressedString = new StringBuilder();
    int total = 1;
    for (int i = 0; i < message.length() - 1; i++){
        if (message.charAt(i) == message.charAt(i+1)){
            total++;
        }
        else if(total==1){
            compressedString.append(message.charAt(i));
        }
        else 
        {
            compressedString.append(message.charAt(i)).append(total);
            total = 1;
        }
    }
    if(message.charAt(message.length()-2) != message.charAt(message.length()-1)
        compressedString.append(message.charAt(message.length()-1));
    else
        compressedString.append(message.charAt(message.length()-1)).append(total);

    return compressedString.toString();
}

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