How does this Java snippet work?

Question

I am studying Java as a beginner, but I did not understand this script.

I have a 'look and say' assignement.
Can anyone tell me how this is working?

public class App {

  public static void main(String[] args) {

    for (int k = 0, len = args.length; k < len; k++) {

      int i, j;


      StringBuilder preNumber = new StringBuilder("1");     // what is the use of StringBuilder?
      StringBuilder AnsNumber = new StringBuilder();

      int n = Integer.parseInt(args[k]);

      for (i = 1; i <= n; i++) {

        AnsNumber = preNumber;
        int l = preNumber.length(), cnt = 1;
        StringBuilder nxtNumber = new StringBuilder();

        for (j = 1; j < l; j++) {

          char ch = preNumber.charAt(j);

          if (preNumber.charAt(j - 1) == preNumber.charAt(j))   // what does charAt(j) means?
          {
            cnt++;
          } 
          else 
          {
            nxtNumber.append(cnt);
            //why '+' isnot using as concatination?

            nxtNumber.append(preNumber.charAt(j - 1));
            cnt = 1;
          }

          // System.out.println(nxtNumber);
        }

        nxtNumber.append(cnt);
        nxtNumber.append(preNumber.charAt(j - 1));
        preNumber = nxtNumber;
      }
      System.out.println(AnsNumber);
    }
  }
}

You can get answers to the first two from Javadocs for `StringBuilder` and `String` respectively. — Andy Turner, Jun 01 '17 at 21:23
Have you read the Java DOC for this. Please read once again. — Janny, Jun 01 '17 at 21:24

iavanish · Accepted Answer · 2017-06-01T21:49:47.763

You can easily get your answers if you look at the Java Doc.

StringBuilder class is used to create mutable strings. It performs way better than normal String class (which is immutable). Reason here: What is difference between mutable and immutable String in java
charAt(j) returns the character at the j'th index within the string (indexing is 0 based).
'+' is used to concat String objects. append() is used to concat StringBuilder objects (add some String to the end of the StringBuilder object).

How does this Java snippet work?

1 Answers1