Compressing adjacent repeated characters

Question:

Write a Java program that reads in a sequence of characters of unknown length, compresses adjacent repeated characters, and prints the output to the screen. The program replaces strings of repeating character sequences by [nX] in the output, where n is an integer count of the number of repetitions , and X is the character.

A sample input and output session is given below:

Enter string: abbcccddddeeeeef
a[2b][3c][4d][5e]f

Code:

import java.util.*;
 
public class CompressString {
  public static void main(String[] args){
 
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter string: ");
    String str = sc.nextLine();
 
    String tmp = "";
    char d = ' ';
    int counter = 0;
    for(int i = 0; i < str.length(); i++){
      counter = 0;
      for(int j = i; j < str.length(); j++){
        if(str.charAt(i) == str.charAt(j)){
          counter++;
        }else{
          if(str.charAt(j-1) == d){
            break;
          }
        }
        if(counter > 1){
          d = str.charAt(i);
        }
      }
      if(counter > 1){
        tmp += "[" + counter + str.charAt(i) + "]";
        i+= counter - 1;
      }else{
        tmp += str.charAt(i);
      }
    }
    System.out.println(tmp);
  }
}

Output:

$ java CompressString
Enter string: abbcccddddeeeeef
a[2b][3c][4d][5e]f