Java 8 # Count of character at given String (aabbbbcccc)

 import java.util.*;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.*;

public class ToStringProgram
{
     public static void main(String[] args)
     {
         String str = "aabbbccccc";
         String charCount = compressString(str);
         System.out.println(charCount);
     }

     private static String compressString(String str)
     {
            List<String> list = new ArrayList<>();
            AtomicInteger count = new AtomicInteger(1);

            IntStream.range(0,str.length())
                    .forEach(i -> {
                        if(i>0 && str.charAt(i)==str.charAt(i-1))
                        {
                            count.incrementAndGet();
                        }
                        else {
                            if(i>0)
                            {
                                list.add(String.valueOf(str.charAt(i-1))+count);
                            }
                            count.set(1);
                        }
                    });
                    list.add(String.valueOf(str.charAt(str.length()-1))+count);
                    return list.stream().collect(Collectors.joining());



     }
}