How to print sequence of characters according to the number next to it?

Input: a2b4c3
Output: aabbbbccc

Here is my code.........which has wrong output:

import java.lang.*;
import java.util.ArrayList;

class hi
{
public static void main(String[] args)
{
String a="a2b4c3";
String alpha="";
ArrayList<Character> num=new ArrayList<Character>();
for(int i=0;i<a.length();i++)
{
  char c=a.charAt(i);
  if(Character.isDigit(c)==true)
   {
     num.add(c);
   }
  else
   {
    alpha+=c;
   }
}
for(int y=0; y<num.size(); y++)
 {
   for(int z=0; z<num.get(y); z++)
    { 
      System.out.print(alpha.charAt(y));
    }
  }
}
}

Whats wrong in this code, I didn't get the output as expected

My output is: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb ccccccccccccccccccccccccccccccccccccccccccccccccccc

Need corrections or code in java

Hi Here your answer.

import java.lang.*;
import java.util.ArrayList;

class Hi
{
    public static void main(String[] args)
    {
        String a="a2b4c3";
        String alpha="";
        ArrayList<Character> num=new ArrayList<Character>();
        for(int i=0;i<a.length();i++)
        {
            char c=a.charAt(i);
            if(Character.isDigit(c)==true)
            {
                num.add(c);
            }
            else
            {
                alpha+=c;
            }
        }
        for(int y=0; y<num.size(); y++)
        {
            int count=Integer.parseInt(String.valueOf(num.get(y)));

            for(int z=0; z<count; z++)
            {
                System.out.print(alpha.charAt(y));
            }
        }
    }
}

Or

import java.lang.*; import java.util.ArrayList;

class Hi
{
    public static void main(String[] args)
    {
        String a="a2b4c3";
        String alpha="";
        ArrayList<Integer> num=new ArrayList<>();
        for(int i=0;i<a.length();i++)
        {
            char c=a.charAt(i);
            if(Character.isDigit(c)==true)
            {
                num.add(Integer.parseInt(String.valueOf(c)));
            }
            else
            {
                alpha+=c;
            }
        }
        for(int y=0; y<num.size(); y++)
        {
            for(int z=0; z<num.get(y); z++)
            {
                System.out.print(alpha.charAt(y));
            }
        }
    }
}