how to copy a string and replace all the spaces with tabs

i attempted this program as follows but am being asked that i do not copy a string. can anyone help

Code:

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public class stringCopy {

        public stringCopy() {
                // TODO Auto-generated constructor stub
        }
  public String CopyString(String s){
         
        String grt= new String();
       
                return  s.replaceAll("\\s+", "\t");
                 
          }
       
  public String subString(String s,String sub){
          String str[]=new String[10];
          if ((s!=null)&&(sub!=null)){
                  s.replaceAll(sub, "");
                  for (int i = 0; i < str.length; i++) {
                        str[i]= s.replaceAll(sub, "");
                }
                 
          }
       
          return  s.replaceAll(sub, "");
  }
 
        /**
        * @param args
        */
        public static void main(String[] args) {
                // TODO Auto-generated method stub
stringCopy str = new stringCopy();

System.out.println(str.CopyString("Strings are good to work with"));


System.out.println(str.subString("Strings are good to work with","work"));
 
        }
}

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Quote:

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am being asked that i do not copy a string

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What does that mean?

What does your code do now and what do you want it to do?
Show what it outputs and explain what is wrong with that output.