Answer:
Check the explanation
Explanation:
CODE:-
import java.util.*;
class UserName{
   ArrayList<String> possibleNames;
   UserName(String firstName, String lastName){
       if(this.isValidName(firstName) && this.isValidName(lastName)){
           possibleNames = new ArrayList<String>();
           for(int i=1;i<firstName.length()+1;i++){
               possibleNames.add(lastName+firstName.substring(0,i));
           }  
       }else{
           System.out.println("firstName and lastName must contain letters only.");
       }
   }
   public boolean isUsed(String name, String[] arr){
       for(int i=0;i<arr.length;i++){
           if(name.equals(arr[i]))
               return true;
       }
       return false;
   }
   public void setAvailableUserNames(String[] usedNames){
       String[] names = new String[this.possibleNames.size()];
       names = this.possibleNames.toArray(names);
       for(int i=0;i<usedNames.length;i++){
           if(isUsed(usedNames[i],names)){
               int index = this.possibleNames.indexOf(usedNames[i]);
               this.possibleNames.remove(index);
               names = new String[this.possibleNames.size()];
               names = this.possibleNames.toArray(names);
           }
       }
   }
   public boolean isValidName(String str){
       if(str.length()==0) return false;
       for(int i=0;i<str.length();i++){
           if(str.charAt(i)<'a'||str.charAt(i)>'z' && (str.charAt(i)<'A' || str.charAt(i)>'Z'))
               return false;
       }
       return true;
   }
   public static void main(String[] args) {
       UserName person1 = new UserName("john","smith");
       System.out.println(person1.possibleNames);
       String[] used = {"harta","hartm","harty"};
       UserName person2 = new UserName("mary","hart");
       System.out.println("possibleNames before removing: "+person2.possibleNames);
       person2.setAvailableUserNames(used);
       System.out.println("possibleNames after removing: "+person2.possibleNames);
   }
}
Kindly check the attached image below for the code output.