public static void main(String[] args) {
    //I print an array with letters from A-Z
    char abc[]=new char[26];
    for(int i=0; i<abc.length; i++) {
        abc[i]=(char)('A'+i);
    System.out.println(abc[i]);
    }
    //I ask for positions and save the results in a String
    do {
        String result="";
        Scanner sn = new Scanner(System.in);
        int num=sn.nextInt();

        result=result + abc[num];
        }while(num!=-1);
}
}

while(true) {
  int num=sn.nextInt();
  if(num == -1) {
     break;
  }
  //Do stuff

int num=sn.nextInt();
//When user inputs -1 to quit num= -1
result=result + abc[num];
//This will call the -1 index of abc

while(true) {
    try {       //Add a try catch to prevent InputMisMatchException
       num=sn.nextInt();
    }catch(Exception e) {
        e.printStackTrace();
    }
    if(num < 0 || num > 25) { //If it is going to be out of bounds
         break;
    }
    if(num>=0) {
         result+=abc[num];
    }
}

System.out.println(result);

public static void main(String[] args) {

    //I print an array with letters from A-Z
    char abc[]=new char[26];
    for(int i=0; i<abc.length; i++) {
        abc[i]=(char)('A'+i);
    System.out.println(abc[i]);
    }
    //I ask for positions and save the results in a String
    Scanner sn = new Scanner(System.in);
    int num;
    String result = "";

    while(num = sn.nextInt() != -1) {
        result = result + abc[num];
    }
    System.out.println(result);
}

Scanner sn = new Scanner(System.in);
    int num;
    String result = "";

    while(true) {
          num=sn.nextInt();
          if(num == -1) {
             break;
          }
          if(num>=0) {
              result+=abc[num];
          }
    }
    System.out.println(result);