Sorting an array of students by last name alphabetically in Java

I currently have this program read the contents of a text file and calculate the averages and amount of test scores taken and print them out neatly in a small data table. These are the names, amount of quizes taken and average of each student:

James Tiberius Kirk              8                         91.63 
Buffy Summers                    7                         83.14 
Tom Baker                       15                        100.00 
Malcolm Reynolds                 9                         84.22 
Elizabeth Bennet                 9                         93.33 
John Blutarsky                   9                          0.00 
Dorthy Gale                      6                         85.83 

All of these Students are stored within the Array named Anames[]. I was wondering if it was at all possible to sort these students alphabetically by last name using the code that I have now. When I run the program it gives me the error:

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1

at java.lang.String.substring(String.java:1927)
at text.reader.TextReader.compareLastNames(TextReader.java:117)
at text.reader.TextReader.main(TextReader.java:94)

Here is the code of my main class:

public static void main(String[] args)throws IOException{

    Double score=0.0;
    int b,j;
    String tempfirst = "";
    String templast = "";
    Student Anames[] = new Student[30];
    Student Temp[] = new Student [1];
    int Stucount = 0;
    Scanner reader = new Scanner(new File("quizScores.txt"));
    boolean runProgram = true;
    PrintWriter writer = new PrintWriter(new File("scoreReport.txt"));
    //prints header for report
    System.out.println("Name                        Number Quizes             Quiz Socres");
    writer.println("Name                        Number Quizes             Quiz Socres");

    //check to see if end of file string
    while (!reader.hasNext("-10")){
        String name="", first="", last="";

        //gets the name from file
        while(!reader.hasNextDouble()){
            last = reader.next();

            while (!reader.hasNextDouble()){
                first = first+reader.next()+" ";
            }
            name=first+last;
        }

        //creates new student with given name
        Student newStudent = new Student(first, last);
        Anames[Stucount] = newStudent;
        Stucount++;

        //gets the quiz scores and makes sure does not averge in the end of file string.
        while (reader.hasNextDouble()&& !reader.hasNext("-10")){
           newStudent.addQuiz(reader.nextDouble());
        }

        //Prints out the formated data
        System.out.printf("%-30s%4.0f%30.2f \n",newStudent.getName(), newStudent.getQuizNumber(), newStudent.getAverage());
        writer.printf("%-30s%4.0f%30.2f",newStudent.getName(), newStudent.getQuizNumber(), newStudent.getAverage());

        writer.println();
    }
    System.out.println("\n");

    for (b = 0; b < Stucount; b++){
        int INTEGERTEMP = b;
        for (j= b+1; j < Stucount; j++){
            int INTEGERTEMP2 = j;
            if ((compareLastNames(Anames[INTEGERTEMP].getLAST(), Anames[INTEGERTEMP2].getLAST()))>0){
                Temp[0] = Anames[b];
                Anames[b] = Anames[j];
                Anames[j] = Temp[0];
            }
        }
    }

    System.out.println("Name                        Number Quizes             Quiz Socres");
    for (int i = 0; i < Stucount; i++) {

            System.out.printf("%-30s%4.0f%30.2f \n", Anames[i].getName(), Anames[i].getQuizNumber(), Anames[i].getAverage());

    }

    writer.close();
}

private static int compareLastNames(String a, String b){
    int index_a = a.lastIndexOf(" ");
    String surname_a = a.substring(index_a);
    int index_b = b.lastIndexOf(" ");
    String surname_b = b.substring(index_b);
    int lastNameCmp = surname_a.compareToIgnoreCase(surname_b);
    return lastNameCmp;
}

Here is the Student.java which contains most of the methods used:

public Student (String inName, String inLast){
    studentName=inName;
    studentLast = inLast;
    quizAverage = 0;
    quizScore=0;
    numberQuizes=0;
}

public void addQuiz(double inQuiz){
    quizScore += inQuiz;
    numberQuizes++;
}

public double getAverage(){
    quizAverage = quizScore/numberQuizes;
    return quizAverage;
}

public String getName(){
    return studentName+studentLast;
}

public double getQuizNumber(){
    return numberQuizes;
}

public String getLAST(){
    return studentLast;
}