Help fix error

Question

I get a run time error in my Insertion sort, i will make the line and tell what the erros is saying.
ERROR: NullPointerException
public class Person {
// Data Memebers
    private String LastName;
    private String FirstName;
    private String StrYearsInSchool;
    private int    yearsInSchool;
    private Person[] a;
// Constructor
    public Person(String LastName, String FirstName, int yearsInSchool) {
        this.LastName      = LastName;
        this.FirstName     = FirstName;
        this.yearsInSchool = yearsInSchool;
        a = new Person[20];
    public String getLast() {
        return LastName;
    public void insert(String LastName, String FirstName, int yearsInSchool) {
        for (int i=0; i<20; i++) {
            a[i] = new Person(LastName, FirstName, yearsInSchool);
// Insertion sort
    public void insertionSort() {
        int in, out;
        for (out=1; out<a.length; out++) {
            Person temp = a[out];
            in = out;
//error            while(in > 0 && a[in-1].getLast().compareTo(temp.getLast()) > 0) {
                a[in] = a[in-1];
                --in;
            a[in] = temp;
    public void display(Person[] a) {
        for (int i=0; i<3; i++) {
            a.formatDisplay();
System.out.println("");
public void formatDisplay() {
if (yearsInSchool == 1) {
StrYearsInSchool = "Freshman";
else if(yearsInSchool == 2) {
StrYearsInSchool = "Sophmore";
else if(yearsInSchool == 3) {
StrYearsInSchool = "Junior";
else if(yearsInSchool == 4) {
StrYearsInSchool = "Senior";
else if(yearsInSchool >= 5) {
StrYearsInSchool = "N/A";
System.out.println("------------------------------");
System.out.println("Student ");
System.out.println(LastName + ", " + FirstName);
System.out.println(StrYearsInSchool);
System.out.println("");

Answer 1

a[in-1] is null.