Java: Expanding array size, can't seem to keep all values in original locations

For my current homework, I'm trying to sort my array through a generic class as the user inserts values into its locations. When the size reads as fully loaded, the array class calls in an expansion method that increases the size of the array while retaining its values in proper locations, which I followed from my Professor's note. For some reason, all my values except for location[0] seem to either be misplaced or erased from the array. I'm leaning that the problem originates in the expansion method but I have no idea how to fix this.

For example, the initial size is currently set to 5 but increments by 3 when expansion method is called. The user can input values 1,2,3,4,5 perfectly. But expansion is called when user inputs new value 6 that outputs an array of 1, 6, null, null, null, null. Any further will lead to the error "Exception in thread "main" java.lang.NullPointerException"

Here is my Sorted Array class:

public class SortedArray {
private int size;
    private int increment;
    private int top;
    Comparable[] a;

public SortedArray(int initialSize, int incrementAmount)
{
        top = -1;
        size = initialSize;
        increment = incrementAmount;
        a = new Comparable [size];
}
public int appropriatePosition(Comparable value)
{
        int hold = top;
        if(hold == -1)
        {
            hold = 0;
        }
        else
        {
            for(int i = 0; i <= top; i++)
            {
               if(value.compareTo(a[i]) > 0)
               {
                   hold = i + 1;
               }
            }
        }
        return hold;
}
public Comparable smallest()
    {
        return a[0];
    }
public Comparable largest()
    {
        return a[top];
    }
public void insert(Comparable value)// the method that my driver calls for.
{
        int ap = appropriatePosition(value);
        //Expansion if full
        if(full() == true)
        {
            expansion();
        }
        //Shifting numbers to top
        for(int i = top; i >= ap ; i--)
        {
            {
                  a[i + 1] = a[i];
            }
        }
        a[ap] = value;
        top++;

    }
public boolean full()
{
    if(top == a.length -1)
    {
        return true;
    }
    else
    {
        return false;
    }
}
public void expansion()//here's where the expansion begins
    {
        int newSize = a.length + increment;
            Comparable[] tempArray = new Comparable[newSize];
            for(int i= 0; i < a.length; i++)
            {
                tempArray[i]= a[i];
                a  = tempArray;
            }
    }

Here's my driver class that calls for the insert method in SortedArray class.

public class In开发者_运维知识库tDriver {
 public static void main(String[] args)
 {
     Scanner keyboard = new Scanner(System.in);
     //Creating variables
     int data;
     boolean check = false;
     int choice;
     int size = 5;
     int increment = 3;
     SortedArray b = new SortedArray(size, increment);
     //Creating Menu
     System.out.println("Please choose through options 1-6.");
     System.out.println("1. Insert\n2. Delete\n3. Clear\n4. Smallest\n5. Largest\n6. Exit\n7.Redisplay Menu");
     while(check == false)
     {
     choice = keyboard.nextInt();
     switch(choice)
         {
         case 1:
             System.out.println("Type the int data to store in array location.");
             data = keyboard.nextInt();
             Integer insertObj = new Integer(data);
             b.insert(insertObj);
             System.out.println("The value " + data + " is inserted");
             b.print();
            break;

In the expansion method, you're replacing a too soon. The replacement should happen after the for loop:

public void expansion()//here's where the expansion begins
    {
        int newSize = a.length + increment;
            Comparable[] tempArray = new Comparable[newSize];
            for(int i= 0; i < a.length; i++)
            {
                tempArray[i]= a[i];
            }
            a  = tempArray;
    }