not sorting or entering into calculation method

This program compiles, however, it only lets me insert one grade before the window closes and it doesn't enter the sort or the calculation of the average grade. I am not sure where I am going wrong, any help would be greatly appreciated.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.text.*;
import java.text.DecimalFormat;

public class Average extends JFrame implements ActionListener
{
	JTextPane gradesPane = new JTextPane();
	JButton enterGrades = new JButton("Enter Grades");
	String grades[] = new String[50];
	int SENTINEL = 0;
	DecimalFormat formatter = new DecimalFormat("######.##");

	public Average()
	{
		super("Grade Average");
	}

	//create the menu system
	public JMenuBar createMenuBar()
	{
		//create an instance of the menu
		JMenuBar mnuBar = new JMenuBar();
		setJMenuBar(mnuBar);

		//construct and populate the File menu
		JMenu mnuFile = new JMenu("File", true);
			mnuFile.setMnemonic(KeyEvent.VK_F);
			mnuFile.setDisplayedMnemonicIndex(0);
			mnuBar.add(mnuFile);

		JMenuItem mnuFileExit = new JMenuItem("Exit");
			mnuFileExit.setMnemonic(KeyEvent.VK_X);
			mnuFileExit.setDisplayedMnemonicIndex(1);
			mnuFile.add(mnuFileExit);
			mnuFileExit.setActionCommand("Exit");
			mnuFileExit.addActionListener(this);
		
		
			return mnuBar;
	}

	//create the content pane
	public Container createContentPane()
	{
		//construct and populate the north panel
		JPanel northPanel = new JPanel();
		northPanel.setLayout(new FlowLayout());
		northPanel.add(enterGrades);
		enterGrades.addActionListener(this);

		//construct and populate the central panel
		JPanel centralPanel = new JPanel();
		centralPanel.setLayout(new FlowLayout());
		centralPanel.add(gradesPane);
		JScrollPane scrollPane = new JScrollPane(gradesPane);
						scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
						scrollPane.setPreferredSize(new Dimension(500,200));
		centralPanel.add(scrollPane);
		gradesPane.setEditable(false);

		//create Container and set attributes
		Container c = getContentPane();
			c.setLayout(new BorderLayout(10,10));
			c.add(northPanel, BorderLayout.NORTH);
			c.add(centralPanel, BorderLayout.CENTER);

		return c;
	}
//event to process user clicks
	public void actionPerformed(ActionEvent e)
	{
		String arg = e.getActionCommand();
		//user clicks the enter by combo box
		if (e.getSource() == enterGrades)
			gradeCollector();
		// user clicks the exit on the file menu
		if (arg == "Exit")
			System.exit(0);
	}

	public void gradeCollector()
	{
		int arrayPop = 0;

		while (SENTINEL != -1)
		{
			String grade = JOptionPane.showInputDialog("Please enter a grade:");
			checkGrade(grade);
			SENTINEL = Integer.parseInt(grade);
			grades[arrayPop] = grade;
			arrayPop++;

			sort(grades);
		}
	}

	public void sort(String tempArray[])
	{
		//loop to control number of passes
		for (int pass = 1; pass < tempArray.length; pass++)
		{
			for (int element = 0; element < tempArray.length - 1; element++)
				if(tempArray[element].compareTo(tempArray[element + 1]) > 0)
				{
					swap(grades, element, element + 1);
				}
		}

		addTextToTextPane();
	}


	public JTextPane addTextToTextPane()
	{
		Document doc = gradesPane.getDocument();
		try
		{
			//clear previous text
			doc.remove(0, doc.getLength());

			//insert detail
			for (int j = 0; j<grades.length; j++)
			{
				doc.insertString(doc.getLength(), grades[j] + "\t",gradesPane.getStyle("Bold"));
			}


		}

		catch(BadLocationException ble)
		{
			System.err.println("Couldn't insert text.");
		}

		return gradesPane;
	}

	public void swap(String swapArray[], int first, int second)
	{
		String hold; //temporary holding area for swap
		hold = swapArray[first];
		swapArray[first] = swapArray[second];
		swapArray[second] = hold;
	}

	public void checkGrade(String value)  //implemented later
	{
		int intValue = 0;

		intValue = Integer.parseInt(value);

		while ((intValue > 100) || (intValue < -1))
		{
			JOptionPane.showMessageDialog(null,"that is not a valid entry.");
			String grade = JOptionPane.showInputDialog("Please enter a grade:");

			intValue = Integer.parseInt(grade);
		}
	}

	public double averager(String genericArray[])
	{
		int total = 0;
		int gradeCount = 0;
		double finalAverage;
		for (int z = 0; z < genericArray.length; z++)
		{
			total += Integer.parseInt(genericArray[z]);
			gradeCount++;

			if (Integer.parseInt(genericArray[z]) <= 0)
			gradeCount --;
		}

		finalAverage = total / gradeCount;

		return finalAverage;
	}

	public String gradeConverter(double decGrade)
	{
		String result = " ";

		if (decGrade > 89)
			result = ("Your final grade is " + formatter.format(decGrade) + " an A.");
		else
			if (decGrade > 79)
				result = ("Your final grade is " + formatter.format(decGrade) + " a B.");
			else
				if (decGrade > 69)
					result = ("Your final grade is " + formatter.format(decGrade) + " a C.");
				else
					if (decGrade > 59)
						result = ("Your final grade is " + formatter.format(decGrade) + " a D.");
					else result = ("Your final grade is " + formatter.format(decGrade) + " an F. Sorry :(/>/>/>/>");

		return result;
	}

	public static void main(String args[])
	{
		Average a = new Average();
		a.setJMenuBar(a.createMenuBar());
		a.setContentPane(a.createContentPane());
		a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		a.setSize(600,375);
		a.setVisible(true);
	}
}