JOptionPane and e.getSource problem

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

public class AverageGrade extends JFrame implements ActionListener
{
	//construct components
	JLabel sortPrompt = new JLabel("Sort by:");
	JComboBox fieldCombo = new JComboBox();
	JTextPane textPane = new JTextPane();

	boolean firstRun = true;
	int arrayLength;
	double[] grades = new double[50];
	DecimalFormat format = new DecimalFormat("##.##");
	double maxGrade = Double.MIN_GRADE;
	double minGrade = Double.MAX_GRADE;


	//construct instance of AverageGrade
	public AverageGrade()
	{
		super("Grade Averages");
	}

	//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");
			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);

		JMenuItem mnuFileNewGrades = new JMenuItem("Insert New Grades");
			mnuFileNewGrades.setMnemonic(KeyEvent.VK_I);
			mnuFileNewGrades.setDisplayedMnemonicIndex(0);
			mnuFile.add(mnuFileNewGrades);
			mnuFileNewGrades.setActionCommand("NewGrades");
			mnuFileNewGrades.addActionListener(this);

		return mnuBar;
	}

	//create the content pane
	public Container createContentPane()
	{
		//populate the JComboBox
		fieldCombo.addItem("Highest Grade");
		fieldCombo.addItem("Lowest Grade");
		fieldCombo.addActionListener(this);
		fieldCombo.setToolTipText("Click the drop-down arrow to display High/Low fields.");

		//construct and populate the north panel
		JPanel northPanel = new JPanel();
			northPanel.setLayout(new FlowLayout());
			northPanel.add(sortPrompt);
			northPanel.add(fieldCombo);

		//create the JTextPane and center panel
		JPanel centerPanel = new JPanel();
			setTabsAndStyles(textPane);
			textPane = addTextToTextPane();
			JScrollPane scrollPane = new JScrollPane(textPane);
				scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
				scrollPane.setPreferredSize(new Dimension(500, 200));
			centerPanel.add(scrollPane);

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

		return c;
	}

	//method to create tab stops and set font styles
	protected void setTabsAndStyles(JTextPane textPane)
	{
		//create Tap Stops
		TabStop[] tabs = new TabStop[2];
			tabs[0] = new TabStop(200, TabStop.ALIGN_LEFT, TabStop.LEAD_NONE);
			tabs[1] = new TabStop(350, TabStop.ALIGN_LEFT, TabStop.LEAD_NONE);
		TabSet tabset = new TabSet(tabs);

		//set Tab Style
		StyleContext tabStyle = StyleContext.getDefaultStyleContext();
		AttributeSet aset =
			tabStyle.addAttribute(SimpleAttributeSet.EMPTY, StyleConstants.TabSet, tabset);
		textPane.setParagraphAttributes(aset, false);

	}

	//method to add new text to the JTextPane
	public JTextPane addTextToTextPane()
	{
		Document doc = textPane.getDocument();
		try
		{
			StyleContext style = StyleContext.getDefaultStyleContext();
			AttributeSet set = style.addAttribute(SimpleAttributeSet.EMPTY,StyleConstants.ALIGN_LEFT,style);

			//clear previous text
			doc.remove(0, doc.getLength());

			//insert detail
			for (int gv = 0; gv<arrayLength; gv++)

			//insert grade

			{
				doc.insertString(doc.getLength(), String.valueOf(grades[gv])+"\n" ,set);

				if(firstRun)
				System.out.println("Insert Grades");
				else
			    {
					double average = average();
				    doc.insertString(doc.getLength(),"Average: " + String.valueOf(format.format(average)) + "\n",set);

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

		    }

		return textPane;
	}

	//event to process user clicks
	public void actionPerformed(ActionEvent e)
	{
		String arg = e.getActionCommand();

		//user clicks the drop down box
		if (e.getSource() == "Highest Grade")
		{
			for (grades>maxGrade)
			{
				maxGrade = grades;
				(JOptionPane.showInputDialog(null, "The highest grade is ", + maxGrade));
			}
		}
		else if (e.getSource() == "Lowest Grade")
		{
			for (grades<minGrade)
			{
				minGrade = grades;
				(JOptionPane.showInputDialog(null, " The lowest grade is ", + minGrade));
			}
		}

		//user clicks the file
		if (e.getSource() == "NewGrades")
		{
			for (int gv = 0; gv<grades.length; gv++)
			grades[gv] = 0.0;
			double grade = 0;
			int counter = 0;

			while (grade != -1 && counter <50)
			{
				try
				{
					grade = Double.parseDouble
					(JOptionPane.showInputDialog(null, "Enter Grade", "Enter Value"));
					grades[counter] = grade;

					if (grade != -1)
					counter++;
				}
				catch (NumberFormatException nfe)
				{
					System.out.println("Incorrect entry");
					JOptionPane.showMessageDialog(null, "You have entered an invalid number. If you are " + "finished, enter -1.");
				}

			}
			arrayLength = counter;

			addTextToTextPane();
		}

		//user clicks Exit on the File menu
		else if (e.getSource() == "Exit")
		{
			System.exit(0);
		}
	}

		public double average()
		{
			double sum = 0, average;
			int totalCount = 0;

			for(int gv = 0; gv < arrayLength; gv++)
			{
				if (grades[gv] > 0)
				{
					sum += grades[gv];
					totalCount++;
				}
			}
			average = sum / (double) totalCount;

			return average;
		}

		//method to sort arrays
		public void sort(String tempArray[])
		{
			boolean change;
			do
			{
				change = false;

			//loop to control number of passes
			for (int gv = 0; gv < arrayLength - 1; gv++)
			{
				if (grades[gv] > grades[gv+1])
					{
						swap(gv);
						change = true;
					}
			}
		}
		while(change);
	}

		//method to swap two elements of an array
		public void swap(int index)
		{
			double temp = grades[index];
			grades[index] = grades[index+1];
			grades[index+1] = temp;
		}

		//main method executes at run time
		public static void main(String[] args)
		{
			AverageGrade m = new AverageGrade();
			m.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
			m.setJMenuBar(m.createMenuBar());
			m.setContentPane(m.createContentPane());
			m.setSize(600,375);
			m.setVisible(true);
			JOptionPane.showMessageDialog(null, "To enter grades, select File, then Enter New Grades.\n" + "To finish, enter -1 in the grade box.", JOptionPane.INFORMATION_MESSAGE);
		}

	}