Matrix multiplying using separate input classes and a GUI

I have written code for the Matrix Multiplication GUI and I have set up two classes that build panels Matrix A and Matrix B. I used a toString() method in both classes, but how do I bring both methods into a multiplication class to do the math and return it to the GUI to display?

Code Java:

---

public class MatrixA extends JPanel
{
	private JPanel panel;
	private JTextArea matrixATextArea;
	public static String inputA;
 
	// constructor
	public MatrixA()
	{
		setBorder( BorderFactory.createTitledBorder("Matrix A"));
		buildPanel();
		add( panel );
		setVisible( true );
	}
 
	private void buildPanel()
	{
		matrixATextArea = new JTextArea( 10, 30 );
		setPreferredSize( new Dimension( 400, 225 ) );
 
		panel = new JPanel();
		panel.setBorder( BorderFactory.createLineBorder( Color.black ) );
		matrixATextArea.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
		panel.add( matrixATextArea );
	}
 
	public String getTextA() 
	{
		 return inputA = matrixATextArea.getText();
	}
 
    public String toString()
    {
        return inputA;
    }
}

---

*Note: Matrix B is using the same exact code except inputB as the variable.

MatrixGUI:

Code Java:

---

public class MatrixGUI extends JFrame 
{
	private MatrixInstructions matrixMain;  // holding instructions.
	private MatrixA matrixA;  // matrix on left side.
	private MatrixB matrixB;  // matrix on right side.
	private JPanel buttonPanel;  // to hold the calculate and exit button.
	private JButton calcButton;  // to calculate the multiplication.
 
	// constructor
	public MatrixGUI()
	{
		// display title
		setTitle( "Matrix Multiplier" );
 
		// specify action for close button
		setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
 
		// create a layout.
		setLayout( new BorderLayout() );
 
		// create custom panels.
		matrixMain = new MatrixInstructions();
		matrixA = new MatrixA();
		matrixB = new MatrixB();
 
		// create button panel
		buildButtonPanel();
 
		// add everything to content pane.
		add( matrixMain, BorderLayout.NORTH );
		add( matrixA, BorderLayout.WEST );
		add( matrixB, BorderLayout.EAST );
		add( buttonPanel, BorderLayout.SOUTH );
 
		pack();
		setVisible( true );
	}
 
	// build the button panel in the south region.
	private void buildButtonPanel()
	{
		// create a panel for buttons
		buttonPanel = new JPanel();
 
		// create the buttons
		calcButton = new JButton( "Calculate" );
 
		// register the action buttons
		calcButton.addActionListener( new MyButtonListener() );
 
		buttonPanel.add( calcButton );
	}
 
    private class MyButtonListener implements ActionListener 
    {
        public void actionPerformed( ActionEvent e ) 
        {
        	JOptionPane.showMessageDialog(null, MatrixMult.resultant);
 
        }
     }
 
    public static void main( String[] args ) 
    {
        new MatrixGUI();
 
    }
}

---

and finally the multiplier, which was supplied by the instructor. This is the class that I am having the most issues with, my program compiles but will not do the calculations. I don't think that I am initializing anything in this class, but I need to pull the string from input A and input B into here and do the calculations, then create a string array and send it to the GUI to display.

Code Java:

---

public class MatrixMult 
{
	public String inputA;
	public String inputB;
	public static String resultant;
 
    public static int[][] convertStringToArray( String str ) 
    {
 
        String[] rows = str.split("\\n"); // The \n character separates rows.
        int numRows = rows.length;
 
        // count the non white-space tokens, then count columns.
        int numCols = str.split("\\s+").length / numRows;
 
        Scanner strScanner = new Scanner( str );
 
        //System.out.println( numRows + " " + numCols );
 
        int[][] a = new int[ numRows ][ numCols ];
        for( int i = 0; i < numRows; i++ ) {
            for( int j = 0; j < numCols; j++ ) 
            {
                a[ i ][ j ] = strScanner.nextInt();
            }
        }
 
        return a;
    }
 
    public static String toString( int arr[][] ) {
        int aRows = arr.length;
        int aColumns = arr[0].length;
        String str = "";
        for(int i = 0; i < aRows; i++) { // aRow
            for(int j = 0; j < aColumns; j++) { // aColumn
                str += arr[ i ][ j ] + " ";
            }
            str += "\n";
        }
        return str;
    }
 
    public static void printArray( int a[][] ) {
        int aRows = a.length;
        int aColumns = a[0].length;
        for(int i = 0; i < aRows; i++) { // aRow
            for(int j = 0; j < aColumns; j++) { // aColumn
                System.out.print( a[ i ][ j ] + " " );
            }
            System.out.println();
        }
    }
 
    public static int[][] multiply( int a[][], int b[][] ) {
 
        int aRows = a.length;
        int aColumns = a[0].length; // all rows have the same length.
        int bRows = b.length;
        int bColumns = b[0].length; // all rows have the same length.
 
        if ( aColumns != bRows ) {
            throw new IllegalArgumentException(
                          "A:Rows: " + aColumns 
                          + " did not match B:Columns " + bRows + ".");
        }
 
        int[][] resultant = new int[aRows][bColumns];
 
        for(int i = 0; i < aRows; i++) { 
            for(int j = 0; j < bColumns; j++) { 
                for(int k = 0; k < aColumns; k++) { 
                    resultant[i][j] += a[i][k] * b[k][j];
                }
            }
        }
 
        return resultant;
    } 
 
    public String toString()
    {
    	return resultant;
    }
}

---

You have more than one variable named:resultant. This confuses me and probably confuses the compiler.
Rename one of them so there names are different.

Using static variables can cause problems. Can you remove the static and call the method to get the value.

ok, I changed all my variable so that they are not static anymore and renamed the string variable from resultant to finalMatrix....

I called it to the MatrixGUI, is that correct?

Code java:

---

 private class MyButtonListener implements ActionListener 
    {
        public void actionPerformed( ActionEvent e ) 
        {
        	MatrixMult matrixC = new MatrixMult();
        	JOptionPane.showMessageDialog(null, matrixC.finalMatrix);
 
        }
}

---

If that was correct, why is the results not being displayed?

What data is in the new MatrixMult class instance you create in the actionPerformed method?
You need to have a reference to the instance of the class that has the data.
When you create an instance of the MyButtonListener class, pass a reference to the MatrixMult instance with the data in the MyButtonHandler class's constructor and save it in the class so it can be used to get to the data.

I guess I am really confused now. There is nothing in the new MatrixMult class, but need to reference the toString in the MatrixMult which should hold the results in a String. I am not understanding what you mean by "When you create an instance of the MyButtonListener class, pass a reference to the MatrixMult instance with the data in the MyButtonHandler class's constructor and save it in the class so it can be used to get to the data." Am I to create a string in the MyButtonHandler class and then reference it to the MatrixMult?

What class creates the String you want in the listener?
How can you get a reference to that class in the listener?

One way is to pass a reference to the class with the String in the listener class's constructor.

create a constructor for the listener class
pass a reference in that constructor
have the constructor save the reference in a class variable
then the listener method can use that reference to call the method to get the String

The class that creates that String is the MatrixMult, and by creating a new MatrixMult referencing the toString return, right? And then in the MatrixMult I create a null reference to finalMatrix?

Change the MyButtonListener class to have a constructor that takes a MatrixMult object as a parameter.
Add a MatrixMult variable to the MyButtonListener class.
Have the constructor save its argument in the class's variable.
Use that variable to access the finalMatrix variable.

I feel like I am making you run in circles.....I am getting an error when I try to add a parameter to the MyButtonListener class stating that I need {} brackets which I have in there already. Adding another variable in this class, do you mean creating another String= null? and then String = finalMatrix?

Have you made the changes I recommended for the MyButtonListener class?

Is this what you are talking about? I am getting an error on this.

Code java:

---

 private class MyButtonListener implements ActionListener 
    {
        public void actionPerformed( ActionEvent e, String matrixC ) 
        {
        	matrixC = new MatrixMult().finalMatrix;
        	JOptionPane.showMessageDialog(null, matrixC);
 
        }
     }

---

No, I said a constructor. I said nothing about changing the parameters to the actionPerformed method.
Add a constructor to the MyButtonListener class that takes a MatrixMult type as a parameter.

Am I on the right track?

Code java:

---

 private class MyButtonListener implements ActionListener 
    {
    	public MyButtonListener()
    	{
 
    	}
        public void actionPerformed( ActionEvent e ) 
        {
 
        	MatrixMult matrixC = new MatrixMult();
        	JOptionPane.showMessageDialog(null, matrixC.finalMatrix);
 
        }
     }

---

Yes, now extend it
Add a constructor to the MyButtonListener class that takes a MatrixMult type as a parameter.

ok, I tried to add a parameter to the constructor, am I not doing this the right way. I have been trying to follow along with my book too.

Code java:

---

calcButton.addActionListener( new MyButtonListener() );
 
		buttonPanel.add( calcButton );
	}
 
    private class MyButtonListener implements ActionListener 
    {
 
    	public MyButtonListener(MatrixMult matrixC)
    	{
    		matrixC = new MatrixMult().finalMatrix;
    	}
        public void actionPerformed( ActionEvent e) 
        {
 
 
        	JOptionPane.showMessageDialog(null, " ");
 
        }
     }

---

I keep getting an error and Eclipse wants to remove the parameters

Now create a class variable of type MatrixMult in the MyButtonListener class
and in the constructor assign it the value of the MatrixMult argument passed in the constructor

ok...am I getting closer?

Code java:

---

// register the action buttons
		calcButton.addActionListener( new MyButtonListener() );
 
		buttonPanel.add( calcButton );
	}
 
    private class MyButtonListener implements ActionListener 
    {
    	MatrixMult c = new MatrixMult();
 
    	public MyButtonListener(MatrixMult finalMatrix)
    	{
    		c = new MatrixMult().finalMatrix;
    	}
 
		public void actionPerformed( ActionEvent e) 
        {
 
 
        	JOptionPane.showMessageDialog(null, " ");
 
        }
     }

---

No. I did not say to create new instances of the MatixMult class.
I said to assign the parameter passed to the constructor (finalMatrix) to the class variable: c.
<REMOVED> // assign the parameter to the local variable

You did define a class variable but then added a new statement.

There are a lot of basic terminology you dont understand:
constructor
define a variable
parameter to a constructor.
assign value of

yes I realize this and I am struggling to get through this class with a decent grade, so I am using my resources to learn from. Thank you for making me feel really stupid here. I am asking for guidance that's all I don't want you to give me the answer!

I have given you guidance in many posts. But you didn't understand the terminology.
Look at posts starting at post#4, 6 and 8.
I have been saying the same thing since then.

Sorry about giving you the answer in post#18. I'll remove it now.

now I am getting this error:

Code java:

---

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at MatrixGUI$MyButtonListener.actionPerformed(MatrixGUI.java:80)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)

---

and here is the code that I think is what you are asking for

Code java:

---

    private class MyButtonListener implements ActionListener 
    {
 
    	MatrixMult c = new MatrixMult();
 
    	public MyButtonListener(MatrixMult finalMatrix)
    	{
    		c = finalMatrix;
    	}
 
 
		public void actionPerformed( ActionEvent e) 
        {
 
 
        	JOptionPane.showMessageDialog(null, c.finalMatrix);
 
        }
     }

---

Now after all that and following examples I am getting a null pointer, so now I need to find out what variable is null, the "c" or finalMatrix right?

ok I added in print statements

Code java:

---

public void actionPerformed( ActionEvent e) 
        {
        	System.out.println("c" + c);
        	System.out.println("c" + c.finalMatrix);
 
        	JOptionPane.showMessageDialog(null, c.finalMatrix);
 
        }
     }

---

and found that cnull printed out but c.finalmatrix did not print out anything. So I need to initialize the finalMatrix variable in MatrixMult right?

What is the value of the variable passed to the constructor? If you pass a null value then you'll get a NPE.
You need to pass a reference to an existing MatrixMult object that has or will have the finalMatrix value set.

the value of the finalMatrix should be the resultant from multiplying the two initial matrices together. So I want to pass the resultant to the toString finalMatrix, does that make sense?

I'm not sure how that is related to the NPE you are getting. You must change the code so a valid non null value is passed to the MyButtonListener constructor.