Thread: Problem with NullPointerException?

Ok, so I'm working with returning an array in my program, StudentGrades

In this block, once my button construct is hit... it creates the array

if(e.getActionCommand().equals("Construct"))
			{
				sg =  new StudentGrades(students);
				scores = new int[students];				
 
				JOptionPane.showMessageDialog(null,"Created");
			}

In the next block, once Action Button 1 is pressed, it should return a copy of the array

if(e.getActionCommand().equals("Action 1"))
			{
				scores = sg.getTestGrades();
 
				for(int i = 0; i < scores.length; i++)
				{
					area[0].setVisible(true);
					area[0].append(scores[i]+ " ");
				}
			}

Here's the getTestGrades method

public int[] getTestGrades()
	{
		int[] testGrades = new int[studentGrades.length];
		int value = 0;
		for(int i = 0; i < studentGrades.length; i++)
		{
			testGrades[i]= studentGrades[i];
 
		}
		return testGrades;
 
	}

And here's the error that comes up

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at StudentGUI.actionPerformed(StudentGUI.java:89)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.jav a:6267)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3267)
at java.awt.Component.processEvent(Component.java:603 2)
at java.awt.Container.processEvent(Container.java:204 1)
at java.awt.Component.dispatchEventImpl(Component.jav a:4630)
at java.awt.Container.dispatchEventImpl(Container.jav a:2099)
at java.awt.Component.dispatchEvent(Component.java:44 60)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4168)
at java.awt.Container.dispatchEventImpl(Container.jav a:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478 )
at java.awt.Component.dispatchEvent(Component.java:44 60)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 599)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

Maybe you never initialized the array.

Or the button.

It could be because scores is defined inside an if statement and may not have a scope beyond that if statement so it could be there but be null.

if(e.getActionCommand().equals("Action 1"))
{
scores = sg.getTestGrades();

for(int i = 0; i < scores.length; i++)
{
area[0].setVisible(true);
area[0].append(scores[i]+ " ");
}
}

hmmmm....did you mean area[i] instead of area[0]?

No, I mean area[0]. I specifically have an array of 8 JTextAreas, used in a JLayeredPane.

Here's the fields in StudentGUI

 private int[] scores;
		private int[] sorted;
		static JButton[] action;
		static JTextArea[] area;

You only need to tell it to set visible once. Sometimes I've done it more than once, by accident, or just to figure out why it wasn't showing up, but a for loop to keep telling it that seems kind of silly.

A good idea would be to define area[], sq, and especially studentGrades.

In fact, it appears that StudentGrades is a class.

While you can make a class an array, I think that perhaps that you should probably change

public int[] getTestGrades()
	{
		int[] testGrades = new int[studentGrades.length];
		int value = 0;
		for(int i = 0; i < studentGrades.length; i++)
		{
			testGrades[i]= studentGrades[i];
 
		}
		return testGrades;
 
	}

to

public int[] getTestGrades()
	{
		int[] testGrades = new int[scores.length];
		int value = 0;
		for(int i = 0; i < scores.length; i++)
		{
			testGrades[i]= scores[i];
 
		}
		return testGrades;
 
	}

and try to get a method that returns the number of students, like

Then define sq in your constructor.

area may not have been defined, I don't have the whole code so I can't exactly say.

I'm assuming area is a JTextArea array.

It appears you only made it for element 0, so why do an array at all?

Even if StudentGrades is an array, it's possible you never put anything in it.

Was sq first defined in your actionPerformed(ActionEvent e) method, inside that if statement for the button "Construct"?

sg = new StudentGrades(students);

If so, then it won't know what sq is because it didn't go to the if statement it won't have been initialized, and it won't throw a compiler error because sq has been defined in your constructor probably, but it didn't have it initialized due to scope, and so you called a method with an uninitialized variable, which throws a Null Pointer Exception.

Define sq before the if statements if you can. That way it'll be accessible in all the if statements.

Did you ever set action[0] to visible?

StudentGrades sg = null;

Why null?

StudentGrades sq = null;
scores = sg.getTestGrades();

yeah, your first if statement may define sq, but if the if condition is false, it won't go there. So it'll still be null.

Because it order for my program requirements to be met, it had to be instantiated at some point, but the program requirements wouldn't have been met if I just had it set as

StudentGrades sg = null;

I've also tested the first if statement, if it's true it'll come up with a box saying created. So it does create it.

Well, you didn't define it before

scores = sg.getTestGrades();

so it's still null.

That's why I was asking why it is null there.

You could always have the method click the construct button if it is null.

if (sq == null)
{
action[0].doClick();
}

scores = sg.getTestGrades();

In short, you never assign sq a value, in that if (e.getActionCommand().equals("Action 1"))

so it's still null.

This is a scope problem. The code may create an instance of StudentGrades, but when the actionPerformed method exits it ends the variable scope - eg no more StudentGrades instance. Move it to be an instance variable of the class (eg outside the method) so once it is created there still is a reference to the object.

Ah, I wasn't thinking that. I'll try that. Thanks.

And thank you javapenguin.

Originally Posted by copeg

This is a scope problem. The code may create an instance of StudentGrades, but when the actionPerformed method exits it ends the variable scope - eg no more StudentGrades instance. Move it to be an instance variable of the class (eg outside the method) so once it is created there still is a reference to the object.

I thought that might be the case.