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
Code :
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
Code :
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
Code :
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
Quote:
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)
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
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]?
Re: Problem with NullPointerException?
No, I mean area[0]. I specifically have an array of 8 JTextAreas, used in a JLayeredPane.
Here's the fields in StudentGUI
Code :
private int[] scores;
private int[] sorted;
static JButton[] action;
static JTextArea[] area;
Re: Problem with NullPointerException?
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
Code java:
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
Code java:
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.
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
Well, I do have it defined.
Here's the full code
Code :
[highlight = Java] import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Random;
public class StudentGUI extends JFrame
implements ActionListener
{
private int[] scores;
private int[] sorted;
static JButton[] action;
static JTextArea[] area;
static JFrame frame;
public StudentGUI()
{
super("StudentGrades.java");
action = new JButton[9];
action[0] = new JButton("Construct");
action[0].addActionListener(this);
action[0].setMnemonic(KeyEvent.VK_C);
for(int i = 1; i < action.length; i++)
{
action[i] = new JButton("Action " + i);
action[i].addActionListener(this);
action[i].setVisible(true);
}
frame = new JFrame("Action Window");
frame.getLayeredPane();
frame.setBounds(200,200,140,140);
frame.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);
frame.setVisible(true);
area = new JTextArea[8];
for(int i = 0; i < area.length; i++)
{
area[i] = new JTextArea();
area[i].setLineWrap(true);
area[i].setEditable(false);
area[i].setVisible(false);
frame.add(area[i]);
}
Container c = getContentPane();
JPanel buttonPanel = new JPanel(new GridLayout(1,9));
for(int i = 0; i < action.length; i++)
{
buttonPanel.add(action[i]);
}
c.setBackground(Color.white);
c.add(buttonPanel, BorderLayout.CENTER);
}
public void actionPerformed(ActionEvent e)
{
Random r = new Random();
StudentGrades sg = null;
int students = r.nextInt(21)+1;
if(e.getActionCommand().equals("Construct"))
{
sg = new StudentGrades(students);
scores = new int[students];
JOptionPane.showMessageDialog(null,"Created");
}
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]+ " ");
}
}
if(e.getActionCommand().equals("Action 2"))
{
sorted = sg.selectionSortB(scores);
for(int i = 0; i < scores.length; i++)
{
area[1].setVisible(true);
area[1].append(sorted[i] + " ");
}
}
/*if(e.getActionCommand().equals(action[3].getText()))
{
area[2].setVisible(true);
if(sg.issEqual(scores)== true)
area[2].append("The Arrays are Equal");
else
area[2].append("The Arrays are not Equal");
}*/
if(e.getActionCommand().equals(action[4].getText()))
{
int highest = sg.highestGrade(sorted);
area[3].setVisible(true);
area[3].append("The Highest Value is " + highest);
}
if(e.getActionCommand().equals(action[5].getText()))
{
int lowest = sg.lowestGrade(sorted);
area[4].setVisible(true);
area[4].append("The lowest Value is " + lowest);
}
if(e.getActionCommand().equals(action[6].getText()))
{
int median = sg.medianGrade(sorted);
area[5].setVisible(true);
area[5].append("The median Value is " + median);
}
if(e.getActionCommand().equals(action[7].getText()))
{
double average = sg.averageGrade(sorted);
area[6].setVisible(true);
area[6].append("The Average Value is " + average);
}
if(e.getActionCommand().equals(action[8].getText()))
{
int mode = sg.modeGrade(sorted);
area[7].setVisible(true);
area[7].append("The Mode is " + mode);
}
}
}[/highlight]
and StudentGrades
Code :
import java.util.Random;
public class StudentGrades
{
private int[] studentGrades;
public StudentGrades(int students)
{
Random r = new Random();
studentGrades = new int[students];
for(int i = 0; i < studentGrades.length; i++)
{
studentGrades[i] = r.nextInt(101);
}
}
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;
}
public int[] selectionSortB(int[] nums)
{
int size = nums.length;
// Same as version A, but sorts in ascending order
int first, current, least, temp;
for(first = 0; first < size; first = first + 1)
{
least = first;
for(current = first+1; current < size; current = current + 1)
{ if (nums[current] < nums[least])
{ least = current; }
}
temp = nums[least];
nums[least] = nums[first];
nums[first] = temp;
}
return nums;
}
/*public boolean isEqual(int[] arr)
{
}*/
public int highestGrade(int[] arr)
{
int max = 0;
for(int i = 0; i < arr.length; i++)
{
if(arr[i] > max)
{
max = arr[i];
}
}
return max;
}
public int lowestGrade(int[] arr)
{
int min = arr[0];
for(int i = 1; i < arr.length; i++)
{
if(arr[i] < min)
min = arr[i];
}
return min;
}
public double averageGrade(int[] arr)
{
double total = 0.0;
for(int i = 0; i < arr.length; i++)
{
total += (double)arr[i];
}
double avg = total / arr.length;
return avg;
}
public int medianGrade(int[] arr)
{
int median = (arr[0] + arr[arr.length - 1]) / 2 ;
return median;
}
public int modeGrade(int[] arr)
{
int[] count = new int[arr.length];
for(int i = 0; i < arr.length; i++)
{
count[i]++;
}
int max = 0;
for(int i = 0; i < count.length; i++)
{
if(count[i] > max)
{
max = count[i];
}
}
return count[max];
}
}
Re: Problem with NullPointerException?
Did you ever set action[0] to visible?
Re: Problem with NullPointerException?
StudentGrades sg = null;
Why null?
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
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;
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
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.
Re: Problem with NullPointerException?
Ah, I wasn't thinking that. I'll try that. Thanks.
And thank you javapenguin.
Re: Problem with NullPointerException?
Quote:
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.
