On 8/28/2013 9:22 AM, Jeremy Isnard wrote:
Hi,
This exercise should throw different exceptions through a box
containing Radio buttons that trigger exceptions of different kinds ;
fact is that for most of them -those using the double[] a declared-
only a NullPointerException is thrown.
I assumed it is because each array item is null as they are never
defined.
I tried to modify the code to trigger the proper exceptions, so I
started to fill the array "a" through a loop (basically a[i]=i).
Problem is that I do not get any exceptions thrown for the
Math.sqrt(-1), the dividing by zero nor for the overflow... and I do
not understand why.
First of all, excellent experimentation.
The following is my explanation of what you've observed
- array size gets determined when an array is created not when an
array is declared
In other words, "private double[ ] a" just declares the type of
array but not create one. So there is no a [0], a[1], yet.
And if you try to access it, either you will experience
ArrayOutOfBounds exception of NullPointexception.
- "double" is a bit funny animal. Dividing a number by double type
like "3/0.0" results in NaN (meaning Not a number)
not ArtithmeticException. Dividing a number by int like "3/0"
results in ArtithmeticException.
So once you create an array with "private double[] = new double[9]", you
no longer experience
NullPoint exception.
Having said this, I realized the example code is not really a good
one. I am modifying the code
as following
public class Main extends JFrame implements ActionListener {
private int[] a = new int[10];
private JRadioButton divideByZeroButton;
private JRadioButton badCastButton;
private JRadioButton arrayBoundsButton;
private JRadioButton nullPointerButton;
private JRadioButton negSqrtButton;
private JRadioButton overflowButton;
private JRadioButton noSuchFileButton;
private JRadioButton throwUnknownButton;
public Main() {
// Create a JPanel and GridLayout
JPanel p = new JPanel();
p.setLayout(new GridLayout(8, 1));
// Create buttons and add them to the panel
ButtonGroup g = new ButtonGroup();
divideByZeroButton = addRadioButton("Divide by zero", g, p);
badCastButton = addRadioButton("Bad cast", g, p);
arrayBoundsButton = addRadioButton("Array bounds", g, p);
nullPointerButton = addRadioButton("Null pointer", g, p);
negSqrtButton = addRadioButton("sqrt(-1)", g, p);
overflowButton = addRadioButton("Overflow", g, p);
noSuchFileButton = addRadioButton("No such file", g, p);
throwUnknownButton = addRadioButton("Throw unknown", g, p);
getContentPane().add(p);
}
private JRadioButton addRadioButton(String s, ButtonGroup g, JPanel
p) {
JRadioButton button = new JRadioButton(s, false);
button.addActionListener(this);
g.add(button);
p.add(button);
return button;
}
// Trigger and catch various exceptions
public void actionPerformed(ActionEvent evt) {
try {
Object source = evt.getSource();
if (source == divideByZeroButton) {
int x = 3 / 0;
} else if (source == badCastButton) {
Frame f = (Frame) evt.getSource();
} else if (source == arrayBoundsButton) {
a[1] = a[10];
} else if (source == nullPointerButton) {
Frame f = null;
f.setSize(200, 200);
} else if (source == negSqrtButton) {
double x = Math.sqrt(-1);
System.out.println("Is Math.sqrt(-1) result NaN? " +
Double.isNaN(x));
} else if (source == overflowButton) {
a[1] = 1000 * 1000 * 1000 * 1000;
int n = (int) a[1];
} else if (source == noSuchFileButton) {
FileInputStream is = new FileInputStream("Java Source
and Support");
} else if (source == throwUnknownButton) {
throw new UnknownError();
}
} catch (RuntimeException e) {
System.out.println("I Caught RuntimeException myself: " + e);
} catch (Exception e) {
System.out.println("I Caught Exception myself: " + e);
}
}
public static void main(String[] args) {
JFrame frame = new Main();
frame.setSize(150, 200);
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
frame.setVisible(true);
}
}
I tried to move by loop around (maybe as it is placed at the beginning
of actionPerformed() it overrides the assigned values of the following
multiple if() section?) but it does not seem to work any better.
Any help ?
*Code : *
import java.awt.Frame;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.FileInputStream;
import javax.swing.ButtonGroup;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JRadioButton;
public class Main extends JFrame implements ActionListener {
*private double[] a = new double[9];*
private JRadioButton divideByZeroButton;
private JRadioButton badCastButton;
private JRadioButton arrayBoundsButton;
private JRadioButton nullPointerButton;
private JRadioButton negSqrtButton;
private JRadioButton overflowButton;
private JRadioButton noSuchFileButton;
private JRadioButton throwUnknownButton;
public Main() {
// Create a JPanel and GridLayout
JPanel p = new JPanel();
p.setLayout(new GridLayout(4, 2));
// Create buttons and add them to the panel
ButtonGroup g = new ButtonGroup();
divideByZeroButton = addRadioButton("Divide by zero", g, p);
badCastButton = addRadioButton("Bad cast", g, p);
arrayBoundsButton = addRadioButton("Array bounds", g, p);
nullPointerButton = addRadioButton("Null pointer", g, p);
negSqrtButton = addRadioButton("sqrt(-1)", g, p);
overflowButton = addRadioButton("Overflow", g, p);
noSuchFileButton = addRadioButton("No such file", g, p);
throwUnknownButton = addRadioButton("Throw unknown", g, p);
getContentPane().add(p);
}
private JRadioButton addRadioButton(String s, ButtonGroup g,
JPanel p) {
JRadioButton button = new JRadioButton(s, false);
button.addActionListener(this);
g.add(button);
p.add(button);
return button;
}
// Trigger and catch various exceptions
public void actionPerformed(ActionEvent evt) {
*for (int i=0; i<9; i++){
a[i]=i;
}*
try {
Object source = evt.getSource();
if (source == divideByZeroButton) {
a[1] = a[1]/(a[1]-a[1]);
} else if (source == badCastButton) {
Frame f = (Frame) evt.getSource();
} else if (source == arrayBoundsButton) {
a[2] = a[10];
} else if (source == nullPointerButton) {
Frame f = null;
f.setSize(200, 200);
} else if (source == negSqrtButton) {
a[3] = Math.sqrt(-1);
} else if (source == overflowButton) {
a[4] = 1000 * 1000 * 1000 * 1000 * 1000;
int n = (int) a[4];
} else if (source == noSuchFileButton) {
FileInputStream is = new FileInputStream("Java Source
and Support");
} else if (source == throwUnknownButton) {
throw new UnknownError();
}
} catch (RuntimeException e) {
System.out.println("I Caught RuntimeException myself: " + e);
} catch (Exception e) {
System.out.println("I Caught Exception myself: " + e);
}
}
public static void main(String[] args) {
JFrame frame = new Main();
frame.setSize(150, 200);
frame.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
frame.pack();
frame.setVisible(true);
}
}
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