Exception Handling

You should refer to the book the book p243-268 or Java tutorial Lesson: Handling Errors with Exceptions

Introduction

What is an exception?

An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.

Examples :

Here are some common exceptions that you may have seen it before

Access an out-of-bounds array element. An ArrayIndexOutOfBoundsException occurs.
Access members of a null referece. A NullPointerException occurs.
An integer "divide by zero". An ArithmeticException occurs.
Parse a string to an integer. A NumberFormatException may occur if the string not in number format.

Here are some more examples:

Open a file that does not exist. A FileNotFoundException occurs.
Input or ouput error. An IOException may occur.

The first four exceptions are in the package java.lang . They all extend RuntimeException. That means, the exceptions occur during the runtime.
Contrary to runtime exceptions are something called checked exceptions. They are checked by the compiler. The compiler checked if the exceptions are properly handled.
We will concentrate on the runtime exceptions in this short introductory notes on exception.

First Encounter with Java Exceptions

ExTest.java

public class ExTest {
    public static void main(String[] args) {
        int a = 5;
	int b = 0;
	System.out.println(a/b);
    }
}

Output

The program can be compiled.
After you type

java 
ExTest

, the output is

Exception is thread "main" java.lang.ArithmeticException:/ by zero. 
        at ExTest.main(ExTest.java:5)

ExTest2.java

public class ExTest2 {
    public static void main(String[] args) {
        String[] countries = {"USA", "Canada", "Mexico", "Japan"};
	System.out.println(countries[10]);
    }
}

Output

The program can be compiled.
After you type

java 
ExTest2

, the output is

Exception is thread "main" java.lang.ArrayIndexOutOfBoundsException
        at ExTest2.main(ExTest2.java:4)

ExTest3.java

import javax.swing.*;

public class ExTest3 {
    public static void main(String[] args) {
        JLabel[] labels = new JLabel[10];
	labels[0].setText("Hi");
    }
}

Output

The program can be compiled.
After you type

java 
ExTest3

, the output is

Exception is thread "main" java.lang.NullPointerException
        at ExTest3.main(ExTest3.java:6)

Try and Catch

Syntax

try {
    // your code
} catch (FirstException ex) {
    // your code for handling the occurrence of FirstException
} catch (SecondException ex) {
    // your code for handling the occurrence of SecondException
} catch(.....) {
  ....
}

The try block

try {
   // your code
}

If an exception occurs within the try statement, that exception is handled by the appropriate exception handler associated with this try statement.
A try statement must be accompanied by at least one catch block or one finally block.

The catch block

You associate exception handlers with a try statement by providing one or more catch blocks directly after the try block:

try {
    . . . 
} catch ( . . . ) {
    . . . 
} catch ( . . . ) {
    . . . 
} . . .

The general form is

catch (SomeException variableName) {
    // your exceptional handler
    // codes for handling occurrence of SomeException
}

The catch block contains a series of legal Java statements.
These statements are executed if the exception handler is invoked. The steps are :
1. when an exception occurs in try block.
2. The exception matches an exception declared in catch.
3. The code inside the catch block will be executed.
4. After the statements inside the catch block are executed, the flow of the program leaves the whole try-catch block.

Examples

In below we rewite the above programs.

ExTest.java

public class ExTest {
    public static void main(String[] args) {
        int a = 5;
	int b = 0;
	try { 
	    System.out.println(a/b);
	} catch (ArithmeticException ex) {
	    System.out.println("You can't divide a number by zero!");
	}
	System.out.println("Program ends");
    }
}

Output

You can't divide a number by zero!
Program ends.

Explanation

a/b is 5/0. An ArithmeticException occurs.
Search through the catch blocks. The first one (and the only one) matches the exception. So the codes in the block are executed.
After the codes are excuted. The next code is the code outside the whole try-catch block.

ExTest2.java

public class ExTest2 {
    public static void main(String[] args) {
        String[] countries = {"USA", "Canada", "Mexico", "Japan"};
	try {
	    System.out.println(countries[10]);
	} catch(ArrayIndexOutOfBoundsException ex) {
	    System.out.println("Array index out of bounds");
	} catch(NullPointerException ex) {
	    System.out.println("Null pointer");
	}
    }
}

Output

Array index out of bounds

Explanation

ArrayIndexOutOfBoundsException occurs in the try block.
compare the exceptions in the catch blocks. The first one is matched.
The codes inside the catch blocked are executed.

ExTest3.java

import javax.swing.*;

public class ExTest3 {
    public static void main(String[] args) {
        JLabel[] labels = new JLabel[10];
	try {
	     labels[0].setText("Hi");
	} catch(ArrayIndexOutOfBoundsException ex) {
	    System.out.println("Array index out of bounds");
	} catch(NullPointerException ex) {
	    System.out.println("Null pointer");
	}
    }
}

Output

Null pointer

Explanation

NullPointerException occurs in the try block.
compare the exceptions in the catch blocks. The second one is matched.
The codes inside the catch blocked are exected.

Division.java

You can get the program here.
Enter in the textfields and click button.
dvi1

   1:import java.awt.*;
   2:import javax.swing.*;
   3:import java.awt.event.*;
   4:
   5:public class Division extends JFrame {
   6:    private JTextField numTextField1 = new JTextField(10);
   7:    private JLabel divideLabel = new JLabel("/", JLabel.CENTER);
   8:    private JTextField numTextField2 = new JTextField(10);
   9:    private JLabel equalLabel = new JLabel("=", JLabel.CENTER);
  10:    private JTextField resultTextField = new JTextField(15);
  11:    private JButton button = new JButton("Click here for result");
  12:    
  13:    public Division() {
  14:       
  15:        getContentPane().setLayout(new FlowLayout());
  16:        getContentPane().add(numTextField1);
  17:        getContentPane().add(divideLabel);
  18:        getContentPane().add(numTextField2);
  19:        getContentPane().add(equalLabel);
  20:        getContentPane().add(resultTextField);
  21:        getContentPane().add(button);
  22:        resultTextField.setEditable(false);
  23:        button.addActionListener(new ButtonListener());
  24:    }
  25:    
  26:    class ButtonListener implements ActionListener {
  27:        public void actionPerformed(ActionEvent e) {
  28:            int num1, num2, result;
  29:            try {
  30:                num1 = Integer.parseInt(numTextField1.getText());
  31:                num2 = Integer.parseInt(numTextField2.getText());
  32:                result = num1/num2;
  33:            } catch (NumberFormatException ex) {
  34:                resultTextField.setText("Please enter numbers");
  35:                return;
  36:            } catch (ArithmeticException ex) {
  37:                resultTextField.setText("Cannot divided by 0");
  38:                return;
  39:            }
  40:            resultTextField.setText(Integer.toString(result));
  41:        }
  42:    }
  43:    
  44:     public static void main(String[] args) {
  45:        JFrame f = new Division();
  46:        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  47:        f.pack();
  48:        f.setVisible(true);
  49:    }
  50:}

Highlight

Line 26-42. Handle different exceptions.