Lab 1: Exception Handling (I)

• Course Name: Advanced Programming
• Course Code: CPIT-305
• Course Instructor: Dr. Rayan Mosli

What is an Exception?

Definition: An exception is an event, which occurs during the execution of a program that disrupts the normal flow of the program’s instructions.

Unchecked Exceptions

Example Code: UcException.java

The following program takes one command line argument and prints it on the console

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// File UcException.java
public class UcException {
    public static void main(String args[]) {
        System.out.println(args[0]);
    }
}

Compile & Execute

If we compile & execute the above program without passing any command line argument, an ArrayIndexOutOfBoundsException would be thrown. This is shown in the following picture.

Why?

Since we have passed size of String args[ ] is zero, and we have tried to access the first element (first element has index zero) of this array. From the output window, you can find out, which code line causes the exception to be raised. In the above example, it is System.out.println(args[0]);

Modify UcException.java

Though it is not mandatory to handle unchecked exceptions we can still handle Unchecked Exceptions if we want to. These modifications are shown in bold.

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// File UcException.java
public class UcException {
    public static void main(String args[]) {
        try {
            System.out.println(args[0]);
        } catch (IndexOutOfBoundsException ex) {
            System.out.println("You forget to pass command line argument");
        }
    }
}

The possible exception that can be thrown is IndexOutOfBoundsException, so we handle it in the catch block.

Compile & Execute

Checked Exceptions

Example Code: CException.java The following program reads a line (hello world) from a file and prints it on the console. The File reading code written inside the main read one line from a file and prints that to console.

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// File CException.java

import java.io.*;
public class CException {
    public static void main(String args[]) {
        FileReader fr = new FileReader("input.txt");
        BufferedReader br = new BufferedReader(fr);
        //read the line from file
        String line = br.readLine();
        System.out.println(line);
    }
}

Compile & Execute

If you try to compile this program, the program will not compile successfully and displays the message of unreported exception. This happens when there is code that can generate a checked exception but you have not handled that exception. Remember checked exceptions are detected by compiler. As we early discussed, without handling Checked exception, our program won’t compile.

Modify CException.java

As we have discussed earlier, it is mandatory to handle checked exceptions. In order to compile the code above, we modify the above program so that file reading code is placed inside a try block. The expected exception (IOException) that can be raised is caught in catch block.

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// File CException.java

import java.io.*;

public class CException {
    public static void main(String args[]) {
        try {
            FileReader fr = new FileReader("input.txt");
            BufferedReader br = new BufferedReader(fr);
            //read the line form file
            String line = br.readLine();
            System.out.println(line);
        } catch (IOExceptoin ex) {
            System.out.println(ex);
        }
    }
}

The code line written inside the catch block will print the exception name on the console if exception occurs Compile & Execute After making changes to your program, it would compile successfully. On executing this program, hello world would be displayed on the console Note: Before executing, make sure that a text file named input.txt must be placed in the same directory where the program is saved. Also write hello world in that file before saving it.

The finally block

The finally block always executes regardless of exception is raised or not while as you remembered the catch block only executes when an exception is raised.

Example Code : FBlockDemo.java

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// File FBlockDemo.java

import java.io.*;

public class FBlockDemo {
    public static void main(String args[]) {
        try {
            FileReader fr = new FileReader("strings.txt");
            BufferedReader br = new BufferedReader(fr);
            //read the line form file
            String line = br.readLine();
            System.out.println(line);
        } catch (IOExceptoin ex) {
            System.out.println(ex);
        } finally {
            System.out.println("finally block always execute");
        }
    }
}

Compile & Execute

The program above, will read one line from string.txt file. If string.txt is not present in the same directory the FileNotFoundException would be raised and catch block would execute as well as the finally block.

If string.txt is present there, no such exception would be raised but still finally block executes. This is shown in the following output diagram.

Exercise:

Write a program in java that takes in a value from a user to find the square of an integer from 0 to 100. If the value is not an integer or within the range, then print the exception.

Deliverables and Submission