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Lab 2: Exception Handling (II)

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

Multiple catch blocks

Possible to have multiple catch clauses for a single try statement

  • Evaluated in the order of the code – Bear in mind the Exception hierarchy when writing multiple catch clauses!

Example

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/* numbers.txt contains numbers. After reading number from file, prints its square on console  */

import java.io.*;

public class MCatchDemo {
    public static void main(String args[]) {
        try {
            // may throw FileNotFound & IOException
            FileReader fr = new FileReader("numbers.txt");
            BufferedReader br = new BufferedReader(fr);
            // read the line
            String s = br.readLine();
            // may throw NumberFormatException, if s is not no.
            int number = Integer.parseInt(s);
            System.out.println(number * number);
        } catch (NumberFormatException nfEx) {
            System.out.println(nfEx);
        } catch (FileNotFoundException fnfEx) {
            System.out.println(fnfEx);
        } catch (IOException ioEx) {
            System.out.println(ioEx);
        } finally {
            br.close();
        }
    }
}

We read everything from a file (numbers, floating values or text) as a String. That’s why we first convert it to numb and then print its square on console.

Compile & Execute

If file numbers.txt is not present in the same directory, the FileNotFoundException would be thrown during execution.

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javac MCatchDemo.java

Screenshot 1

If numbers.txt present in the same directory and contains a number, than hopefully no exception would be thrown.

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java MCatchDemo.java

Screenshot 2

throwsclause

  • If method is not interested in handling the exception than it can throw back the exception to the caller method using throws keyword.
  • Any exception that is thrown out of a method must be specified as such by a throws clause.

The following code examples will introduce you with writing & using throws clause.

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// File ThrowsDemo.java

import java.io.*;

public class ThrowsDemo {
    // contains file reading code
    public static void method2() {
        try {
            FileReader fr = new FileReader("strings.txt");
            BufferedReader br = new BufferedReader(fr);
            //read the file line by line
            String s = br.readLine();
            System.out.println(s);
        } catch (IOException ioEx) {
            ioEx.printStackTrace();
        }
    }

    public static void method1() {
        method2();
    }

    public static void main(String args[]) {
        ThrowsDemo.method1();
    }
}

Example Code: ThrowsDemo.java

The ThrowsDemo.java contains two methods namely method1 & method2 and one main method. The main method will make call to method1 and than method1 will call method2. The method2 contains the file reading code. The program looks like one given below

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// File ThrowsDemo.java

import java.io.*;

public class ThrowsDemo {
    // contains file reading code
    public static void method2() throws IOEception {
        FileReader fr = new FileReader("strings.txt");
        BufferedReader br = new BufferedReader(fr);
        //read the line form file
        String s = br.readLine();
        System.out.println(s);
    }

    // calling method2 & handling incoming exception
    public static void method1() {
        try {
            method2();
        } catch (IOException ioEx) {
            ioEx.printStackTrace();
        }
    }

    public static void main(String args[]) {
        ThrowsDemo.method1();
    }
}

Compile & Execute

If file strings.txt is not present in the same directory, method2 will throw an exception that would be caught by method1 and the printStackTrace method will print the full calling history on console. The above scenario is shown in the output below:

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javac ThrowsDemo.java
java ThrowsDemo

If file strings.txt exist there, then hopefully a line would be displayed on the console.

Screenshot 3 Screenshot 4

throw

To manually throw an exception, keyword throw is used.

Exercise Solution:

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import javax.swing.*;

public class Main {


    public static void main(String[] args) {
        // TODO code application logic here
        JOptionPane j = new JOptionPane();

        String in = j.showInputDialog("Enter number to power it:");
        int x;

        try {
            x = Integer.parseInt(in);
            System.out.println(x + " * " + x + " = " + Power(x));
        } catch (OutOfRange e) {
            System.err.println(e.getMessage());
        } catch (NumberFormatException e) {
            System.err.println(e.getMessage());
        }finally {
            System.out.println("This is Finally Block");
        }
        System.out.println("This is Next Statement");
    }

    public static int Power(int x) throws OutOfRange {
        if (x < 0 || x > 100) {
            throw new OutOfRange(0, 100);
        }
        return x * x;
    }

}

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class OutOfRange extends Exception {

    private int min;
    private int max;
    public OutOfRange(int min, int max) {
        this.min = min;
        this.max = max;
    }

    @Override
    public String getMessage() {
        return "Number must be between " + min + " and " + max;
    }
}

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. To handle the outOfRange exception, create a custom exception class that can be thrown in case a number is out of range.

Deliverables and Submission

Submission Instructions


Submit on GitHub Classroom

Section: IT1 Section: IT2 Section: IT3

If your instructor is using GitHub classroom, then you should click on your class submission link, link your GitHub username to your name if you have not already done so, accept the assignment, clone the repository into your local development environment, and push the code to the remote repository on GitHub. Please make sure that your written answers are included in either a README (Markdown) file or a PDF file.

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