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An exception is an unusual situation that could
occur in your program. As a programmer, you should
anticipate any abnormal behavior that could be caused by the user
entering wrong information that could otherwise lead to unpredictable
results. The ability
to deal with a program’s abnormal behavior is called
exception handling. Java provides many keywords and built-in classes to handle an exception.
You proceed with a top-down approach.
To start handling an exception, create a section of code
that includes the normal flow of the program. This section starts with the try
keyword followed by an opening curly bracket and a closing curly bracket. The
section in the curly brackets is the body. In it, you write the normal code.
Here is an example:
import java.util.Scanner;
public class Exercise {
public static void main(String[] args) {
double side;
Scanner scnr = new Scanner(System.in);
try {
System.out.println("Square Processing");
System.out.print("Enter Side: ");
side = scnr.nextDouble();
System.out.println("\nSquare Characteristics");
System.out.printf("Side: %.2f\n", side);
System.out.printf("Perimeter: %.2f\n", side * 4);
}
}
}
Just after the try section, that is, after the closing curly
bracket of try, you create a new section that uses the catch keyword.
This new section uses the following formula:
catch(ExceptionClass) { WhatToDo }
As you can see, this section starts with the catch
keyword with parentheses. The parentheses behave like those of a method. You
must pass an argument. At a minimum, you can pass a class named Exception
and a name for the argument. After the closing the parenthesis, create a body
for the section. Like a normal body, this starts with an opening curly bracket
and ends with a closing curly bracket.
Based on this, the basic formula of exception handling is:
try {
// Try the program flow
}
catch(Exception arg) {
// Catch the exception
}
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Here is an example of a program that handles an exception:
import java.util.Scanner;
public class Exercise {
public static void main(String[] args) {
double side = 0;
Scanner scnr = new Scanner(System.in);
try {
System.out.println("Square Processing");
System.out.print("Enter Side: ");
side = scnr.nextDouble();
System.out.println("\nSquare Characteristics");
System.out.printf("Side: %.2f\n", side);
System.out.printf("Perimeter: %.2f\n", side * 4);
}
catch(Exception e) {
}
}
}
Here is an example of running the program:
Square Processing
Enter Side: 22.98
Square Characteristics
Side: 22.98
Perimeter: 91.92
This is another example of running the same program:
Square Processing
Enter Side: Yes
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 Practical
Learning: Introducing Vague Exceptions
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- Change the Central.java file as
follows:
package geometry8b;
import java.util.Scanner;
import Quadrilaterals.Parallelograms.Rectangle;
public class Central {
private static double specifyMeasure(String name) {
double value = 0.00;
Scanner scnr = new Scanner(System.in);
try {
System.out.print("Enter the " + name + ": ");
value = scnr.nextDouble();
}
catch(Exception exc) {
}
return value;
}
private static void show(Rectangle figure) {
System.out.println("Rectangle Characteristics");
System.out.printf("Length: %.2f\n", figure.getLength());
System.out.printf("Height: %.2f\n", figure.getHeight());
System.out.printf("Perimeter: %.2f\n", figure.calculatePerimeter());
System.out.printf("Area: %.2f\n", figure.calculateArea());
}
public static void main(String[] args) {
double dLength = specifyMeasure("length");
double dHeight = specifyMeasure("height");
Rectangle recto = new Rectangle(dLength, dHeight);
show(recto);
}
}
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- Press F6 to execute the application
- When asked to enter the length, type What and press Enter
- When asked to enter the height, type 24.55 and press Enter
Enter the length: What
Enter the height: 24.55
Rectangle Characteristics
Length: 0.00
Height: 24.55
Perimeter: 49.10
Area: 0.00
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Exceptions and Custom Messages
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As mentioned already, if an error occurs when processing the
program in the try section, the compiler transfers the processing to the next
catch section. You can then use the catch section to deal with the error. At a
minimum, you can display a message to inform the user.
To display a message resulting from an exception, create it
in the catch block. Here is an example:
import java.util.Scanner;
public class Exercise {
public static void main(String[] args) {
double side = 0;
Scanner scnr = new Scanner(System.in);
try {
System.out.println("Square Processing");
System.out.print("Enter Side: ");
side = scnr.nextDouble();
System.out.println("\nSquare Characteristics");
System.out.printf("Side: %.2f\n", side);
System.out.printf("Perimeter: %.2f\n", side * 4);
}
catch(Exception exc) {
System.out.println("The value you provided for the side is invalid");
}
}
}
Here is an example of running the program:
Square Processing
Enter Side: 24
Square Characteristics
Side: 24.00
Perimeter: 96.00
Here is another example of running the program:
Square Processing
Enter Side: 26GW
The value you provided for the side is invalid
You can provide any message you want. The idea is to be as
help and explicit as possible so the user may know what the problem was.
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Practical
Learning: Displaying Custom Messages |
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- To display custom messages to the user, change the OrderProcessing.cs file as
follows:
package geometry8b;
import java.util.Scanner;
import Quadrilaterals.Parallelograms.Rectangle;
public class Central {
private static double specifyMeasure(String name) {
double value = 0.00;
Scanner scnr = new Scanner(System.in);
try {
System.out.print("Enter the " + name + ": ");
value = scnr.nextDouble();
}
catch(Exception exc) {
System.out.println("Something went wrong!!!");
}
return value;
}
private static void show(Rectangle figure) {
System.out.println("Rectangle Characteristics");
System.out.printf("Length: %.2f\n", figure.getLength());
System.out.printf("Height: %.2f\n", figure.getHeight());
System.out.printf("Perimeter: %.2f\n", figure.calculatePerimeter());
System.out.printf("Area: %.2f\n", figure.calculateArea());
}
public static void main(String[] args) {
double dLength = specifyMeasure("length");
double dHeight = specifyMeasure("height");
Rectangle recto = new Rectangle(dLength, dHeight);
show(recto);
}
}
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- Execute the application to test it
- When asked to enter the values, type anything you want and press Enter
each time
Enter the length: What
Something went wrong!!!
Enter the height: 44.68
Rectangle Characteristics
Length: 0.00
Height: 44.68
Perimeter: 89.36
Area: 0.00
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- Execute the application again and test it with other values
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