Question

(Catching Exceptions with Superclasses) Use inheritance to create an exception superclass. (called ExceptionA) and exception subclasses ExceptionB and ExceptionC, where ExceptionB inherits from ExceptionA and ExceptionC inherits from ExceptionB. Write a program to demonstrate that the catch block for type ExceptionA catches exceptions of types ExceptionB and ExceptionC.

Short answer

Define a superclass `ExceptionA` and subclass exceptions `ExceptionB` and `ExceptionC` where `ExceptionC` extends `ExceptionB`, which in turn extends `ExceptionA`. A try-catch block with a catch for `ExceptionA` will catch exceptions of both `ExceptionB` and `ExceptionC`. Running the program will demonstrate this behavior.

Step-by-step solution

Define the Exception Superclass

Create a class named `ExceptionA` that extends the built-in Java `Exception` class. Since it's a simple exception class, it won't include additional functionality beyond what `Exception` provides.

Define the First Subclass of ExceptionA

Create another class named `ExceptionB` that extends `ExceptionA`. This subclass represents a more specific type of exception that is still caught by catch blocks looking for `ExceptionA`.

Define the Second Subclass of ExceptionA

Create a third class named `ExceptionC` that extends `ExceptionB`. This class demonstrates another layer of exception inheritance and will also be caught by catch blocks designed for `ExceptionA` and `ExceptionB`.

Write a Program to Demonstrate Catching Exceptions

Write a program with a main method and a try-catch block. The try block should throw an exception of type `ExceptionC`, and the catch block should be looking for exceptions of type `ExceptionA`. This will demonstrate that `ExceptionA` can catch `ExceptionB` and `ExceptionC` because they are subclasses of `ExceptionA`.

Execute the Program

Run the program. The catch block for `ExceptionA` should successfully catch the `ExceptionC` object. You may include print statements to confirm which block is executing.

Key concepts

Inheritance in Java

Inheritance is a cornerstone of Object-Oriented Programming (OOP) and Java utilizes this concept extensively. It allows a class to inherit fields and methods from another class, termed the superclass or parent class, enabling code reusability and the creation of a hierarchical relationship between classes.

In context of exceptions, inheritance enables the creation of an organized structure of exception types. A superclass of exceptions can be defined, with subclasses representing more specific error conditions. This hierarchy indicates that the subclasses are specialized versions of their superclass.

For example, when a 'ExceptionA' is used as a superclass for 'ExceptionB' and 'ExceptionC', it allows catch blocks looking for 'ExceptionA' to also intercept any of its subclasses, thereby providing a versatile mechanism for error handling. This not only streamlines the code but also ensures that related exception types are managed together, making your programs more robust and maintainable.

Custom Exceptions

Java allows the creation of custom exceptions to represent and handle errors specific to an application's domain. By extending the Java 'Exception' class, you can create your own exception types that are tailored to your program's needs.

Creating custom exceptions involves defining a new class that inherits from 'Exception' or one of its subclasses. This enables the integration of additional data and methods into the exception. For instance, you might include extra information or provide methods to retrieve detailed error messages relevant to the thrown exception.

In our textbook exercise, 'ExceptionA', 'ExceptionB', and 'ExceptionC' demonstrate a hierarchy of custom exceptions. Each one is built upon the foundation of its predecessor, allowing for a catch block intended for 'ExceptionA' to capture all associated exceptions, enhancing the error handling capabilities of a Java application.

Try-Catch Blocks

The try-catch block is the fundamental mechanism in Java for catching and handling exceptions. A 'try' block surrounds the code that might throw an exception, while a 'catch' block is used to specify the action to take when an exception occurs.

When an exception is thrown within the 'try' block, the flow of execution is immediately transferred to the 'catch' block that matches the type of thrown exception. If no such block exists, the program will terminate abruptly.

One key point to remember is that the catch block for a superclass of exceptions will catch not only exceptions of that exact type but also any exception that inherits from that class. This feature was demonstrated in the exercise by catching 'ExceptionC' with a catch block for 'ExceptionA', which is the parent class for both 'ExceptionB' and 'ExceptionC'. Highlighting this concept helps understand polymorphism in error handling, a valuable feature of Java's exception mechanism.