Chap02

Data Structures Using Java 1
Chapter 2
Inheritance and Exception Handling

Chapter Objectives
• Learn about inheritance
• Learn about sub- and superclasses
• Explore how to override the methods of a
superclass
• Examine how constructors of super- and
subclasses work
• Examine abstract classes
• Learn about composition

Chapter Objectives
• Learn about exceptions
• Become aware of exception classes and their
hierarchy
• Learn about checked and unchecked exceptions
• Learn how to handle exceptions within a program
• Examine try/catch blocks
• Discover how to throw and rethrow an exception

Inheritance
• “is-a” relationship
• Single inheritance
– subclass derived from one existing class (superclass)
• Multiple inheritance
– Subclass derived from more than one superclass
– Not supported by Java
– In Java, a class can only extend definition of one class

Inheritance

Inheritance
• Private members of superclass
– private to superclass; cannot be accessed
directly by subclass
• Subclass can override method of superclass;
redefinition applies only to object of
subclass

Inheritance
• To write method definition of subclass to specify
call to public method of superclass
– If subclass overrides public method of superclass, to
specify call to public method of superclass:
super.MethodName(parameter list)
– If subclass does not override public method of
superclass, to specify call to public method of
superclass: MethodName(parameter list)

Defining Constructor of Subclass
• Call to constructor of superclass
– must be first statement
– specified by: super parameter list

The class Object
• Directly or indirectly superclass of every
class in Java
• Public members of class Object can be
overridden/invoked by object of any class
type

The class Object

Objects of Superclasses and
Subclasses
• Cannot automatically make reference variable of
subclass type point to object of superclass
• Dynamic binding: method executed determined at
execution time, not compile time
• Operator instanceof : determines whether
reference variable that points to object is of
particular class type
• ClassCastException thrown if class cast is not
allowed

The Operator instanceof
• Reference of superclass type can point to
objects of its subclass
• Can determine if a reference variable points
to an object using operator instanceof

Abstract Methods and Classes
• Abstract method: method that has only the
heading with no body
– must be declared abstract
• Abstract class: class that is declared with the
reserved word abstract in its heading

Abstract Class
• Can contain instance variables,
constructors, finalizer, abstract and
nonabstract methods
• Cannot instantiate object of abstract class
type; can only declare reference variable
• Can instantiate object of subclass of abstract
class, but only if subclass gives definitions
of all abstract methods of superclass

Composition
• Another way to relate two classes
• One or more members of a class are objects
of another class type
• “has-a” relation between classes

Exception
• Definition: an occurrence of an undesirable
situation that can be detected during
program execution
• Examples
– division by zero
– trying to open an input file that does not exist is
an exception
– an array index that goes out of bounds

Java Exception Hierarchy

The class Throwable

The class Exception and its
Subclasses from java.lang

Subclasses from java.util

Subclasses from java.io

Java’s Exception Class
• Class Exception
– Subclass of class Throwable
– superclass of classes designed to handle exceptions
• Various types of exceptions
– I/O exceptions
– Number format exceptions
– File not found exceptions
– Array index out of bounds exception
• Various exceptions categorized into separate
classes and contained in various packages

Constructors

Java Exception Classes

Exceptions Thrown by Methods

Checked Exceptions
• Checked Exception: any exception that can
be analyzed by the compiler
• Example
– IOExceptions

Unchecked Exceptions
• Unchecked Exception: exception that cannot be
analyzed when the program compiles (must be
checked for by programmer)
• Examples
– Division by zero
– Array index out of hounds
• Syntax
– throws ExceptionType1, ExceptionType2, …
*ExceptionType1, ExceptionType2, etc are names of exception
classes

Handling Exceptions within a
Program
• try/catch/finally block used to handle exceptions
within a program
• try block:
– Includes statements that may generate an exception
– Includes statements that should not be executed if an
exception occurs
– followed by zero or more catch blocks
– May or may not be followed by finally block

Handling Exceptions within a
Program
• Catch block:
– heading specifies type of exception it can catch
– contains exception handler; completely handles
exception
– can catch all exceptions of a specific type or all types of
exceptions
– may or may not be followed by a finally block
• Finally block
– Code contained in this block always executes
– try block with no catch block has finally block

Order of Catch Blocks
• If exception in try block caught by first
catch block, reaming catch blocks ignored
• Must be careful about order catch blocks
are listed

Rethrowing and Throwing an
Exception
• Useful when
– Catch block catches exception but is unable to
handle it
– Catch block decides exception should be
handled by calling environment
• Allows programmer to provide exception
handling code in one place

Exception Handling Techniques
• Terminate program
– Output appropriate error message upon termination
• Fix error and continue
– Repeatedly get user input/output appropriate error
message until valid value is entered
• Log error and continue
– Write error messages to file and continue with program
execution

Creating Your Own Exception
Classes
• Exception class you define extends class
Exception or one of its subclasses
• Syntax to throw your own exception object
– throw new ExceptionClassName(messageString);

Programming Example: Grade
Report
• Main algorithm
– Declare variables
– Open input file
– Open output file
– Get number students registered and tuition rate
– Load students’ data
– Print grade reports

Report
• Components: student, course
• Operations on course
– Set course information
– Print course information
– Show credit hours
– Show course number
– Show grade

Report
• Operations on student
– Set student information
– Print student information
– Calculate number of credit hours taken
– Calculate GPA
– Calculate billing amount
– Sort the courses according to the course number

Chapter Summary
• Inheritance
– Single and multiple
– Rules and Uses
– Superclasses/subclasses (objects)
– Overriding/overloading methods
• The class Object
– Constructors
– Rules
• Abstract methods and classes

Chapter Summary
• Composition
• Exception
• Checked and Unchecked Exceptions
• Creating your own exception classes
• Exception handling techniques
• Handling exceptions within a program

Chap02

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