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Chapter 2
Inheritance and Exception Handling
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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
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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
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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
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Inheritance
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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
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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)
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Defining Constructor of Subclass
• Call to constructor of superclass – must be first statement– specified by: super parameter list
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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
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The class Object
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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
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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
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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
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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
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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
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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
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Java Exception Hierarchy
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The class Throwable
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The class Exception and its Subclasses from java.lang
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The class Exception and its Subclasses from java.util
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The class Exception and its Subclasses from java.io
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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
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The class Exception and its Constructors
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Java Exception Classes
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Exceptions Thrown by Methods
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Exceptions Thrown by Methods
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Checked Exceptions
• Checked Exception: any exception that can be analyzed by the compiler
• Example– IOExceptions
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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
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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
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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
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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
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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
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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
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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);
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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
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Programming Example: Grade Report
• Components: student, course
• Operations on course– Set course information– Print course information– Show credit hours– Show course number– Show grade
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Programming Example: 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
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Chapter Summary
• Inheritance– Single and multiple– Rules and Uses– Superclasses/subclasses (objects)– Overriding/overloading methods
• The class Object– Constructors– Rules
• Abstract methods and classes
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Chapter Summary
• Composition
• Exception
• Checked and Unchecked Exceptions
• Creating your own exception classes
• Exception handling techniques
• Handling exceptions within a program