| PPS

Java Programming Language
Objectives

In this session, you will learn to:
Create final classes, methods, and variables
Create and use enumerated types
Use the static import statement
Create abstract classes and methods
Create and use an interface
Define exceptions
Use try, catch, and finally statements
Describe exception categories
Identify common exceptions
Develop programs to handle your own exceptions
Use assertions
Distinguish appropriate and inappropriate uses of assertions
Enable assertions at runtime

Ver. 1.0 Session 7 Slide 1 of 24

The final Keyword

The final keyword is used for security reasons.
It is used to create classes that serve as a standard.
It implements the following restrictions:
– You cannot subclass a final class.
– You cannot override a final method.
– A final variable is a constant.
– All methods and data members in a final class are implicitly
final.
You can set a final variable once only, but that
assignment can occur independently of the declaration;
this is called a blank final variable.


Blank Final Variables

A final variable that is not initialized in its declaration; its
initialization is delayed:
A blank final instance variable must be assigned in a
constructor.
A blank final local variable can be set at any time in the body of
the method.
It can be set once only.


Enumerated Types

• An enum type field consist of a fixed set of constants.
• You can define an enum type by using the enum keyword.
For example, you would specify a days-of-the-week enum
type as:
public enum Day { SUNDAY, MONDAY, TUESDAY,
WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }
• The enum class body can include methods and other fields.
• The compiler automatically adds some special methods
when it creates an enum.
• All enums implicitly extend from java.lang.Enum. Since
Java does not support multiple inheritance, an enum cannot
extend anything else.


Static Imports

Imports the static members from a class:
import static
<pkg_list>.<class_name>.<member_name>;
OR
import static <pkg_list>.<class_name>.*;
Imports members individually or collectively:
import static cards.domain.Suit.SPADES;
OR
import static cards.domain.Suit.*;
There is no need to qualify the static constants:
PlayingCard card1 = new PlayingCard(SPADES,
2);


Abstract Classes

• An abstract class is declared with abstract access
specifier and it may or may not include abstract methods.
• Abstract classes cannot be instantiated, but they can be
subclassed. For example:

Shape

Circle Rectangle Hexagon


Abstract Classes (Contd.)

An abstract class defines the common properties and
behaviors of other classes.
It is used as a base class to derive specific classes of the
same type. For example:
abstract class Shape
{
public abstract float calculateArea();
}

The preceding abstract method, calculateArea, is inherited
by the subclasses of the Shape class. The subclasses
Rectangle, Circle, and Hexagon implement this method in
different ways.


Abstract Classes (Contd.)

A simple example of implementation of Abstract Method:
public class Circle extends Shape
{
float radius;
public float calculateArea()
{
return ((radius * radius)* (22/7));
}
}
In the preceding example, the calculateArea() method has
been overridden in the Circle class.


Interfaces

A public interface is a contract between client code and the
class that implements that interface.
A Java interface is a formal declaration of such a contract in
which all methods contain no implementation.
Many unrelated classes can implement the same interface.
A class can implement many unrelated interfaces.
Syntax of a Java class declaration with interface
implementation is as follows:
<modifier> class <name> [extends
<superclass>]
[implements <interface> [,<interface>]* ]
{ <member_declaration>*
}


Interfaces (Contd.)

Interfaces are used to define a behavior protocol (standard
behavior) that can be implemented by any class anywhere
in the class hierarchy. For example:
Consider the devices TV and VDU. Both of them require a
common functionality as far as brightness control is
concerned. This functionality can be provided by
implementing an interface called BrightnessControl which is
applicable for both the devices.
Interfaces can be implemented by classes that are not
related to one another.
Abstract classes are used only when there is a kind-of
relationship between the classes.


Interfaces (Contd.)

Uses of Interfaces:
Declaring methods that one or more classes are expected to
implement
Determining an object’s programming interface without
revealing the actual body of the class
Capturing similarities between unrelated classes without
forcing a class relationship
Simulating multiple inheritance by declaring a class that
implements several interfaces


Exceptions and Assertions

Exceptions are a mechanism used to describe what to do
when something unexpected happens. For example:
When a method is invoked with unacceptable arguments
A network connection fails
The user asks to open a non-existent file
Assertions are a way to test certain assumptions about the
logic of a program. For example:
To test that the value of a variable at a particular point is
always positive


Exceptions

Conditions that can readily occur in a correct program are
checked exceptions.
– These are represented by the Exception class.
Severe problems that normally are treated as fatal or
situations that probably reflect program bugs are unchecked
exceptions.
– Fatal situations are represented by the Error class.
Probable bugs are represented by the RuntimeException
class.
The API documentation shows checked exceptions that can
be thrown from a method.


Exceptions (Contd.)

Consider the following code snippet:
public void myMethod(int num1 , int num2)
{
int result;
result = num2 / num1;
System.out.println(“Result:” + result);
}
• In the preceding piece of code an exception
java.lang.ArithmeticException is thrown when the
value of num1 is equal to zero. The error message
displayed is:
Exception in thread “main”
java.lang.ArithmeticException: / by zero at
<classname>.main(<filename>)


The try-catch Statement

The try-catch block:
– The try block governs the statements that are enclosed within
it and defines the scope of the exception-handlers associated
with it.
– A try block must have at least one catch block that follows it
immediately.
– The catch statement takes the object of the exception class
that refers to the exception caught, as a parameter.
– Once the exception is caught, the statements within the catch
block are executed.
– The scope of the catch block is restricted to the statements in
the preceding try block only.


The try-catch Statement (Contd.)

An example of try-catch block:
public void myMethod(int num1 , int num2)
{
int result;
try{
result = num2 / num1;
}
catch(ArithmeticException e)
{
System.out.println(“Error…division by
zero”);
}
System.out.println(“Result:” + result);
}


Call Stack Mechanism

If an exception is not handled in the current try-catch block,
it is thrown to the caller of the method.
If the exception gets back to the main method and is not
handled there, the program is terminated abnormally.


The finally Clause

• The characteristics of the finally clause:
– Defines a block of code that always executes, regardless of
whether an exception is thrown.
– The finally block follows the catch blocks.
– It is not mandatory to have a finally block.


Creating Your Own Exceptions

Characteristics of user-defined exceptions:
– Created by extending the Exception class.
– The extended class contains constructors, data members and
methods.
– The throw and throws keywords are used while
implementing user-defined exceptions.


Demonstration

Let see how to create a custom Exception class, and use it in a
Java program.


Assertions

Syntax of an assertion is:
assert <boolean_expression> ;
assert <boolean_expression> :
<detail_expression> ;
• If <boolean_expression> evaluates false, then an
AssertionError is thrown.
• The second argument is converted to a string and used as
descriptive text in the AssertionError message.


Assertions (Contd.)

Recommended Uses of Assertions:
Use assertions to document and verify the assumptions and
internal logic of a single method:
Internal invariants
Control flow invariants
Postconditions and class invariants
Inappropriate Uses of Assertions:
Do not use assertions to check the parameters of a public
method.
Do not use methods in the assertion check that can cause
side-effects.


Summary

In this session, you learned that:
– final classes cannot be subclassed,final methods cannot
be overriden, and final variables are constant.
– An enum type is a type whose fields consist of a fixed set of
constants.
– An abstract class defines the common properties and
behaviors of other classes. It is used as a base class to derive
specific classes of the same type:
• Abstract classes allow implementation of a behavior in different
ways. The implementation is done in subclasses.
• An abstract class cannot be instantiated.
• Subclasses must override the abstract methods of the super class.
– Interfaces are used to define a behavior protocol that can be
implemented by any class anywhere in the class hierarchy.


Summary (Contd.)

An exception is an abnormal event that occurs during the
program execution and disrupts the normal flow of instructions.
You can implement exception handling in your program by
using the following keywords:
• try
• catch
• throws/throw
• finally
Assertions can be used to document and verify the
assumptions and internal logic of a single method:
Internal invariants
Control flow invariants
Postconditions and class invariants


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