Applying OO Concepts Using Java

In this class, we will cover:

• Overriding a method• Overloading a method• Constructors• Mutator and accessor methods• The import statement and using prewritten classes• Packages and the protected access modifier• The finalize() method

Overriding a Method

Overriding:• If you declare a variable within a class, and use

the same variable name within a method of the class, then the variable used inside the method takes precedence, or overrides, the first variable

Overloading a Method

Overloading:• Involves using one term to indicate diverse

meanings• Writing multiple methods with the same name, but

with different arguments• Overloading a Java method means you write

multiple methods with a shared name• This is polymorphism in action.

Constructors

• Constructors are a special type of method.

• Used to create an instance of the class.

– e.g. Employee e = new Employee( );This calls the Employee constructor.

• Java automatically provides a constructor method when you create a class

• Programmers can write their own constructor classes• Programmers can also write constructors that receive

arguments– Such arguments are often used for initialization purposes

when values of objects might vary

Overloading Constructors

• If you create a class from which you instantiate objects, Java automatically provides a constructor

• But, if you create your own constructor, the automatically created constructor no longer exists

• As with other methods, you can overload constructors– Overloading constructors provides a way to create

objects with or without initial arguments, as needed

Example ofOverloading a Constructor

• public class Employee { public Employee (String n, double a) { name = n; salary = a; } public Employee ( ) { name = “ “; salary = 0; }}

Mutator and Accessor Methods• Often referred to as get/set methods.• Mutator methods modify fields in a class.

– Example: public void setName (String n) { empName = n; }

• Accessor methods retrieve fields in a class– Example:

public String class getName () { return empName; }

• Be careful not to return mutatable objects in your accessor methods. Why is this important?

Mutator and Accessor Methods

• Returning mutatable objects in public accessor methods breaks encapsulation!!!!

• Even if the data element is private, outside classes can now modify it.

• You should return immutable objects (Strings, ints, etc).

• If you must return a reference to a mutatable object, you should clone it first.

• See pg. 112 in book for example of this rogue code.

The finalize() Method

• Inherited from the Object class.

• Called by the VM before an object is destroyed and it’s memory is released

• Use it to release resources that might not otherwise be released (e.g. files)

• Use it to record the fact that an object has been destroyed

The Import Statement and Using Pre-written Classes

• The creators of Java wrote nearly 500 classes – For example:

• System, Character, Boolean, Byte, Short, Integer, Long, Float, and Double are classes

• These classes are stored in packages, or a library of classes, which is a folder that provides a convenient grouping for classes

• java.lang – The package that is implicitly imported into every Java program and contains fundamental classes, or basic classes

• Fundamental classes include:– System, Character, Boolean, Byte, Short, Integer, Long,

Float, and Double

• Optional classes – Must be explicitly named

The Import Statement and Using Pre-written Classes

The Import Statement and Using Pre-written Classes

• To use any of the prewritten classes (other than java.lang):– Import the classOR– Import the package which contains the class you are using

• To import an entire package of classes use the wildcard symbol - *

• For example:– import java.util.*; //imports all java.util classes– import java.util.Vector; //imports the Vector class

– Represents all the classes in a package

Packages

• Creating packages encourages others to reuse software because it makes it convenient to import many related classes at once

• Packages are used to:– maintain the uniqueness of class names

• Using Packages in your programs prevent class name scope conflicts if multiple classes of the same name are used.

• e.g. both java.util and java.sql have a Date class, so if you are using both packages you need to reference with java.util.Date or java.sql.Date.

– group classes to make them more easily accessible to your classes– reference classes in a particular scope

• What access modifier helps you limit access to packages?

Packages and theProtected Access Modifier

• the protected access modifier:– Provides you with an intermediate level of security

between public and private access– Is used to limit access to classes within the same package– If you create a protected data field or method, it can be

used:• within its own class

• in any classes extended from that class

• or in classes in the same package

• but it cannot be used by “outside” classes

Putting Your Class in a Package

• To include your class into a package, use the package statement– The package statement must appear outside the class definition

– The package statement looks like this:

• package <name of package>;

• example: package MC697; class Person { ... }

Packages and Directory Structure

• Packages map to the directory structure.– Example:

package com.MC697;public class Test { public static void main (String[] args) { System.out.println(“Testing packages”); }}

– This package statement maps to the directory: <base directory>/com/MC697 where base directory is the directory you are going to compile and execute the class file from

Compiling and ExecutingUsing Packages

• When using packages you must compile and run from the base directory.

• So, let’s say c:\temp is the base directory we want to use. The file in the example should be saved to c:\temp\com\MC697\Test.java.

• To compile:– cd to c:\temp

javac com/MC697/Test.java

• To execute:– cd to c:\temp

java com.MC697.Test

Setting the Classpath

• When using packages or classes outside the java. packages, you need to modify the classpath.

• What is the classpath?– It is a environment variable that points the jvm to the needed class files.

• How do you set the classpath?– You can set it using the -classpath option in the javac or java command.

• This is temporary. You will need to do this every time you use those commands.

• e.g. javac -classpath c:\user\classdir PackageTest.java java -classpath c:\user\classdir PackageTest

– Another way is to modify the environment variable on your machine.• e.g. SET CLASSPATH=c:\com\MC697

• Different operating systems have different ways of setting this variable.

• See pg. 137 in book for examples of how to set this on different platforms.

JAR Files

• Jar files are Java’s version of the zip file.

• They group packages and class files together in a unit to make it easier to deploy.

• Can be viewed using Winzip or similar utility.

• You can point your classpath variable to jar’s that contain needed classes.– e.g. SET CLASSPATH=c:\j2sdk1.4.1\jre\lib\rt.jar;%CLASSPATH%

Javadocs

• Javadocs are documentation for class files.

• Javadoc is a utility built into the sdk to automatically build documentation from the java files.– e.g. javadoc VectorDemo.java

• Remember the /** …. */ documentation symbols?– These are used to denote documentation comments.– Put these before a method or field to include comments about these in the javadocs.– Special tags can be used:

• @author• @version

• @param for methods

– See pg. 139 in book for more information