CS 116 OBJECT ORIENTED PROGRAMMING II LECTURE 2_1 GEORGE KOUTSOGIANNAKIS Copyright: 2014 Illinois Institute of Technology/George Koutsogiannakis 1.

CS 116

OBJECT ORIENTED PROGRAMMING IILECTURE 2_1

GEORGE KOUTSOGIANNAKIS

Copyright: 2014 Illinois Institute of Technology/George Koutsogiannakis1

OOP Review• In previous lecture we discussed:

– User defined template classes• Default constructor/ non default constructor(s).• Accessor mutator methods.• toString method.• Other methods as required by the specification.

– Client classes– How to use a static instance variable.– Java packages

• Note: You must bring your text to the lectures

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Java Packages

• User defined packaging.– Suppose that we created a package for our user

defined template class Auto :package name1.name2.name3;import java.util.Scanner;

import java.io.File;public class Auto

{ ………………………………

}

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Java Packages

• We compile Command Line using the special command:

• >javac –d . Auto.javaNotice that there is space between javac and –d and also between –d and

the dot. There is also space between the dot and the name of the file.

• This command tells the compiler to create the folders first name1 then name 2 (inside name1 folder) and then name 3 (inside name 2 folder) with respect to the current directory

• and place Auto.class inside folder name3.

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Java Packages

Current Directory (folder)

name1(folder)

name2 (folder)

name3 (folder) Auto.class

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Source code files

Auto.java AutoClient.java are in Current

Directory

Java Packages• Therefore right now our service class Auto.class file is in folder name3 in our

example.• Suppose that we want to use the class Auto in the class AutoClient and also place

AutoClient in the folder name2 (not the same folder as the Auto class):– We must import the class Auto in the class AutoClient by using an import statement in class

AutoClient because the two classes are going to be in different folders i.e. package name1.name2 //Notice that AutoClient resides in folder name2 import name1.name2.name3.Auto; public class AutoClient

{//now we can instantiate objects of

//class Auto and invoke its methods inside //the main method of this class

}

– File AutoClient.class will be found in path name1.name2 with respect to the current directory.

– Note: If both classes reside in the same folder then there is no need to import the Auto class

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Java Packages

Current Directory (folder)

Auto.java , AutoClient.java name1(folder)

name2 (folder)AutoClient.class

name3 (folder)

Auto.class

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Java Packages

• How do we call the interpreter for class AutoClient from the current directory (folder)?

• We need to list the path to the AutoClass when we use the interpreter command:

i.e >java name1.name2.AutoClient• Notice that the reason we call the AutoClient class is

because this class has the main method. The Auto class will be called automatically.

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Java Packages

• Pre defined library classes are grouped into packages according to their functionality:

i.e.

package java.lang Provides classes that are fundamental to the design of the Java programming language.Every java program automatically imports all the classes of this package without the need for an import statement.– Includes such classes as :

• String, Math , System, Integer, Float and others.

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Java Packages

package java.util– Must explicitly be imported with an import

statement.– Provides such classes as:• Scanner, StringTokenizer, Vector, Calendar, Date and

others.

java.text– Must imported explicitly.– Provides numeric formatting classes such as:• Format, NumberFormat and others

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Java Packages

• Import statements can include the entire package or a selective class out of the package

i.e. import java.util.*; imports all classes in package util or import java.util.Scanner; imports only the Scanner class (saves memory usage)

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Java Packages

• There are over 200 packages in the java library– Number of classes varies in each package with

some having over 20 classes.– Remember that a package can include something

called “interfaces”.– For now let us think of an interface as a class whose

methods have no code (no implementation). We will come back to the subject of interfaces later.

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Default Initial ValuesIf the constructor does not assign values to the instance variables, they

receive default values depending on the instance variable data type.

Data Type Default Value

byte, short, int, long 0

float, double 0.0

char space

boolean false

Any object reference (for example, a String)

null

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main Method

• A client program must contain a method called main()

• Execution always begins with the first statement in method main and ends with the last statement in method main.

• Between the first and last statement of method main other methods are called for execution as needed.

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Object Reference this

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• How does a method know which object's data to use?

• this is an implicit parameter sent to methods. this is an object reference to the object for which the method was called.

• When a method refers to an instance variable name, this is implied. Thus,

variableName is understood to be this.variableName Example in the Auto class:

modelName is understood to be this.modelName

Using this in a Mutator Method

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public void setInstanceVariable( dataType instanceVariableName ){ this.instanceVariableName = instanceVariableName;}

Example:public void setModel( String model ){ this.model = model;}

this.model refers to the instance variable whose value is being set.

model refers to the parameter.

Using this

• Suppose in AutoClient class (which uses class Auto) we instantiate an object:– Auto a1=new Auto(); (globally -outside any method)– Suppose that we create a new method that it is

responsible for setting the instance fields of a1 and getting their values by using also a globally declared parameter i.e int x;

– We can call the value of x using this.x

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Using thispackage folder1.folder2;import folder1.folder2.folder3.Auto;public class AutoClient{

Auto a1=new Auto("Ford", “Mario”, 100, 20);int x=300;public static void main(String[] args) {

//because we can NOT directly call a non static method from a static//we need to use an object of AutoClientAutoClient ac=new AutoClient();ac.usingThis();Auto a2=new Auto("Infinity", “Jim”, 150, 18);System.out.println(a2.toString());a2.setCurrentID(15);System.out.println(“the value of x ia:”+ac.x);

}public void usingThis(){

a1.setCurrentID(15);int x=a1.getCurrentID();System.out.println("The current id for a1 is now"+" "+this.x);System.out.println("The value of the current id variable for object a1 is"+" "+x);

}} 18

Using this• The output of the previous program is:C:\CS116\FALL2010\PracticeExercises\Lecture 2\ExampleUsingthis>javac -d

. AutoClient.javaC:\CS116\FALL2010\PracticeExercises\Lecture 2\ExampleUsingthis>java

folder1.folder2.AutoClientThe current id for a1 is now 300The value of the current id variable for object a1 is15The model is Infinity The owner is Jim The miles driven are 150.0 The gas

mileage is 18.0 The id is 2The value of x is:300

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Using this

• Practice Questions:• What object is the refers this refers to in the

Program AutoClient.java?• Explain the value of all attributes in the

output!

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The equals Method used for objects

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• Determines if the data encapsulated in another object is equal to the data encapsulated in this object.

• Example client code using Auto references auto1 and auto2:

if ( auto1.equals( auto2 ) ) System.out.println( "auto1 equals auto2" );

Return value Method name and argument list

boolean equals( Object obj )

returns true if the data in the Object obj is the same as in this object; false otherwise

The equals Method used for objects

• All java objects regardless if the class is user defined or pre defined get to use (the term is “inherit”) methods of a library class called Object.

• All classes get to use (the term is “inherit”) the toString and equals methods of the Object class.

• However, quite often we create (as we know) our own toString method. We can also create our own equals method for objects of our user defined template class.– When we create our version of a method that already exists in

the library we call the technique “method overriding”.

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The instanceof Operator

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Because the equals method’s parameter is of type Object, we need to determine if the parameter is an Auto object. (Object is general- we need to narrow it down to the specific type of Object).

We can use the instanceof operator to determine if an object reference refers to an object of a particular class.

Syntax: objectReference instanceof ClassName evaluates to true if objectReference is of ClassName type; false otherwise.

Auto Class equals Method (method overriding of Object class’ equals method).

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public boolean equals(Object o ){ // if o is not an Auto object, return false if ( ! ( o instanceof Auto ) ) return false; else { // type cast o to an Auto object Auto objAuto = ( Auto ) o; if ( this.modelName.equals( objAuto.modelName ) &&this.milesDriven== objAuto.milesDriven && Math.abs( this.mileage - objAuto.mileage ) < 0.0001 ) return true; else return false; }}

See text Examples 7.10 Auto.java and 7.11 AutoClient.java

Auto Class equals Method (method overriding of Object class’ equals method).

• When in the AutoClient class we compare two objects i.e. auto1 and auto2 of Auto class as in the statement

if (auto1.equals(auto2)) this.modelName in the equals method refers

to the value of instance variable modelName of auto1 object.

The same applies to the other instance variables.

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Class Scope

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• Instance variables have class scope– Any constructor or method of a class can directly

refer to instance variables.

• Methods also have class scope– Any method or constructor of a class can call any

other method of a class (without using an object reference).

Local Scope

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• A method's parameters have local scope, meaning that: – a method can directly access its parameters.– a method's parameters cannot be accessed by other

methods.• A method can define local variables which also have

local scope, meaning that:– a method can access its local variables.– a method's local variables cannot be accessed by other

methods.

Example of Class vs local scopepublic class Person {

String firstName;static int I d;int currentid;

public Person(){

firstName="John"; id++; currentid=id;

}

public void method1(){ int currentid;

currentid = this.currentid*2;System.out.println("The local value of currentid is"+" "+currentid);System.out.println("The class scope of variable currentid is"+" "+this.currentid);

}}

currentid defined in method1() has local scopethis.currentid refers to the currentid variable with global class scope as defined at the top of the class and outside any

method.28

Example of Class vs local scopepublic class PersonClient {

public static void main(String[] args) {

Person p1=new Person();Person p2=new Person();p1.method1();p2.method1();

}}What is the output ?

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Example of Class vs Local scope

• ---------- for object p1 ----------– The local value of currentid is 2– The class scope of variable currentid is 1

• ---------- for object p2 ----------– The local value of currentid is 4– The class scope of variable currentid is 2

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enum Types

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Used to define a set of constant objects. Built into java.lang (no import statement needed)Syntax: enum EnumName { obj1, obj2,… objn };

Example: enum Days { Sun, Mon, Tue, Wed, Thurs, Fri, Sat };

A constant object is instantiated for each name in the list. Thus, each name is a reference to an object of type Days.

enum is a keyword

enum Types

• Enumerated Types are classes with special properties.

• They have a finite number of instances (as for example Days in previous slide).

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Useful enum Methods

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Return value Method name and argument list

int compareTo( Enum eObj ) compares two enum objects and returns a negative number if this object is less than the argument, a positive number if this object is greater than the argument, and 0 if the two objects are equal.

int ordinal( )returns the numeric value of the enum object. By default, the value of the first object in the list is 0, the value of the second object is 1, and so on.

boolean equals( Object eObj )

returns true if this object is equal to the argument eObj; returns false otherwise

String toString( )

returns the name of the enum constant

Example enum enum PersonType { ADULT_MALE, CHILD_MALE, ADULT_FEMALE, CHILD_FEMALE };public class Persons {

PersonType pt;String firstName;static int id;int currentid;

public Persons(){

firstName="John"; id++;currentid=id;

}

public void setPersonType(PersonType pertyp){

this.pt=pertyp;}

public PersonType getPersonType(){

return pt;}

} 34

Example enum public class PersonsClient {

public static void main(String[] args) {

Persons p1=new Persons();Persons p2=new Persons();p1.setPersonType(PersonType.ADULT_FEMALE);p2.setPersonType(PersonType.CHILD_MALE);System.out.println("p1 is of person type:"+" "+p1.getPersonType());System.out.println("p2 is of person type:"+" "+p2.getPersonType());

}}

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Example enum

– ---------- Output ----------– p1 is of person type: ADULT_FEMALE– p2 is of person type: CHILD_MALE

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Javadocs

• Automatic generation of documentation for your user defined classes in html format.

• It is another tool available in the jdk (see bin subfolder in the installation folder of the jdk).

• To generate documentation use the command javadoc and the nam eof your class followed by the .java extension:– >javadoc Persons.java or– >javadoc *.java (means all files in the folder that

end with .java extansion).

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Javadocs

• The tool reads all comments added (/** to */)• plus additional information that we add in the

source code file (i.e. describe parameters using the symbol @) i.e.– @ param id denotes the id number of a person advancing for each new Person object instantiated.

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Javadoc Example/** This class describes Persons */public class Person { /** @param firstName provides the first name of a Person */

String firstName;static int id;int currentid;

public Person(){

firstName="John";id++;currentid=id;

}

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Output html File• Package Class Tree Deprecated Index Help • PREV CLASS NEXT CLASS FRAMES NO FRAMES All Classes All Classes SUMMARY: NESTED | FIELD |

CONSTR | METHOD DETAIL: FIELD | CONSTR | METHOD • Class Person• java.lang.Object Person public class Person extends java.lang.Object This class describes Persons • Constructor Summary Person()

Method Summary void method1() Methods inherited from class java.lang.Object clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

• Constructor Detail Person• public Person() Method Detail method1• public void method1() Package Class Tree Deprecated Index Help • PREV CLASS NEXT CLASS FRAMES NO FRAMES All Classes All Classes

SUMMARY: NESTED | FIELD | CONSTR | METHOD DETAIL: FIELD | CONSTR | METHOD

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Examples

• If you click on the Examples link next to lecture2.ppt link ( in the Lectures page of the course’ s web site) you will access example files dealing with javadocs and enumeration.

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STUDY GUIDE

• Read Chapter 7– Section 7.9– Section 7.10– Section 7.13– Section 7.15– Section 7.16

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