Defining classes and methods Recitation 09/(25,26)/2008 CS 180 - - PowerPoint PPT Presentation

Defining classes and methods

Recitation – 09/(25,26)/2008

CS 180 Department of Computer Science, Purdue University

Announcements

Project 4 is out

2 week project Milestone Due October 1, 10:00pm Final Submission Due October 8, 10:00pm

Exam 1

Wednesday, October 1, 6:30 – 7:30 pm, CL50

224

Covers chapters 1-5 Sample exam on course webpage. Exam review at the study group meeting:

Tuesday, September 30, 5-7 pm, LWSN B116

Announcements (contd.)

Consulting hours

Students are expected to go through the project

spec in detail before approaching the TAs for help.

Do not approach TAs during help hours with errors

in programs unless you have tried enough to fix the problem yourself.

Recall the way to go about fixing errors:

• Syntax error – check the highlighted line carefully
• Logic error – Use print statements to identify the problem
• Runtime error – Check the line where the exception is being

thrown.

Classes

Ways to organize data

Use to create objects containing data

Data can be of primitive type and objects of other

classes.

Contain methods to perform certain operations

Accessors Mutators

Java program consists of objects of class

type

Objects can interact with one another Program objects can represent objects in real

world

Terminology

Class – a blueprint for an object Object – an instance of a class

Real world analogy – Your TA’s hummer is an

instance of a class Car

Method/function – means to perform

• perations with the class

Instance variables – data held within an

• bject of a class

Methods

• Used to provide an interface to the class
• Helper methods – break up your code into

understandable chunks

• Types of methods

Void - do not return anything

• Return statements are optional
• E.g - return;

Methods returning a value

• Mandatory return statements
• Usage: Name of the object followed by a dot followed

by the name of the method.

• E.g: keyboard.nextInt() or myCar.getCarType()

Methods - example

public class Car { private int carType; public int getCarType(int n) { …… return carType;

//mandatory return statement

} }

Return type Parameters

Car myCar = new Car(); ....... int type = myCar.getCarType(5);

Scope

• Can only use variables within the current scope
• Loop/conditional scope: if a variable is declared within an if/else

block or any type of loop, it is not accessible outside the block

• Includes any declarations in the for loop declaration
• Method scope: local variables declared within a method are not

accessible outside the method

• Includes argument list
• Class scope: manageable with public/private modifiers
• When given the choice, Java will always choose the

variable with closest scope

• If you have a class variable x and a local variable x declared, x

refers to the local variable, this.x refers to the class variable

this keyword

• this is a pointer to an object’s self
• Always used implicitly, but sometimes useful to be used

explicitly

• In case of ambiguity resulting from having a local

variable with the same name as a class variable, use this to refer to the class’s variable.

class A { private int a; public int add(int a) { return this.a + a; } } A more clear version avoids ambiguity class A { private int a; public int add(int b) { return a + b; } }

The public and private modifiers

Type specified as public

Any other class can directly access that

variable/method by name

Classes generally specified as public Instance variables usually specified as

private

Private members not directly accessible from

• utside the class

Mutators and accessors

• Mutator
• Method to modify the value of a private instance variable.

public void setCarType(int carType) { this.carType = carType; }

• Accessor
• Method to access the value of a private instance variable.

public int getCarType() { return carType; }

Mutators and accessors

Note:

Obviously, mutators and accessors should be

declared as public methods.

Accessor methods

• Usually no parameter
• Return type same as that of the variable to be accessed.

Mutator methods

• Usually one parameter with the new value of the

instance variable.

• Usually of void return type

What about instance variables that we do not want

to be accessed or changed from outside?

• Do not have to write an accessor or mutator method

Variables of a Class Type

Data of primitive type stored in the memory

location assigned to the variable

For class type, the object itself is not stored in the

instance variable

Stored elsewhere in memory Variable contains address of where it is stored (i.e

the reference to the object)

Car a = new Car(“hummer”);

The two variables might contain the same data, but in different locations. The value of each variable is the address of the memory location where the objects are stored – which is different. Recall that (a==b) will only compare the addresses, resulting in the value false.

More on class types

Assignment operator used with objects

• Only memory address is copied
• This creates an alias.
• E.g

Car a = new Car(“hummer”); Car b = a; // Now b and a refer to the same object in the memory Parameters of class type

• Memory address of actual parameter passed to formal

parameter

• Actual parameter thus can be changed by class

methods

Return type of a method as class type

• When a method returns a class object, only the

memory address is returned.

Equality Comparison

For primitives, compare for equality with == For objects, when to use ==, when to use

equals() method?

• When using ==, you are comparing addresses
• True: if they are aliases
• The two variables refer to the same object in the memory
• False: if they are not aliases (even if they hold the same data)
• Write your own equals method for your classes.
• For example, for class Car, compare if they belong to the

same make, same model, etc.

Exam review

Chapter 1: Introduction to Computers and Java

• Object-oriented programming principles
• Programming Languages and Compilers
• Java Byte-Code etc.

Chapter 2 : Basic Computation

• Java data types, variables
• assignment statements, variables and expressions
• Class String – string processing
• Keyboard and Screen I/O

Exam review

Chapter 3: Flow of Control: Branching

• branching statements - Boolean type and expressions
• If-else and switch statements
• Enumerations

Chapter 4 : Flow of Control: Loops

• Use while, do, and for in a program
• Use the for-each with enumerations

Exam review

Chapter 5: Defining classes and methods

Define a Java class, its methods Describe use of parameters in a method Use modifiers public, private Define accessor, mutator class methods