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, CL  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 operations with the class Instance variables – data held within an object 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 typeParameters 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 outside 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”); d Car b = 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