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 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; Return type Parameters public int getCarType(int n) { …… return carType; //mandatory return statement } 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 A more clear version avoids ambiguity class A { { private int a; private int a; public int add(int a) public int add(int b) { { return a + b; return this.a + a; } } } }
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
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