Transcript Powerpoint Slides - Purdue University

Defining classes and methods
Recitation – 09/(25,26)/2008
CS 180
Department of Computer Science,
Purdue University
Announcements
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Project 4 is out
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2 week project
Milestone Due October 1, 10:00pm
Final Submission Due October 8, 10:00pm
Exam 1
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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.)
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Consulting hours
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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:
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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
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Ways to organize data
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Use to create objects containing data
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Contain methods to perform certain operations
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Data can be of primitive type and objects of other
classes.
Accessors
Mutators
Java program consists of objects of class
type
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Objects can interact with one another
Program objects can represent objects in real
world
Terminology
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Class – a blueprint for an object
Object – an instance of a class
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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
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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
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Methods returning a value
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Return statements are optional
E.g - return;
Mandatory return statements
Usage: Name of the object followed by a dot followed
by the name of the method.
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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
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Can only use variables within the current scope
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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
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Method scope: local variables declared within a method are not
accessible outside the method
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Includes any declarations in the for loop declaration
Includes argument list
Class scope: manageable with public/private modifiers
When given the choice, Java will always choose the
variable with closest scope
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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
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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
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Type specified as public
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Any other class can directly access that
variable/method by name
Classes generally specified as public
Instance variables usually specified as
private
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Private members not directly accessible from
outside the class
Mutators and accessors
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Mutator
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Method to modify the value of a private instance variable.
public void setCarType(int carType)
{
this.carType = carType;
}
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Accessor
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Method to access the value of a private instance variable.
public int getCarType()
{
return carType;
}
Mutators and accessors
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Note:
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Obviously, mutators and accessors should be
declared as public methods.
Accessor methods
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Mutator methods
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Usually no parameter
Return type same as that of the variable to be accessed.
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?
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Do not have to write an accessor or mutator method
Variables of a Class Type
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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
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Stored elsewhere in memory
Variable contains address of where it is stored (i.e
the reference to the object)
Car a = new Car(“hummer”);
Car b = new Car(“hummer”);
d
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
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Assignment operator used with objects
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Only memory address is copied
This creates an alias.
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Parameters of class type
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Car a = new Car(“hummer”);
Car b = a; // Now b and a refer to the same object in
the memory
E.g
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
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When a method returns a class object, only the
memory address is returned.
Equality Comparison
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For primitives, compare for equality with ==
For objects, when to use ==, when to use
equals() method?
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When using ==, you are comparing addresses
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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.
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For example, for class Car, compare if they belong to the
same make, same model, etc.
Exam review
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Chapter 1: Introduction to Computers and Java
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Object-oriented programming principles
Programming Languages and Compilers
Java Byte-Code etc.
Chapter 2 : Basic Computation
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Java data types, variables
assignment statements, variables and expressions
Class String – string processing
Keyboard and Screen I/O
Exam review
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Chapter 3: Flow of Control: Branching
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branching statements - Boolean type and expressions
If-else and switch statements
Enumerations
Chapter 4 : Flow of Control: Loops
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Use while, do, and for in a program
Use the for-each with enumerations
Exam review
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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