swe 510: object oriented programming in java1 defining classes 1 this slide set was compiled from...

SWE 510: Object Oriented Programming in Java 1

Defining Classes 1

This slide set was compiled from the Absolute Java textbook and the instructor’s own class materials.

Outline

Introduction/Overview Constructors Information Hiding Overloading Recursion Homework!


Primitive Data Types versusAbstract Data Types (Classes) Primitive data types

a single piece of data byte, char, short, int, long, float, double, boolean Variables declared before their use:

int a = 10, b = 20;

Classes a collection of multiple pieces of data + methods Objects defined before their use (“instantiation”)

Including the same methods Including the same set of data, but Maintaining different values in each piece of dataMyClass object1 = new MyClass( );

MyClass object2 = new MyClass( );

object1.data = 10;

object2.data = 20;

10

a

20

b

10data

method( )

object1

20data

method( )

object2


Primitive Data Types versus Classes—comparison “Primitive” data type (String is not primitive in java, but to some extent we

can treat it like one) String can be used like a primitive data typeString myString;

myString = “test string”;

String myString = “test string”;

String myString = someOtherString;

Abstract data type (class) The String class is much more powerful than a primitive typeString myString = new String (“test string”);

myString.length();

myString.valueOf();

myString.equals(someOtherString);

myString.compareTo(someOtherString);

myString.split(regularExpression);


Java Doc

Google Use Google to locate the particular class or language feature of

interest

Dive Straight in Go straight to the Java documentation

http://download.oracle.com/javase/6/docs/ http://download.oracle.com/javase/6/docs/api/

The Contents of a Class Definition

A class definition specifies the data items and methods that all of its objects will have

These data items and methods are sometimes called members of the object

Data items are called fields or instance variables

Instance variable declarations and method definitions can be placed in any order within the class definition

Methods define an object’s behavior


Members, Instance Variables, and Methods

public class Course { public String department; // SWE, IAS, CS, etc public int number; // course number public char section; // A, B, C ... public int enrollment; // the current enrollment public int limit; // max number of students public double textPrice1; // primary textbook public double textPrice2; // secondary textbook public double courseFee; // laboratory fee

public int availableSpace( ) { return limit - enrollment;

}

public double capacity( ) {return ( double )enrollment / limit;

}

public double totalExpenditure( ) {return textPrice1 + textPrice2 + courseFee;

}}

members

Instance variables:(data members)Each object has its ownvalues in these variables.

Methods:Each object has the same methods (actions, computations).


Object Instantiation with new

Declare a reference variable (a box that contains a reference to a new instance.)ClassName object; object has no reference yet, (= null).

Create a new instance from a given class.object = new ClassName( ); object has a reference to a new instance.

Declare a reference variable and initialize it with a reference to a new instance created from a given classClassName object = new ClassName( );


Class Instantiation--Examplepublic class CourseDemo { public static void main( String[] args ) { Course myCourse1 = new Course( ); Course myCourse2 = new Course( );

myCourse1.department = "CSS"; myCourse1.number = 161; myCourse1.section = 'A'; myCourse1.textPrice1 = 111.75; myCourse1.textPrice2 = 37.95; myCourse1.courseFee = 15;

myCourse2.department = "CSS"; myCourse2.number = 451; myCourse2.section = 'A'; myCourse2.textPrice1 = 88.50; myCourse2.textPrice2 = 67.50; myCourse2.courseFee = 0;

System.out.println( "myCourse1(" + myCourse1.department + myCourse1.number + myCourse1.section + ") needs $" + myCourse1.totalExpenditure( ) ); System.out.println( "myCourse2(" + myCourse2.department + myCourse2.number + myCourse2.section + ") needs $" + myCourse2.totalExpenditure( ) ); }}

public class Course { public String department; // CSS, IAS, BUS, NRS, EDU public int number; // course number public char section; // A, B, C ... public double textPrice1; // primary textbook public double textPrice2; // secondary textbook public double courseFee; // laboratory fee

public double totalExpenditure( ) {return textPrice1 + textPrice2 + courseFee;

}}

Multiple pieces of data

Method

myCourse1myCourse2

164.7 = 117.75 + 37.95 + 15 156.0 = 88.50 +67.50 + 0

instantiatedinstantiated

department: CSSnumber: 161section: AtextPrice1: $111.75textPrice2: $37.95courseFee: $15totalExpenditure( )

department: CSSnumber: 451section: AtextPrice1: $88.50textPrice2: $67.50courseFee: $0totalExpenditure( )

myCourse1(CSS161A) needs $164.7myCourse2(CSS451A) needs $156.0

The this Parameter

All instance variables are understood to have <the calling object>. in front of them

If an explicit name for the calling object is needed, the keyword this can be used myInstanceVariable always means and is

always interchangeable with this.myInstanceVariable

The this Parameter

this must be used if a parameter or other local variable with the same name is used in the method Otherwise, all instances of the variable name

will be interpreted as local

int someVariable = this.someVariable

local instance

The this Parameter

The this parameter is a kind of hidden parameter

Even though it does not appear on the parameter list of a method, it is still a parameter

When a method is invoked, the calling object is automatically plugged in for this

A Constructor has a this Parameter

Outline


Constructors

A constructor is a special kind of method that is designed to initialize the instance variables for an object:Public ClassName(anyParameters){code} A constructor must have the same name as

the class A constructor has no type returned, not even void

Constructors are typically overloaded

Constructors

A constructor is called when an object of the class is created using new:ClassName objectName = new ClassName(anyArgs); The name of the constructor and its parenthesized list of

arguments (if any) must follow the new operator This is the only valid way to invoke a constructor: a

constructor cannot be invoked like an ordinary method If a constructor is invoked again (using new), the first object is

discarded and an entirely new object is created If you need to change the values of instance variables of

the object, use mutator methods instead

You Can Invoke Another Method in a Constructor The first action taken by a constructor is to create an

object with instance variables Therefore, it is legal to invoke another method within

the definition of a constructor, since it has the newly created object as its calling object For example, mutator methods can be used to set the

values of the instance variables It is even possible for one constructor to invoke

another

Include a No-Argument Constructor

If you do not include any constructors in your class, Java will automatically create a default or no-argument constructor that takes no arguments, performs no initializations, but allows the object to be created

If you include even one constructor in your class, Java will not provide this default constructor

If you include any constructors in your class, be sure to provide your own no-argument constructor as well

Default Variable Initializations

Instance variables are automatically initialized in Java boolean types are initialized to false Other primitives are initialized to the zero of

their type Class types are initialized to null

However, it is a better practice to explicitly initialize instance variables in a constructor

Note: Local variables are not automatically initialized


Accessing Instance Variables Declaring an instance variable in a class

public type instanceVariable; // accessible from any methods (main( ))

private type instanceVariable; // accessible from methods in the same class Examples:

public String department; public int number; public char section; public int enrollment;

Assigning a value to an instance variable of a given object objectName.instanceVariable = expression; Examples:

myCourse1.department = "CSS";myCourse1.number = 161;myCourse1.section = 'A';myCourse1.enrollment = 24;

Reading the value of an instance of a given object Operator objectName.instanceVariable operator Example:

System.out.println( "myCourse1 = " + myCourse1.department + myCourse1.number + ")" );


Class Names and Files

Each class should be coded in a separate file whose name is the same as the class name + .java postfix.

Example Source code

Course.java

CourseDemo.java

Compilation (from DOS/Linux command line.)javac Course.java javac CourseDemo.java

javac CourseDemo.java javac Course.java

Compiled codeCourse.class

CourseDemo.class

Execution (from DOS/Linux command line.)java CourseDemo Start with the class name that includes main( ).

Either order is fine. Compiling CourseDemo.java firstautomatically compiles Course.java, too.

Information Hiding and Encapsulation

Information hiding is the practice of separating how to use a class from the details of its implementation Abstraction is another term used to express the

concept of discarding details in order to avoid information overload

Encapsulation means that the data and methods of a class are combined into a single unit (i.e., a class object), which hides the implementation details Knowing the details is unnecessary because

interaction with the object occurs via a well-defined and simple interface

In Java, hiding details is done by marking them private

Outline


A Couple of Important Acronyms: API and ADT The API or application programming

interface for a class is a description of how to use the class A programmer need only read the

API in order to use a well designed class

An ADT or abstract data type is a data type that is written using good information-hiding techniques

Encapsulation

public and private Modifiers

The modifier public means that there are no restrictions on where an instance variable or method can be used

The modifier private means that an instance variable or method cannot be accessed by name outside of the class

It is considered good programming practice to make all instance variables private

Most methods are public, and thus provide controlled access to the object

Usually, methods are private only if used as helping methods for other methods in the class


public and private

Public Methods and instance variables

accessible from outside of their class

Private Methods and instance variables

accessible within their class

public type method1( ) {

}


}

private type utility( ) {

}

public String department;

public int number;

private int enrollment;

private int gradeAverage;

public class Course

css263.method1( )

css263.method2( )

css263.utility( )

css263.department

css263.number

css263.enrollment


Encapsulating Data in Class

Which design is more secured from malicious attacks?


}


}

private type utility( ) {

}

private String department;

private int number;

private int enrollment;

private int gradeAverage;

public class Course


}


}

public type utility( ) {

}

public String department;

public int number;

public int enrollment;

public int gradeAverage;

public class Course

css263.method1( )css161.method1( )

css161.utility( )

css161.number = 263

Accessor and Mutator Methods

Accessor methods allow the programmer to obtain the value of an object's instance variables The data can be accessed but not changed The name of an accessor method typically starts

with the word get Mutator methods allow the programmer to change

the value of an object's instance variables in a controlled manner Incoming data is typically tested and/or filtered The name of a mutator method typically starts with

the word set

Mutator Methods Can Return a Boolean Value Some mutator methods issue an error

message and end the program whenever they are given values that aren't sensible

An alternative approach is to have the mutator test the values, but to never have it end the program

Instead, have it return a boolean value, and have the calling program handle the cases where the changes do not make sense

Outline


Overloading

Overloading--two or more methods in the same class have the same method name

Overloading--Rules

All definitions of the method name must have different signatures A signature consists of the name of a method

together with its parameter list Differing signatures must have different numbers

and/or types of parameters The signature does not include the type returned Java does not permit methods with the same

signature and different return types in the same class


Overloading—example

Familiar overloaded method

System.out.println()

System.out.println(boolean)

System.out.println(char)

System.out.println(char[])

System.out.println(double)

System.out.println(float)

System.out.println(int)

System.out.println(long)

System.out.println(java.lang.Object)

System.out.println(java.lang.String)

Overloading and Automatic Type Conversion If Java cannot find a method signature that

exactly matches a method invocation, it will try to use automatic type conversion

The interaction of overloading and automatic type conversion can have unintended results

In some cases of overloading, because of automatic type conversion, a single method invocation can be resolved in multiple ways Ambiguous method invocations will produce

an error in Java

Outline


Outline

Introduction/Overview Constructors Information Hiding Overloading Recursion Homework! Odds and Ends


Nothing to return

The same data type

A variable, a constant, an expression, or an object

Two kinds of methods Methods that only perform an action

public void methodName( ) { /* body */ } Example

public void writeOutput( ) {

System.out.println(month + " " + day + ", " + year); }

Methods that compute and return a resultpubic type methodName( ) {

/* body */return a_value_of_type;

} Example

public double totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee;

}

More About Methods


return Statement

Method to perform an action void Method( ) Return is not necessary but

may be added to end the method before all its code is ended

Examplepublic void writeMesssage( ) { System.out.println( “status” ); if ( ) { System.out.println( “nothing” ); return; } else if ( error == true ) System.out.print( “ab” ); System.out.println( “normal” );}

Method to perform an action type Method( ) Return is necessary to return a

value to a calling method. Examplepublic double totalExpenditure( ) { return textPrice1 + textPrice2 + courseFee;}


Any Method Can Be Used As a void Method A method that returns a value can also

perform an action

If you want the action performed, but do not need the returned value, you can invoke the method as if it were a void method, and the returned value will be discarded:objectName.returnedValueMethod();

Methods That Return a Boolean Value

An invocation of a method that returns a value of type boolean returns either true or false

Therefore, it is common practice to use an invocation of such a method to control statements and loops where a Boolean expression is expectedif-else statements, while loops, etc.

swe 510: object oriented programming in java1 defining classes 1 this slide set was compiled from...

Documents

object oriented programming

data memberseach object

set of data

object instantiation

public int enrollment

public double totalexpenditure

public double capacity

havethese data items