unit 2 - basic java and object oriented

8/8/2019 Unit 2 - Basic Java and Object Oriented

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UNIT 2BASIC JAVA & OBJECT ORIENTEDFaculty of Computer Science 2007Universitas Indonesia

Objectives

Introduction to Class

Statement Control in java

Initialization

Object creation and lifetime

Constructor


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Introduction to Class Diagram

A class diagram shows the existence of classes and

their relationships in the logical view of a system


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Example


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Javas Control Statement

Java's syntax is made to mimic C

Control Statements in Java is similar with C.

Control Structures

Two basic types of control structures: Selection: Given one or more possible choices: choose which section (if

any) of an algorithm to execute.

Iteration: Repeat a section of an algorithm provided required conditionsare met. Also known as looping.

Selection Control: The IfThen_Else statement. Provides up to two possible alternatives.

The Case statement. Provides any number of possible alternatives. Repetition Control:

while

dowhile

for


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Exercise

What is the output of the following :

int k = 100;

for (int i = 1; i 5; k--)

{

System.out.println(k);}

System.out.println(k + "\n");

}

Exercise

Consider the following commission scheme:

Create the control statement to determine the

correct monthly income


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Exercise

for(int m = 10; m>5; m--)

{

for(int i=0; i target)

return +1;

else if(testval < target)

return -1;

else

return 0;

}

public static void main(String[] args) {System.out.println(test(10, 5));System.out.println(test(5, 10));System.out.println(test(5, 5));

}

}


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Conditional Operator (? : )

Javas only ternary operator (takes three operands)

? : and its three operands form a conditionalexpression

Entire conditional expression evaluates to the secondoperand if the first operand is true

Entire conditional expression evaluates to the thirdoperand if the first operand is false

Conditional Operator (? : )

int ternary(int i) {

return i < 10 ? i * 100 : i * 10;

}

int alternative(int i) {

if (i < 10)return i * 100;

else

return i * 10;

}


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switch Multiple Selection Statement

When the expression matches a case, the statements

in the case are executed until:

A break statement is encountered or

The end of the switch statement is encountered

The default clause is optional

If the default clause is supplied then it is executed if

the switch expression does not match any of the case

constants

switch Multiple Selection Statement

switch(c) {case 'a':case 'e':case 'i':case 'o':case 'u': System.out.println("vowel"); break;case 'y':case 'w':System.out.println("Sometimes a vowel");break;default: System.out.println("consonant");

}


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Mistakes are easy with switch

Care must be taken to ensure that the break statements areplaced where they are required. The compiler cannot checkthis for you because the break statement is not required tofollow each case.switch ( choice)

{

case Y: case y:

repeatAgain = true;

case N: case n:

repeatAgain = false;

}

What is the error in the above statement?

Corrected Version

The correct version should be:switch ( choice)

{

case Y: case y:

repeatAgain = true;

break;case N: case n:

repeatAgain = false;

}


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break and continue Statement

break/continue

Alter flow of control

break statement

Causes immediate exit from control structure

Used in while, for, dowhile or switch statements

continue statement

Skips remaining statements in loop body

Proceeds to next iteration Used in while, for or dowhile statements

break Statement

1 // Fig. 5.12: BreakTest.java2 // break statement exiting a for statement.3 public class BreakTest4 {5 public static void main( String args[] )6 {7 int count; // control variable also used after loop terminates8

9 for ( count = 1; count


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continue Statement

1 // Fig. 5.13: ContinueTest.java 2 // continue statement terminating an iteration of a for statement. 3 public class ContinueTest4 {5 public static void main( String args[] )6 {7 for ( int count = 1; count


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Primitive vs Reference Variables

Primitive variables: Direct access to the data

Modification of the data involves modification to thecontents of the variable

Reference variables: Indirect access to the dataModification of the data within an object must be done

via object method

Example:frogName = froggie.getName();

froggie.setName(Kermit);

Initialization

What is stored in a variable when it is created?

Java auto-initializes variables

Primitive variables:

Numeric set to zero

char set to blank

boolean set to false

Object variables: Set to null

null represents an invalid memory address

Not all programming languages auto-initialize so it is extremely badprogramming practice to rely on auto-initialization

You should always explicitly initialize variables


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Initialization and Clean Up

Many C bugs occur when the programmer forgets to

initialize a variable. This is especially true withlibraries when users dont know how to initialize a

library component, or even that they must.

Cleanup is a special problem because its easy to

forget about an element when youre done with it,

since it no longer concerns you.

Thus, the resources used by that element areretained and you can easily end up running out of

resources (most notably, memory).

Order of Initialization

Within a class, the order of initialization is

determined by the order that the variables are

defined within the class.

The variable definitions may be scattered

throughout and in between method definitions,but the variables are initialized before any

methods can be calledeven the constructor


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Example:

class Counter {

int i;

Counter() { i = 7; }

// . . .

The variable i will be first initialized to 0, then to 7.

This is true with all the primitive types and with objectreferences, including those that are given explicit

initialization at the point of definition.

Constructors

Used to perform any initialization required.

The name of the constructors must be identical to the name of theclass.

Even though the constructor is returning a value to the calling module,we do not specify a return type for the method (as it must be apointer to the object).

When an object is instantiated, storage is allocated and theconstructor is called

It is guaranteed that an object is initialized before you can use it.

The object reference returned by the new operator is essentially returned

by the constructor

After we have used a constructor to create an object, we call theobject an instance of a class.


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The Constructor

//: c04:SimpleConstructor.java

// Demonstration of a simple constructor.

class Rock {

Rock() { // This is the constructor

System.out.println("Creating Rock");

}

}

public class SimpleConstructor {

public static void main(String[] args) {

for(int i = 0; i < 10; i++)

new Rock();

}

}

Like any method, the constructor can have

arguments.

The constructor arguments allow you to provide

initialization parameters when instantiating an

object


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//: c04:SimpleConstructor2.java

// Constructors can have arguments.

class Rock {

int myNumber;

Rock(int i) { // This is the constructor

myNumber = i;

System.out.println("Creating Rock No: " + myNumber);

}

}

public class SimpleConstructor {


for(int i = 0; i < 10; i++)new Rock(i);

}

}

There may be a variety of ways in which the objectcan be created.

A constructor should be defined for each case.

Method overloading is used to select the correctconstructor for each instantiation.


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The Constructor

// Constructors can be overloaded.

class Rock {

int myNumber;

Rock() { // This is the constructor

// myNumber is left at 0

}

Rock(int i) { // This is the constructormyNumber = i;

}

} // end class Rock

The new operator must call a constructor

The constructor must both exist and be exposed(remember access control?) or the compiler will givean error

If you don't code a constructor in the class definition,

the compiler supplies a default constructor with noarguments

also called the "no args" constructor

If you define any constructors, the compiler will notprovide a default!


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Default Constructor

//: c04:DefaultConstructor.java

class Bird {

int i;

}

public class DefaultConstructor {


Bird nc = new Bird(); // default!}

}

Default Constructor

// Oops! NoDefaultConstructor.java

class Bird {

int wingspan;

Bird(int i) {

wingspan = i;

} // end constructor

}

public class NoDefaultConstructor {


Bird nc = new Bird(); // Compiler error!

}

}


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Default Constructor

A constructor which has no parameters is called adefault constructor.

If a class definition does not include any constructorsthen a default constructor is assumed.

If a class has not explicitly been defined to inheritfrom a specified super class then it is assumed toinherit from the predefined class java.lang.Object.

The default constructor for the super class is always

called prior to executing the constructor for the subclass.

The Java String Class

Generally a string is a collection of 1 or more characters

The Java String class provide us with the facility tohandle strings

String variables are objects but they can be used as ifthey were primitive variablesString unitName = Design and Programming;

Can also be treated as an objectString unitName = new String(Design and Programming);

String can also be concatenated using the plus operator Example: int x=42;

String message;

message = x = + x;


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Exercise

Create a class with a no-args constructor that

prints a message.

Include a main method that instantiates an

instance of the class.

Add a constructor that takes a String as an

argument and prints that.

Referring to a Object from Within Itself:this

It is sometimes necessary to for an object to refer to

itself.

This is a problem for a Java programmer because

they need to express this selfreference in the class

design. i.e. They cannot write the object!

In the class design, whenever the selfreference is

required the reserved word this is used.

When an object is created from the class design then

this will be replaced by the actual address of that

object.


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The Two Uses of this

The two situations where selfreferencing is required are: When the address of this object needs to be passed as a parameter to

a method which resides outside of the class specification for the object.

When a Java programmer wishes to name a local method IMPORTparameter with the same name as its corresponding class field.

Example of when the address of this object needs to bepassed on to some other part of the program:

An object which will display a GUI has a close button object declaredwithin in it.

The close button needs a reference to this object so that it can inform it

when the button has been pressed.

quitButton.addActionListener( this);

The this Keyword

// Simple use of the "this" keyword

public class Leaf {

int i = 0;

Leaf increment() {

i++;

return this;

} // end method increment

void print() {System.out.println("i="+i);


Leaf l = new Leaf();

l.increment().increment().print();

} // end method main

} // end class Leaf


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The this Keyword

// Another use of the "this" keyword

public class Leaf {

int i = 0; // i is part of the object

void increment(int i) { // i as local, too

this.i = this.i + i; // The compiler knows!

} // end method increment} // end class Leaf

Constructor Calling Constructor

When you define a class with several

constructors, you'd often like to call one

constructor from another to avoid duplicating

code.

A constructor is called using: this You can, using this with an argument list:

this(); // calls no-args constructor

this(27); // constructor with numeric arg

But there are rules!


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A constructor can call another constructor only

in the first statement of the constructor.

This implies that a constructor can only call one

other constructor!


Modify your class so that the no-args

constructor, instead of printing a specified

value, calls the other constructor


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The Meaning of Static

Now that we've seen the this keyword, we're more

able to deal with static .

Members that are part of a particular object can

be referenced only with an object reference

(including this ).

Members that are part of a class - and shared by

all objects of that type - are not referred to by

object reference. Java: Static method and static variable

C: Global function and global variable

Static Data Initialization

When you say something is static, it means that data

or method is not tied to any particular object instance

of that class.

So even if youve never created an object of that class

you can call a static method or access a piece of static

data.

With ordinary, non-static data and methods, you must

create an object and use that object to access the

data or method, since non-static data and methods

must know the particular object they are working with.


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Because static members exist whether or not

any object of the type is instantiated, static

members can reference only other static

members

static methods can call only other static methods

static methods can reference only static variables


Static methods are also referred to as "class

methods", and static variables as "class

variables, because they are shared by all

objects in the class.

Is this a way of cheating on the "everything isobjects" and "there are no globals?" Only until

you get a more complete understanding of

how things such as class loading really work.


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Specifying Initialization

You can specify an initialization expression at the

point you define a variable.

class Measurement {

boolean b = true;

char c = 'x';

short s = 0xff;

int i = 999;

}

You can initialize non-primitives in the same way.

String s = new String("Hello");

Specifying Initialization

You can even call a method:

String s = getDefaultString();

The method can have arguments, but the arguments

cannot be other class members that haven't been

initialized yet.

Order is important!


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Static variable initialization

The first time an object of a given type is

instantiated, or a static member of that class

is referenced, the JVM locates the .class

file and loads it into memory.

When the class is loaded, all static initializers

are run in the order they are encountered inthe .class file.


Explicit static initialization

Java allows you to group static initializations inside

a static block:

static int i;

static {i = 999;

}

This block is executed the first time you instantiate

an object of the class type, or the first time you

access a static member of the class.


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Non-static variable allocation and

initialization.

Constructor is called.

Naming Variables, Methods, and Classes

Programmer constantly has to invent names for classes, methods,variables, constants, etc

We call these names identifiers

You need to give considerable thought to every name you createthey are your bricks glued together with the mortar of the operators

Names should be:

Unique

Meaningful

Unambiguous

Consistent

Enhance by case

There are also reserved words (which are always in lower case):identifiers with special meaning which must be used in predefinedways e.g. class, public, etc


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Rules for Identifiers

Consist only of letters, digits, _ or $

Cannot start with a digit

Cannot be a reserved word

Case sensitive, example:

StNo, stno, STNO, Stno are all different variables

Can be any length

Guidelines for Identifers

Meaningful: give a name that truly reflects the natureof value it hold e.g. studentNo is different tonoOfStudents

Readable: e.g. studentNo not stdnbr

Consistent: be consistent in all aspectsabbreviation,case, identation, etc

Avoid verbosity:identification_Number_Of_The_Student that length is unlimited avoid overly long names

Use underscore to good effect

Use capitalization to good effect


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Java Identifier Naming Convention

Constants should be completely uppercase Example:public static final int MAXSTUDENTS =30000;

Class name should be capitalized Example:public class ThisIsAClass

Methods and variables should be internallycapitalized Example:public void thisIsAMethod();

private double thisIsAVariable;

Classes in Java: .java Files and Class Names

The naming conventions in java specify that a class nameshould be capitalised.

Each class definition should be in its own .java file.

The name of the .java file should follow the sameconvention.

Problems can sometimes arise when using an operating system

which is not case sensitive (i.e. MSDOS).

The act of compiling an application will result in a .classfile being generated for each Java class defined. Thisshould mean for each .java file.

The class files will contain the Java byte code to beinterpreted when the application is run using the Javainterpreter.


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Classes in Java: .java Files and Class Names

A reasonable size application can have hundreds of .java files!

This means that you have to stay organised

Keep all of the .java files for an application in one folder

DO NOTkeep any .java files which are not associated with theapplication in the folder.

Name the folder with the same name as the application.

Much of the Java that you write might be applicable across anumber of applications.

These general purpose classes should be grouped together in a

library which is accessible to all of your applications. In Java we call such a library a package.

Nouns and Verbs

Like algorithm design, the determination of what classes should be used isstill, by and large, an art form.

One shallow technique is the nouns and verb approach:

Nouns are mapped to classes.

Verbs are mapped to sub modules within classes

The definition of noun and verb gets stretched to cover collections of words.

Result is that: sub module names should always describe an action (i.e. getName)

Class names should always describe a thing (e.g. PersonClass)

It is important to note that the set of classes proposed will change over thetime the software is being designed.

This is exactly the same principle as the steps in algorithms changing as thealgorithm is being refined.

This approach is both shallow and naive but its a very good starting point.

unit 2 - basic java and object oriented

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