Object Oriented Concepts through Java

8/7/2019 Object Oriented Concepts through Java

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Object Oriented ConceptsObject Oriented Concepts

and Programming throughand Programming through

JAVAJAVA


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Access SpecifiersAccess Specifiers Javas access specifiers are

public,

private,

Protected and

a default access level.

When a member of a class is specified as public, then that

member can be accessed by any other code.

When a member of a class is specified as private, then that

member can only be accessed by other members of its class.

Thats why main( ) has always been preceded by the public

specifier.

It is called by code that is outside the programthat is, by

the Java run-time system.


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Access SpecifiersAccess Specifiers

When no access specifier is used, then by default the

member of a class is public within its own package,

but cannot be accessed outside of its package.

Protected used when inheritance is there. Example


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ContCont

Instance variables declared as static are, essentially,

global variables.

When objects of its class are declared, no copy of a static

variable is made.

Instead, all instances of the class share the same static

variable.

Methods declared as static have several restrictions:

They can only call otherstatic methods.

They must only access static data.

They cannot refer to this or super in any way.


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InheritanceInheritance

It allows the creation of hierarchical classifications.

you can create a general class that defines traits or

properties common to a set of related items.

This class can then be inherited by other, more specificclasses, each adding those things that are unique to it.

In the terminology of Java, a class that is inherited is

called a superclass.

The class thatdoes the inheriting is called a subclass. A subclass is a specialized version of a superclass.

It inherits all of the instance variables and methods

defined by the superclass and adds its own, unique

elements.


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ContCont

To inherit a class, we incorporate the definition of one class

into another by using the extends keyword.

class A{}

class B extends A{}

Example

Superclass can be used as a complete independent,

stand-alone class.

A subclass can be a superclass for another subclass.

The general form

class subclass-name extends superclass-name {

// body of class

}


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Member Access and InheritanceMember Access and Inheritance

A subclass includes all of the members of its

superclass, it cannot access those members

of the superclass that have been declared as

private.


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Using super()Using super()

There will be times when you will want to create a

superclass that keeps the details of its implementation

to itself.

That is, that keeps its data members private. Whenever a subclass needs to refer to its immediate

superclass, it can do so by use of the keyword super.

super has two general forms.

The first calls the superclass constructor. The second is used to access a member of the

superclass that has been hidden by a member of a

subclass.


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ContCont Using super to Call Superclass Constructors:

super(parameter-list);

super( ) must always be the first statement executed

inside a subclass constructor.

When a subclass calls super( ), it is calling the constructorof its immediate superclass.

Thus, super( ) always refers to the superclass immediately

above the calling class.

A Second Use for super : super.member

This second form ofsuper is most applicable to

situations in which member names of a subclass hide

members by the same name in the superclass.


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Constructor calling sequenceConstructor calling sequence

In a class hierarchy, constructors are called in order of

derivation, from superclass to subclass.

Since super( ) must be the first statement executed

in a subclass constructor, this order is the samewhether or not super( ) is used.

If super( ) is not used, then the default or

parameterless constructor of each superclass will be

executed.


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Method OverridingMethod Overriding

When a method in a subclass has the same name and type

signature as a method in its superclass, then the method in the

subclass is said to override the method in the superclass.

When an overridden method is called from within a subclass, it

will always refer to the version of that method defined by thesubclass.

If you wish to access the superclass version of an overridden

function, you can do so by using super.

Method overriding occurs only when the names and the type

signatures of the two methods are identical. If they are not, thenthe two methods are simply overloaded.

Overridden methods allow Java to support run-time

polymorphism.


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ContCont

Polymorphism is essential to object-orientedprogramming for one reason: it allows a general classto specify methods that will be common to all of itsderivatives, while allowing subclasses to define the

specific implementation of some or all of thosemethods.

Overridden methods are another way that Javaimplements the one interface, multiple methodsaspect of polymorphism.


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Using Abstract ClassesUsing Abstract Classes

Sometime you want to define a superclass that declares the

structure of a given abstraction without providing a complete

implementation of every method.

Sometimes you will want to create a superclass that only definesa generalized form that will be shared by all of its subclasses,

leaving it to each subclass to fill in the details.

Such a class determines the nature of the methods that the

subclasses must implement.

One way this situation can occur is when a superclass is unableto create a meaningful implementation for a method.

Figure class area() method

You may have methods which must be overridden by the

subclass in order for the subclass to have any meaning.


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Cont

Sometime you want some way to ensure that a subclass

does, indeed, override all necessary methods. Javas

solution to this problem is the abstract method.

You can require that certain methods be overridden bysubclasses by specifying the abstract type modifier.

These methods are sometimes referred to as subclasser

responsibilitybecause they have no implementation

specified in the superclass.

Thus, a subclass must override themit cannot simply use

the version defined in the superclass.

To declare an abstract method, use this general form:

abstract type name(parameter-list);


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Cont Any class that contains one or more abstract methods must also

be declared abstract.

To declare a class abstract, you simply use the abstract keywordin front of the class keyword at the beginning of the classdeclaration.

There can be no objects of an abstract class.

That is, an abstract class cannot be directly instantiated with thenew operator.

Such objects would be useless, because an abstract class is notfully defined.

Also, you cannot declare abstract constructors, or abstract staticmethods.

Any subclass of an abstract class must either implement all of theabstract methods in the superclass, or be itself declaredabstract.


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Using final with Inheritance Using final to Prevent Overriding

There will be times when you will want to prevent it from

occurring.

To disallow a method from being overridden, specify final as a

modifier at the start of its declaration. Methods declared as final cannot be overridden.

Early and late binding

Using final to Prevent Inheritance

Sometimes you will want to prevent a class from beinginherited.

To do this, precede the class declaration with final. Declaring

a class as final implicitly declares all of its methods as final,

too.

It is illegal to declare a class as both abstract and final


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Packages

Packages are containers for classes that are used to

keep the class name space compartmentalized.

For example, a package allows you to create a class

named List, which you can store in your own packagewithout concern that it will collide with some other class

named List stored elsewhere.

Packages are stored in a hierarchical manner and are

explicitly imported into new class definitions. Name collision without packages.

Java provides a mechanism for partitioning the class

name space into more manageable chunks.


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Cont

This mechanism is the package. The package is both a

naming and a visibility control mechanism.

You can define classes inside a package that are not

accessible by code outside that package. You can also define class members that are only

exposed to other members of the same package.

This allows your classes to have intimate knowledge of

each other, but not expose that knowledge to the restof the world.


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String Class

Every string you create is actually an object of type String.

Objects of type String are immutable; once a String object is

created, its contents cannot be altered.

This may seem like a serious restriction, it is not, for two reasons:

If you need to change a string, you can always create a new

one that contains the modifications.

Java defines a peer class ofString, called StringBuffer,

which allows strings to be altered, so all of the normal string

manipulations are still available in Java. String myString = "this is a test";

System.out.println(myString);


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Cont

Operator forString objects: +. It is used to

concatenate two strings.

String myString = "I" + " like " + "Java.";

The String Constructors

String s = new String();

String(charchars[ ])

String(charchars[ ], int startIndex, int numChars)

String(String strObj)

String(byte asciiChars[ ])

String(byte asciiChars[ ], int startIndex, int

numChars)


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Cont The length of a string is the number of characters that it contains. int length( ); is used to get length.

char chars[] = { 'a', 'b', 'c' };

String s = new String(chars);

System.out.println(s.length()); String concatenation is done with various data types.

String age = 9;

String s = "He is " + age + " years old.";

System.out.println(s);

What will be the output of following code?

String s = "four: " + 2 + 2;

System.out.println(s);


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Cont

String Conversion and toString( )

Java converts data into its string representation during

concatenationby using overloaded versions of the string

conversion method valueOf( ) defined by String.

valueOf( ) is overloaded for all the simple types and for

type Object.

For objects, valueOf( ) calls the toString( ) method on the

object.

Every class implements toString( ) because it is defined byObject.

General form is String toString( );

Example of Box Class


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Cont

Character Extraction charAt()

Extract a single character from a string

char charAt(int where)

ch = "abc".charAt(1);

getChars()

To extract more than one character at a time

void getChars(int sourceStart, int sourceEnd, char target[

], int targetStart)

getBytes()

To stores the characters in an array of bytes.

byte[ ] getBytes( )


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Cont

toCharArray()

To convert all the characters in a String object into a

character array

char[ ] toCharArray( )


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String Comparison

equals() and equalsIgnoreCase()

To compare two strings for equality, use equals( ).

boolean equals(Object str)

It returns true if the strings contain the same

characters in the same order, and false otherwise.

The comparison is case-sensitive.

To perform a comparison that ignores case

differences, call equalsIgnoreCase( ).

boolean equalsIgnoreCase(String str)


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Cont

String s1 = "Hello";


String s3 = "Good-bye";

String s4 = "HELLO";

s1.equals(s2);

s1.equals(s3);

s1.equals(s4);

s1.equalsIgnoreCase(s4);


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Cont regionMatches()

Compares a specific region inside a string with another

specific region in another string.

boolean regionMatches(int startIndex, String str2, int

str2StartIndex, int numChars)boolean regionMatches(boolean ignoreCase, int

startIndex, String str2, int str2StartIndex, int numChars)

startsWith() and endsWith()

The startsWith( ) method determines whether a givenString begins with a specified string.

The endsWith( ) determines whether the String in

question ends with a specified string.

Second form of startsWith() is boolean startsWith(String str,

int startIndex)


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equals() Versus ==

the equals( ) method compares the characters

inside a String object.

The == operator compares two object referencesto see whether they refer to the same instance.


String s2 = new String(s1);

System.out.println(s1 + " equals " + s2 + " -> " +

s1.equals(s2));

System.out.println(s1 + " == " + s2 + " -> " + (s1 ==

s2));


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Cont

Searching Strings

indexOf( ) Searches for the first occurrence of a

character or substring.

lastIndexOf( ) Searches for the last occurrence of acharacter or substring.

int indexOf(int ch)

int indexOf(String str)

int indexOf(int ch, int startIndex)

int indexOf(String str, int startIndex)


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Cont

Modifying a String

substring( )

You can extract a substring using substring( ).

String substring(int startIndex) String substring(int startIndex, int endIndex)

result = org.substring(0, 7);

concat( )

You can concatenate two strings using concat( ) String concat(String str)

replace( )

The replace( ) method replaces all occurrences of one

character in the invoking string with another character.


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Cont

String replace(charoriginal, charreplacement)

String s = "Hello".replace('l', 'w');

trim( )

The trim( ) method returns a copy of the invoking stringfrom which any leading and trailing whitespace has been

removed.

String s = " Hello World ".trim();

The trim( ) method is quite useful when you process usercommands.


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Cont

Data Conversion Using valueOf() The valueOf( ) method converts data from its internal

format into a human-readable form.

static String valueOf(double num)

static String valueOf(long num)

static String valueOf(Object ob)

static String valueOf(charchars[ ])

Changing the Case of Characters String toLowerCase( )

String toUpperCase( )


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Cont

StringBuffer

String represents fixed-length, immutable character

sequences. In contrast,

StringBufferrepresents growable and writeable character

sequences.

StringBuffermay have characters and substrings inserted in

the middle or appended to the end.

StringBufferwill automatically grow to make room for such

additions and often has more characters preallocated than areactually needed, to allow room for growth.

StringBuffer( )

StringBuffer(int size)

StringBuffer(String str)


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Cont

int length( )

int capacity( )

void setLength(int len)

len value must be nonnegative. If you call setLength( ) with a value less than the current

value returned by length( ), then the characters stored

beyond the new length will be lost.

char charAt(int where) void setCharAt(int where, charch)

getChars( )

void getChars(int sourceStart, int sourceEnd, chartarget[

],int targetStart)


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Cont

append()

The append( ) method concatenates the string representation

of any other type of data to the end of the invoking

StringBufferobject.

StringBuffer append(String str)

StringBuffer append(int num) StringBuffer append(Object obj)

insert()

The insert( ) method inserts one string into another.

StringBuffer insert(int index, String str) StringBuffer insert(int index, charch)

StringBuffer insert(int index, Object obj)

StringBuffer sb = new StringBuffer("I Java!");

sb.insert(2, "like ");


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Cont

reverse( )

StringBuffer reverse( )

This method returns the reversed object on which it was

called. delete() and deleteCharAt( )

StringBuffer delete(int startIndex, int endIndex)

StringBuffer deleteCharAt(int loc)

replace() It replaces one set of characters with another set inside a

StringBufferobject.

StringBuffer replace(int startIndex, int endIndex, String

str)


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Cont

substring()

Returns a portion of a StringBuffer.

String substring(int startIndex)

String substring(int startIndex, int endIndex)

Object Oriented Concepts through Java

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