1 part1 faadooengineers.com. 2 day 2 coverage inheritance method overriding abstract class interface...

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Part1

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Day 2 Coverage

• Inheritance

• Method Overriding

• Abstract Class

• Interface

• Overview of predefined packages

• Creating User Defined Packages

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Inheritance, Interface & Abstract Class

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Objectives

• Inheritance

• Method Overriding

• Use of super keyword

• Interface

• Abstract Class

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No Multiple Inheritance In Java

Inheritance

• The ability of a class of objects to inherit the properties and methods from another class, called its parent or base class.

• The class that inherits the data members and methods is known as subclass or child class.

• The class from which the subclass inherits is known as base / parent class.

• In Java, a class can only extend one parent.

What is Inheritance

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public class Person {

private String name;

public void setName(String name) {

this.name = name;

}

public String getName() {

return name;

}

}

public class Employee extends Person {

private String eid;

public void setEid(String eid) {

this.eid=eid;

}

public String getEid() {

return eid;

}

}

Parent Child

Inheritance (Continued)

Inheritance (Example)

Here ‘extends’ keyword,signaling

inheritance

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Example


Types of Inheritance

1. Single Level Inheritance

Derives a subclass from a single superclass.

Class Person

Class Employee Class Student

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Example


Types of Inheritance (Continued)

2. Multilevel Inheritance

Inherits the properties of another subclass.

Class Person

Class Employee

Class Regular

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• Method overriding is defined as creating a method in the subclass that has the same return type and signature as a method defined in the superclass.

Signature of a method includes

name number sequence type

of arguments in a method

Method Overriding

Overriding Members

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• You can override (most) methods in a parent class by defining a method with the same name and parameter list in the child class.

• This is useful for changing the behavior of a class without changing its interface.

• You cannot override the static and final methods of a superclass.

• A subclass must override the abstract methods of a superclass.

Method Overriding (Continued)

Overriding Members (Continued)

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public class Person { private String name; private String ssn; public void setName(String name) { this.name = name; } public String getName() { return name; }

public class Employee extends Person { private String empId; public void setID(String empID) { this.empID=empID; }

public String getId() {

return ssn;

}

}

public String getId() {

return empID;

}

}

getID method is overridde

n

Example on Method Overriding

Overriding Members (Continued)

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• Allows you to access methods and properties of the parent class,

public class Person { private String name;

public Person(String name) { this.name = name; } public String getName() { return name; }}

public class Employee extends Person { private String empID; public Student(String name) { super(name); // Calls Person constructor } public void setID(String empID){ this.empID=empID; } public String getID(){ return empID; }}

Use of super keyword

Example

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Restricting Inheritance

Parent

Child

Restricting Inheritedcapability

Using modifier with a class will restrict the inheritance capability of that class

final

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Final Classes: A way for Preventing Classes being extended

• We can prevent an inheritance of classes by other classes by declaring them as final classes.

• This is achieved in Java by using the keyword final as follows:

final class Regular

{ // members

}

final class Employee extends Person

{ // members

}

• Any attempt to inherit these classes will cause an error.

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Final Members: A way for Preventing Overriding of Members in Subclasses

• All methods and variables can be overridden by default in subclasses.

• This can be prevented by declaring them as final using the keyword “final” as a modifier. For example:

final int marks = 100;

final void display();

• This ensures that functionality defined in this method cannot be altered any. Similarly, the value of a final variable cannot be altered.

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• A programmer’s tool for specifying certain behaviors that an object must have in order to be useful for some particular task.

• Interface is a conceptual entity.

• Can contain only constants (final variables) and abstract method (no implementation).

• Use when a number of classes share a common interface.

• Each class should implement the interface.

What is an Interface

Interface

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• For example, you might specify Driver interface as part of a Vehicle object hierarchy.

A human can be a Driver, but so can a Robot.

Interface Example

Interface (Continued)

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interface InterfaceName {

// Constant/Final Variable Declaration// Methods Declaration – only method body

}

Interface Declaration


public interface Driver { void turnWheel (double angle); void pressBrake (double amount); } Syntax for Interface

DeclarationExample

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public class BusDriver extends Person implements Driver { // include each of the two methods from Driver }

Interface Implementation


class ClassName implements Interface1, Interface2,…., InterfaceN {

// Body of Class }

Syntax for Interface Implementation

Example

• Interfaces are used like super-classes who properties are inherited by classes. This is achieved by creating a class that implements the given interface as follows:

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speak()

Politician Priest

<<Interface>>Speaker

speak() speak()

Lecturer

speak()


More Examples on Interface

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• Declare the Speaker Interface with one method as speak.

• Define three class namely,

Politician

Priest

Lecturer

that implements the Speaker interface and defines the speak method.


Exercise

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• Like classes, interfaces can also be extended. The new sub-interface will inherit all the members of the super-interface in the manner similar to classes.

• This is achieved by using the extends keyword.

interface InterfaceName2 extends InterfaceName1 {

// Body of InterfaceName2 }


Inheriting Interfaces

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• An Abstract class is a conceptual class.

• An Abstract class cannot be instantiated – objects cannot be created.

• Abstract classes provides a common root for a group of classes, nicely tied together in a package.

• When we define a class to be “final”, it cannot be extended. In certain situation, we want properties of classes to be always extended and used. Such classes are called Abstract Classes.

Abstract Class

What is an Abstract Class

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• A class with one or more abstract methods is automatically abstract and it cannot be instantiated.

• A class declared abstract, even with no abstract methods can not be instantiated.

• A subclass of an abstract class can be instantiated if it overrides all abstract methods by implementing them.

• A subclass that does not implement all of the superclass abstract methods is itself abstract; and it cannot be instantiated.

• We cannot declare abstract constructors or abstract static methods.

Abstract Class (Continued)

Properties of an Abstract Class

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Abstract Class Example

Shape

Circle Rectangle

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abstract class ClassName

{

...…abstract DataType MethodName1();……DataType Method2() {

// method body}

}


Declaration of an Abstract Class

Syntax

abstract public class Shape {

public abstract double area();

public void move() { // non-abstract method // implementation }}

Example

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• Is the following statement valid?

Question


Shape sh = new Shape();

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public Circle extends Shape {private double r;private static final double PI =3.1415926535;public Circle() { r = 1.0; }public double area() { return PI * r * r; }

… } public Rectangle extends Shape {

private double l, b;public Rectangle() { l = 0.0; b=0.0; }public double area() { return l * b; }

... }


Implementation of an Abstract Class

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Exercise

• Declare the Train class with the following list of methods:

OrgName()

TrainNo()

TrainName()

FromTo()

• Define three class namely,

DreamExpress

LiveExpress

TimeExpress

• that extends from Train class and define the TrainNo(), TrainName and FromTo() method.

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• Inheritance is the concept of extending data members and methods of a superclass in a subclass.

• You can derive data members and methods from a single superclass that is a subclass of another superclass.

• Java does not support multiple inheritance directly.

• You can use the concept of method overriding to override the superclass method with the subclass method having same names.

• Interface is a concept of creating data members and methods that can be derived by multiple classes in Java.

• Interfaces also allow you to declare set of constants that can be imported into multiple classes.

Summary

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• The methods declared in the interface are defined in the class implementing that interface.

• The methods in an interface are only abstract methods.

• Abstract classes provides a common root for a group of classes, nicely tied together in a package.

Summary (Continued)

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1. The class that inherits the data members and method is known as _ _ _ _ _ _.

a. Abstract Class

b. Sub class

c. Base class

d. Inner class

2. Which keyword is used to define the constants in an interface?

a. super

b. public

c. private

d. final

Test Your Understanding

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3. Which concept of object-oriented programming is used in method overriding?

a. Polymorphism

b. Abstraction

c. Encapsulation

d. Inheritance

4. What is the access specifier of the methods declared in an interface?

a. Default

b. private

c. protected

d. public

Test Your Understanding (Continued)

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Packages

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Objectives

• Packages in Java

• Overview of java.lang package

• Overview of java.util package

• Creating User Defined Package

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• A package is a set of classes that are stored in a directory, which has the same name as the package name.

• Packages enable you to organize the class files provided by Java.

Packages in Java

Java packages are classified into

Java defined / Built inpackages

User defined packages

Packages in Java

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Built in Java Packages

Packages in Java (Continued)

Java Package Name

Description

java.langIncludes various classes, such as Object, System, Thread etc.

java.io Includes all Input-Output Stream related classes.

java.util Provides various classes that support Date, Collection.

java.sql Provides API for Database operation

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Built in Java Packages


Java Package Name

Description

java.awtProviding Supporting classes for Graphic User Interface components.

java.appletProvides the Applet class to create web based application.

java.netIncludes classes that support network programming such as Socket, DatagramSocket.

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Java

java.lang java.io java.util java.awt java.applet java.net java.sql

Hierarchy of Java Packages

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• The java.lang package provides various classes and interfaces that are fundamental to Java programming.

• The java.lang package contains various classes that represent primitive data types, such as int, char, long, and double.

• Classes provided by the java.lang package:

Exploring java.lang package

Class Description

ObjectAll the classes in the java.lang package are the subclasses of the Object class.

SystemIt provides a standard interface to input output and error devices, such as Keyboard & VDU.


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Exploring java.lang package (Continued)

Class Description

ClassSupports runtime processing of the class information of an object.

StringProvides functionality for String manipulation.

IntegerProvides methods to convert an Integer object to a String object.

MathProvides functions for statistical, exponential operations.


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Exploring java.lang.Wrapper Class

Method Name Description

public int intValue()Returns the value of an object as the primitive data type int.

public float floatValue()Returns the value of an object as the primitive data type float.

public double doubleValue()Returns the value of an object as the primitive data type double.


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Exploring java.lang.Math Class


public static int max (int n1 int n2)

public static float max (float n1, float n2)

public static double max (double n1,double n2)

Returns the maximum of two numbers.

public static int min (int n1 int n2)

public static float min (float n1, float n2)

public static double min (double n1,double n2)

Returns the minimum of two numbers.

public static double sqrt (double x)Returns the positive square root of a number passed as an argument.


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Exploring java.lang.Math Class (Continued)


public static int round (float x)

public static long round (double x)

Returns the integer value closest to the parameter.

public static double ceil (double x)Returns the next highest integer of the supplied parameter.

public static double floor (double x)Returns the next smallest integer of the supplied parameter.


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Exploring java.lang.Math Class (Continued)


public static int abs (int n1)

public static float abs (float n1)

public static double abs (double n1)

Returns the absolute value of that parameter.

public static double random()

Accepts no argument and returns a positive, double, value randomly generated, greater than or equal to 0.0 and less than 1.0.


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Exploring java.lang.String Class


public int length()Returns the length of a String object.

public String toUpperCase()Converts all the characters in the string object in uppercase.

public String toLowerCase()Converts all the characters in the string object in lowercase.

public String toString()Returns the String representation of the object.


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Exploring java.lang.String Class (Continued)


public Boolean equals (Object ob)Returns the length of a String object.

public int compareTo (String str2)

Compares current String object with another String object. If the Strings are same the return value is 0 else the return value is non zero.If str1 > str2 then return value is a positive numberIf str1<str2 then return value is a negative number


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public int indexOf (String str)

public int indexOf (String str, int startindex)

searches for the first occurrence of a character or a substring in the invoking String and returns the position if found else return -1.

public int lastIndexOf(String str)

public int lastIndexOf(String str, int startindex)

searches for the last occurrence of a character or a substring in the invoking String and returns the position if found else return -1.


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public String trim()Returns a copy of the string, with leading and trailing whitespace omitted.

public char charAt( int index)Returns the char value at the specified index.

public String concat(String str)Concatenates the specified String to the end of the String.

public String subString(int startInd)

public String subString(int startInd, int endInd)

Returns a new String that is a substring of a String.


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public boolean startsWith(String prefix)Tests if the String starts with the specified prefix.

public boolean endsWith(int suffix)Tests if the String end with the specified suffix.

public char[] toCharArray()Converts this String to a new character array.

public void getChars(int scrBegin,int srcEnd,char[] destination,int dstBegin)

Copies characters from this string into the destination character array.


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• The java.util package provides various utility classes and interfaces that support date and calendar operations, String manipulations and Collections manipulations.

• Classes provided by the java.util package:

Exploring java.util package

Class Description

Date Encapsulates date and time information.

Calendar Provides support for date conversion.

GregorianCalendarIt is a subclass of Calendar class, provides support for standard calendar used worldwide.


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Exploring java.util package (Continued)

Class Description

Random Generated a stream of random numbers.

ArrayListAn indexed sequence that grows and shrinks dynamically.

LinkedListAn ordered sequence that allows efficient insertions and deletions at any location

HashSetAn unordered collection that rejects duplicates.

HashMapA data structure that stores key/value associations.


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Exploring java.util package (Continued)

Interface Description

ListAn ordered list of elements that may consist of duplicate elements.

Set A group of objects with no duplication.

SortedSetA sorted group of objects with no duplication.

EnumerationProvides an abstract mechanism for visiting elements in an arbitrary container

MapProvides an object that maps keys to values.


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• A collection is an object that contains a group of objects within it.

• These objects are called the elements of the collection. The elements

of a collection descend from a common parent type.

• Collections have an advantage over arrays that collections can grow to

any size unlike arrays.

Collection Interface


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Hierarchy of Collection Interface


Collection

Set List

SortedSet

ArrayList LinkedListHashSet

TreeSet

Map

HashMap TreeMap

Interface

Class

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• The classes of the java.util package were updated to support the concept

of collection framework.

• These classes are referred to as the legacy classes.

• The various legacy classes defined by the java.util package are:

• Vector

• Stack

• Hashtable

• Properties

Legacy Classes & Interfaces


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• When you write a Java program, you create many classes. You can

organize these classes by creating your own packages.

• The packages that you create are called user-defined packages.

• A user-defined package contains one or more classes that can be imported

in a Java program.

User Defined Packages

User Defined Packages

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• Creating a user-defined package

Syntax & Example

Syntax for Creating Package

package <package_name>

// Class definition

public class <classname1>

{

// Body of the class.

}

User Defined Packages (Continued)

Example

package land.vehicle;

public class Car

{

String brand;

String color;

int wheels;

}

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• Create a source file containing the package definition

• Create a folder having the same name as package name and save the source file within the folder.

• Compile the source file.

Steps To Create User Defined Packages


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• You can include a user-defined package or any java API using the import statement.

• The following syntax shows how to implement the user-defined package, vehicle from land in a class known as MarutiCar

import land.vehicle.Car;

public class MarutiCar extends Car

{

// Body of the class.

}

Importing Packages


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• The CLASSPATH variable points to the directories in which the classes that are available to import are present.

• CLASSPATH enables you to put the class files in various directories and notifies the JDK tools of the region of these classes.

CLASSPATH Variable

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• Packages enable you to organize the class files provide by Java.

• The various built-in packages of the Java programming language are:

• java.lang

• java.util

• java.io

• java.applet

• java.awt

• java.net

• The packages created by users are called user-defined packages.

• The import statement with the package name enables you to inform the compiler about the region of classes.

• The java.lang package provides a number of classes and interfaces that are fundamental to Java programming.

Summary

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• Some of the classes defined in the java.lang package are :

• The various built-in packages of the Java programming language are:

• Object

• Class

• System

• Wrapper

• Character

• Integer

• Math

• String

• The java.util package provides various utility classes and interfaces that support date/calendar operations, String manipulation.

Summary (Continued)

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• Some of the classes defined in the java.util package are :

• Date

• Calendar

• GregorianCalendar

• Random

• ArrayList

• LinkedList

• HashSet

• TreeSet

• The various interfaces defined in the java.util package are:

• Collection

• List

• Set

• SortedSet

Summary (Continued)

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Test Your Understanding

a. Character class

b. Integer class

c. Math class

d. String class

e. StringBuffer class

iii. Used for manipulating strings.

iv. Wrapper class for the primitive data type char.

v. Wrapper class for the primitive data type int.

i. Collection of useful numeric constants.

ii. Supports operations on strings of characters.

Match Group A with B

Group A Group B

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