02-OOP With Java

8/12/2019 02-OOP With Java

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Amir Kirsh

Object Oriented Programming with Java

Written by Amir Kirsh
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Classes and Objects

A class will look like this: class MyClass {

// field, constructor, and method declarations}

To instantiate an object we will do:

MyClass instance = new MyClass();


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Accessibility Options

Example: public class Person {

private String name; protected java.util.Date birthDate;

String id; // default accessibility = package public Person() {}

}

Four accessibility options: public (default) = package ** protected * private

* protected is also accessible by package** called also package -private or package -friendly


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Static

Example:

public class Widget {static private int counter;static public getCounter() {return counter;}

}

int number = Widget.getCounter();

Static member can be accessed without an instance(same as in C++)

Called sometimes class variable as opposed to instance variable


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The this keyword

Example: public class Point {

private int x, y; public Point(int x, int y) {

this.x = x;this.y = y;

}}

In Java this is a reference to myself(in C++ it is a pointer )

The this keyword is also used to call another constructor ofthe same class we will see that later


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Defining constants

Example: public class Thingy {

public final static doodad = 6; // constant public final id; // constant variable

public Thingy(int id) {this.id = id;} // OK// public set(int id) {this.id = id;} // error!

}

Though const is a reserved word in Javait's actually not in use!However the final keyword let's you defineconstants and const variables


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d a All that is to know on class syntax

Constructors and Initializers Inheritance and Polymorphism

Interfaces

Nested Classes

Enums

Exercise


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Constructors

Examples in following slides

Constructors in Java are very similar to C++

You can overload constructors (like any other method)

A constructor which doesn't get any parameteris called empty constructor

You may prefer not to have a constructor at all,in which case it is said that you have by defaultan empty constructor

A constructor can call another constructorof the same class using the this keyword

Calling another constructor can be done onlyas the first instruction of the calling constructor


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Constructors

Example 1: public class Person {

String name = ""; // fields can be initialized!Date birthDate = new Date();

public Person() {} // empty constructor public Person(String name, Date birthDate) {

this(name); // must be first instructionthis.birthDate = birthDate;

}

public Person(String name) {this.name = name;

}}


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Constructors


String name = "";Date birthDate = new Date();

public Person(String name, Date birthDate) {this.name = name;this.birthDate = birthDate;

}}

Person p; // OK p = new Person(); // not good compilation error


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Initializer

Initializer is a block of instructions performedright after the fields creation and before callingthe constructor

A class does not have to have an initializer and

indeed it usually doesn't

Example: public class Thingy {

String s;// the block underneath is an initializer{ s="Hello"; }

} Usually initializer would doa more complex job


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Interfaces

Nested Classes

Enums

Exercise


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Inheritance

Some Terms

A class that is derived from another class is called a subc lass (also a derived c lass , extend ed c lass , or ch i ld c lass ).

The class from which the subclass is derived is called asuperclass (also a base c lass or a parent c lass ).

Excepting java.lang.Object , which has no superclass,every class has exactly one and only one direct superclass(single inheritance).In the absence of any other explicit superclass, every class isimplicitly a subclass of Object.

A class is said to be descended from all the classes in itsinheritance chain stretching back to Object.


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Inheritance


Class Object is the ancestor base class of all classes in Java

There is no multiple inheritance in Java

Inheritance is always public thus type is not stated(no private or protected inheritance as in C++)

Class can implement several interfaces (contracts)

Class can be abstract

Access to base class is done using the super keyword

Constructor may send parameters to its base using the super keyword as its first instruction

If the base class does not have an empty constructor thenthe class is required to pass parameters to its super


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Inheritance


private String name; public Person(String name) {

this.name = name;}// Override toString in class Object

public String toString() {return name;

}}


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Inheritance

Example 1 (cont) : public class Employee extends Person {

private Employee manager; public Employee(String name, Employee manager) {

super(name); // must be firstthis.manager = manager;

}// Override toString in class Person

public String toString() {

return super.toString() +(manager!=null? ", reporting to: " + manager :

" - I'm the big boss!");}

}


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Inheritance

Example:

abstract public class Shape { final public void setFillColor(Color color)

{}}

The final keyword is used to forbid a method from beingoverride in derived classes

Above is relevant when implementing a generic algorithm in thebase class, and it allows the JVM to linkage the calls to themethod more efficiently

The final keyword can also be used on a class to prevent theclass from being subclassed at all

of course, final and abstractdont go together (why?)


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Interfaces

Nested Classes

Enums

Exercise


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Interfaces


Interface is a contract

An interface can contain method signatures (methods without implementation) and static constants

Interface cannot be instantiated, it can only be implemented by classes and extended by other interfaces

Interface that do not include any method signature is calleda marker interface

Class can implement several interfaces (contracts)

Class can announce on implementing an interface,without really implementing all of the declared methods,but then the class must be abstract


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Interfaces

Example 1 using interface Comparable :// a generic max function static public Object max(Comparable... comparables) {

int length = comparables. length ; if (length == 0) { return null ; } Comparable max = comparables[0];

for ( int i=1; i


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Interfaces

Example 4 new IHaveName interface:

public interface IHaveName {String getName();

}

Exercise:

Create the IHaveName interface andlet class Person implement it

To allow name investigation we want to create a new IHaveNameinterface:


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Interfaces

Nested Classes

Enums

Exercise


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Nested Classes


Nested Classes are divided into two categories:static and non-static.

Nested classes that are declared static are simply calleds ta t ic nested c lasses Non-static nested classes are called inner c lasses

Inner classes that are defined without having their own nameare called anony m ous c lasses


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Nested Classes

Example 1: public class OuterClass {

private int a;static public class InnerStaticClass {

public int b;}

public class InnerClass { public void setA(int a1) {

a = a1; // we have access to a !!!

}}

}


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Nested Classes

Example 1 (cont) :OuterClass.InnerStaticClass obj1 =

new OuterClass.InnerStaticClass();

OuterClass.InnerClass obj2 =new OuterClass().new InnerClass();

obj2.setA(3); // we modify a of OuterClass!!!


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Nested Classes

Example 2 anonymous class: public interface IHaveName {

String getName();}

void someFunction(IHaveName someoneWithName) {System.out.println(someoneWithName.getName());

}

public static void main(String[] args) {

someFunction(new IHaveName() { public String getName() { return "Momo"; }

});}


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Interfaces

Nested Classes

Enums

Exercise


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Enums


Structure for Constant Enumeration

Not an integer! - May represent data (= have fields)- May implement methods (member and static)

Automatically extends the Enum abstract type

Cannot extend other Classes or Enums,but can implement interfaces

Cannot be extended (Enums are final )


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Enums

Example 1: public class Card {

public enum Rank { DEUCE , THREE , FOUR , FIVE , SIX , SEVEN , EIGHT , NINE ,TEN , JACK , QUEEN , KING , ACE

}

public enum Suit {CLUBS , DIAMONDS , HEARTS , SPADES

}

private final Rank rank ; private final Suit suit ;

private Card(Rank rank, Suit suit) {this . rank = rank;this . suit = suit;

}


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Enums

Example 1 (cont) : public class Card {

public String toString() { return rank + " of " + suit ; }

private static final List _deck =new ArrayList();

// Initialize the static deck static {

for (Suit suit : Suit. values ()) for (Rank rank : Rank. values ())

_deck .add( new Card(rank, suit)); }

public static ArrayList newDeck() { // Return copy of prototype deck return new ArrayList( _deck );

} }


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Enums

Example 2: public enum Operation {

PLUS , MINUS , TIMES , DIVIDE ;

// Do arithmetic op represented by this constant double eval( double x, double y) {

switch ( this ) { case PLUS : return x + y; case MINUS : return x - y; case TIMES : return x * y; case DIVIDE : return x / y;

} throw new AssertionError( "Unknown op: " + this );

} }


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Interfaces

Nested Classes

Enums

Exercise


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Exercise 2

Write the necessary classes to support the following main:

static public void main(String[] args) {

Expression e =

new Sum(

new Exponent(new Number(2.0), new Number(3.0)),

new Sum(

new Number(1.0), new Number(-3.0)));

System.out.println(e + " = " + e.evaluate());}


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That concludes this chapter

amirk at mta ac il
http://www.comverse.com/http://www.comverse.com/

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02-OOP With Java