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CSE260, Computer Science B: Honors

Stony Brook University

http://www.cs.stonybrook.edu/~cse260

Recapitulate CSE160: Java Classes, Objects,

Inheritance, Abstract Classes and Interfaces

1 (c) Paul Fodor (CS Stony Brook) & Pearson

Objectives

To refresh information from CSE160 about

classes, objects, inheritance, abstract classes and interfaces 2 (c) Paul Fodor (CS Stony Brook) & Pearson

OO Programming Concepts

An object represents an entity in the real world that can be distinctly identified.

An object has a unique state and behaviors

the state of an object consists of a set of data fields (properties) with their current values the behavior of an object is defined by a set of methods 3

Class Name: Circle

Data Fields:

radius is _______

Methods:

getArea()

Circle Object 1

Data Fields:

radius is 1

Circle Object 2

Data Fields:

radius is 25

Circle Object 3

Data Fields:

radius is 125

A class template

Three objects of

the Circle class (c) Paul Fodor (CS Stony Brook) & Pearson

Classes

Classes are templates that define objects of

the same type.

A Java class uses:

variables to define data fields and methods to define behaviors

A class provides a special type of methods

called constructorswhich are invoked to construct objects from the class 4 (c) Paul Fodor (CS Stony Brook) & Pearson5 class Circle { /** The radius of this circle */ private double radius = 1.0; /** Construct a circle object */ public Circle() { /** Construct a circle object */ public Circle(double newRadius) { radius = newRadius; /** Return the area of this circle */ public double getArea() { return radius * radius * 3.14159;

Data field

Method

Constructors

Classes

(c) Paul Fodor (CS Stony Brook) & Pearson6 public class TestCircle { public static void main(String[] args) {

Circle circle1 = new Circle();

Circle circle2 = new Circle(25);

Circle circle3 = new Circle(125);

System.out.println( circle1.getArea() );

System.out.println( circle2.getArea() );

System.out.println( circle3.getArea() );

//System.out.println( circle1.radius ); //System.out.println( circle2.radius ); //System.out.println( circle3.radius );

Classes

(c) Paul Fodor (CS Stony Brook) & Pearson7

UML Class Diagram

Circle

radius: double

Circle()

Circle(newRadius: double)

getArea(): double circle1: Circle radius = 1.0

Class name

Data fields

Constructors and

methods circle2: Circle radius = 25 circle3: Circle radius = 125

UML Class Diagram

UML notation

for objects (c) Paul Fodor (CS Stony Brook) & Pearson

Constructors

Constructors must have the same name as the class itself. Constructors do not have a return typenot even void. Constructors are invoked using the newoperator when an object is created they initialize objects to reference variables:

ClassNameo = new ClassName();

Example:

Circle myCircle= new Circle(5.0);

A class may be declared without constructors: a no-argdefault constructorwith an empty body is implicitly declared in the class 8 (c) Paul Fodor (CS Stony Brook) & Pearson

Accessing Objects

objectRefVar.data

Example:myCircle.radius

objectRefVar.methodName(arguments)

Example:myCircle.getArea()

9 (c) Paul Fodor (CS Stony Brook) & Pearson10

Default values

public class Test { public static void main(String[] args) { intx; // x has no default value

String y; // y has no default value

System.out.println("x is " + x);

System.out.println("y is " + y);

}Compilation error: the variables are not initialized

BUT it assigns default values to data fields!

Java assigns no default value to a local variable inside a method. (c) Paul Fodor (CS Stony Brook) & Pearson11

Reference Data Fields

The data fields can also be of reference types

Example:

public class Student {

String name; // name has default value null

intage; // age has default value 0 booleanisScienceMajor; // isScienceMajorhas default value false char gender; // c has default value '\u0000' If a data field of a reference type does not reference any object, the data field holds a special literal value: null. public class Test { public static void main(String[] args) {

Student student= new Student();

System.out.println("name? " + student.name); // null

System.out.println("age? " + student.age); // 0

System.out.println("isScienceMajor? " + student.isScienceMajor); // false System.out.println("gender? " + student.gender); // (c) Paul Fodor (CS Stony Brook) & Pearson12

Differences between Variables of

Primitive Data Types and Object Types

1 Primitive type int i = 1 i

Object type Circle c c reference

Created using new Circle()

c: Circle radius = 1 (c) Paul Fodor (CS Stony Brook) & Pearson13

Copying Variables of Primitive Data

Types and Object Types

i

Primitive type assignment i = j

Before:

1 j 2 i

After:

2 j 2 c1

Object type assignment c1 = c2

Before:

c2 c1

After:

c2 c1: Circle radius = 5 c2: Circle radius = 9 c1: Circle radius = 5 c2: Circle radius = 9 -The object previously referenced by c1 is no longer referenced it is called garbage -Garbage is automatically collected by JVM = garbage collection (c) Paul Fodor (CS Stony Brook) & Pearson static vs. non-static

Static methods & variables are scoped to a

class one static variable for all objects to share!

Non-static (object) methods & variables

are scoped to a single object each object owns its non-static methods & variables 14 (c) Paul Fodor (CS Stony Brook) & Pearson15

Static Variables,

Constants and Methods

Circle

-radius: double -numberOfObjects: int +getNumberOfObjects(): int +getArea(): double

1 radius

circle1 radius = 1 numberOfObjects = 2 instantiate instantiate

Memory

2

5 radius

numberOfObjects

UML Notation:

+: public variables or methods underline: static variables or methods circle2 radius = 5 numberOfObjects = 2

After two Circle

objects were created, numberOfObjects is 2. (c) Paul Fodor (CS Stony Brook) & Pearson public class StaticExample{ public intnonStaticCounter= 0; public static intstaticCounter= 0; public StaticExample() { nonStaticCounter++; staticCounter++; public static void main(String[] args) {

StaticExampleex;

ex = new StaticExample(); ex = new StaticExample(); ex = new StaticExample();

System.out.println(ex.nonStaticCounter);

System.out.println(staticCounter);

16

StaticExample.java

(c) Paul Fodor (CS Stony Brook) & Pearson public class StaticExample{ public intnonStaticCounter= 0; public static intstaticCounter= 0; public StaticExample() { nonStaticCounter++; staticCounter++; public static void main(String[] args) {

StaticExampleex;

ex = new StaticExample(); ex = new StaticExample(); ex = new StaticExample();

System.out.println(ex.nonStaticCounter);

System.out.println(staticCounter);

17

StaticExample.java

Output: 1

3 (c) Paul Fodor (CS Stony Brook) & Pearson staticusage

Can a staticmethod:

-staticmethod in the same class? directly call a staticmethod in the same class? directly reference a non-staticvariable in the same class? directly reference a staticvariable in the same class?

Can a non-staticmethod:

-staticmethod in the same class? directly call a staticmethod in the same class? directly reference a non-staticvariable in the same class? directly reference a staticvariable in the same class? 18 (c) Paul Fodor (CS Stony Brook) & Pearson staticusage

Can a staticmethod:

-staticmethod in the same class? No directly call a staticmethod in the same class? Yes directly reference a non-staticvariable in the same class? No directly reference a staticvariable in the same class? Yes

Can a non-staticmethod:

-staticmethod in the same class? Yes directly call a staticmethod in the same class? Yes directly reference a non-staticvariable in the same class? Yes directly reference a staticvariable in the same class? Yes 19 (c) Paul Fodor (CS Stony Brook) & Pearson

1 public class Nothing {

2private intnada;// Errors?

3private static intnothing;

4

5public void doNada(){ System.out.println(nada);}

6public static void doNothing(){ System.out.println("NOTHING"); }

7

8public static void myStaticMethod(){

9doNada();

10doNothing();

11nada = 2;

12nothing = 2;

13Nothing n = new Nothing();

14n.doNada();

15n.nada= 2;

16n.nothing= 6;

17}

18public void myNonStaticMethod() {

19doNada();

20doNothing();

21nada = 2;

22nothing = 2;

23Nothing n = new Nothing();

24n.doNada();

25n.nada= 2;

26}}20

(c) Paul Fodor (CS Stony Brook) & Pearson

1 public class Nothing {

2private intnada;

3private static intnothing;

4

5public void doNada(){ System.out.println(nada);}

6public static void doNothing(){ System.out.println("NOTHING"); }

7

8public static void myStaticMethod(){

9doNada();

10doNothing();

11nada = 2;

12nothing = 2;

13Nothing n = new Nothing();

14n.doNada();

15n.nada= 2;

16n.nothing= 6;

17}

18public void myNonStaticMethod() {

19doNada();

20doNothing();

21nada = 2;

22nothing = 2;

23Nothing n = new Nothing();

24n.doNada();

25n.nada= 2;

26}}21

(c) Paul Fodor (CS Stony Brook) & Pearson

Visibility Modifiers and

Accessor/MutatorMethods

By default, the class, variable, or method can be

accessed by any class in the same package. 22
public The class, data, or method is visible to any class in any package. private The data or methods can be accessed only by the declaring class -To protect data!

The get and set methods are used to read and

modify private properties. (c) Paul Fodor (CS Stony Brook) & Pearson23

Array of Objects

Circle[] circleArray= new Circle[10];

An array of objects is an array of reference variables (like the multi-dimensional arrays seen before) reference

Circle object 0 circleArray[0]

circleArray circleArray[1] circleArray[9]

Circle object 9

Circle object 1

(c) Paul Fodor (CS Stony Brook) & Pearson

Immutable Objects and Classes

Immutable object: the contents of an object cannot be changed once the object is created -its class is called an immutable class Example immutable class: no set method in the Circle class public class Circle{ private double radius; public Circle() { } public Circle(double radius) { this.radius= radius; public double getRadius() { return radius; radius is private and cannot be changed without a set method A class with all private data fields and without mutatorsis not necessarily immutable!24 (c) Paul Fodor (CS Stony Brook) & Pearson

What Class is Immutable?

1.It must mark all data fields private!

2.Provide no mutator methods!

3.Provide no accessor methods that would

return a reference to a mutable data field object! 25
(c) Paul Fodor (CS Stony Brook) & Pearson26

Example mutable

public class Student { private intid; privateBirthDatebirthDate; public Student(intssn, intyear, intmonth, intday) { id = ssn; birthDate= new BirthDate(year, month, day); public intgetId() { return id; public BirthDategetBirthDate() { return birthDate; public class BirthDate{ private intyear; private intmonth; private intday; public BirthDate(intnewYear, intnewMonth, intnewDay) { year = newYear; month = newMonth; day = newDay; public void setYear(intnewYear) { year = newYear; public intgetYear() { return year; public class Test { public static void main(String[] args) { Student student= new Student(111223333, 1998, 1, 1);

BirthDatedate = student.getBirthDate();

date.setYear(2050); // Now the student birth year is changed: System.out.println(student.getBirthDate().getYear());// 2050 (c) Paul Fodor (CS Stony Brook) & Pearson

The thisKeyword

The thiskeyword is the name of a

reference that refers to an object itself

Common uses of the thiskeyword:

1.

2.To enable a constructor to invoke another

constructor of the same class as the first statement in the constructor. 27
(c) Paul Fodor (CS Stony Brook) & Pearson28

Reference the Hidden Data Fields

public class Foo { private int i = 5; private static double k = 0; void setI(int i) { this.i = i; static void setK(double k) {

Foo.k = k;

Suppose that f1 and f2 are two objects of Foo.

Invoking f1.setI(10) is to execute

this.i = 10, where this refers f1

Invoking f2.setI(45) is to execute

this.i = 45, where this refers f2 (c) Paul Fodor (CS Stony Brook) & Pearson29

Calling Overloaded Constructor

public class Circle { private double radius; public Circle(double radius) { this.radius = radius; public Circle() { this(1.0); public double getArea() { return this.radius * this.radius * Math.PI; Every instance variable belongs to an instance represented by this, which is normally omitted this must be explicitly used to reference the data field radius of the object being constructed this is used to invoke another constructor (c) Paul Fodor (CS Stony Brook) & Pearson30

The Date Class

Java provides a system-independent encapsulation of date and time in the java.util.Dateclass. The toStringmethod returns the date and time as a string java.util.Date +Date() +Date(elapseTime: long) +toString(): String +getTime(): long +setTime(elapseTime: long): void

Constructs a Date object for the current time.

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