objects and classes chapter 6 csci 1302. csci 1302 – objects and classes2 outline introduction...

Objects and Classes

Chapter 6CSCI 1302

CSCI 1302 – Objects and Classes 2

Outline

• Introduction• Defining Classes for Objects• Constructing Objects• Accessing Objects

– Reference Variables and Types– Accessing an Object’s Data and Methods– The null Value– Differences Between Primitive and

Reference Variable Types


Outline

• Using Classes from the Java Library• Visibility Modifiers, Accessors, and

Mutators• Data Field Encapsulation• Immutable Objects and Classes• Passing Objects to Methods• Static Variables, Constants, and

Methods


Outline

• The this keyword• Array of Objects• Class Abstraction and Encapsulation• Inner Classes• Case Study: StackofIntegers


Introduction

• Procedural programming – data and operations are separate

• Object-oriented programming (OOP) – groups data and operations into a single entity

• Improves reusability, development, and maintenance


Defining Classes for Objects

• An object represents a real-world entity• Has a unique identity, state, and

behaviors• State consists of data fields (properties)• Behavior is defined by a set of methods• State defines the object, behavior

defines what it does



data field 1

method n

data field m

method 1

(A) A generic object

...

...

State (Properties)

Behavior

radius = 5

findArea()

Data field, State

Properties

Method, Behavior

(B) An example of circle object



• Constructs that define objects of the same type

• Java classes use variables to define data fields and methods to define behaviors

• Also provide constructors used to create objects from the class


Defining Classes for Objects class Circle {

/** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double findArea() { return radius * radius * 3.14159; }

}


Defining Classes for Objects class Circle {

/** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double findArea() { return radius * radius * 3.14159; }

}

Data field

Method

Constructors


Constructing Objects

Circle() {}

Circle(double newRadius) {radius = newRadius;

}

Constructors with no parameters are referred to as no-arg constructors.



Constructors• Have the same name as the defining

class• Can be overloaded• No return type• Invoked using the new operator• If no constructor is explicitly declared, a

default no-arg constructor is created with an empty body



ExamplesGeneral

new ClassName();

Specific new Circle();new Circle(5.0);



circle1: Circle

radius = 2

new Circle()

circlen: Circle

radius = 5

new Circle()

...

UML Graphical notation for classes

UML Graphical notation for objects

Circle radius: double findArea(): double

UML Graphical notation for fields

UML Graphical notation for methods


Accessing Objects

• Can be accessed through reference variables

ClassName objectRefVar;Circle myCircle;

• Class defines a reference typemyCircle = new Circle();

• One statementCircle myCircle = new Circle();

• An object created without a reference is an anonymous object


Accessing Objects

Accessing Data and Methods• Can be accessed using dot notation

objectRefVar.data;objectRefvar.method(arguments);

• Circle class:myCircle.radius;myCircle.findArea();

• See the code in Circle.java and TestCircle.java for examples


The null value

• Default literal value for reference type variables

• Default values of data fields:

• No default values for local variables inside a method

Reference type null

Numeric type 0

boolean false

char ‘\u0000’


Differences Between Variable Types

• Two main types of variables: Primitive and Reference

• Primitive– Value is of the primitive type– Assignment sets the real value, copies

contents

• Reference– Value is a reference to where an object

is located– Assignment sets reference


Differences Between Variable Types

• Garbage collection reclaims previous c1 object automatically (can assign null to force collection) See code in TestVarTypes.java

1

c1: Circle

radius = 5

Primitive type assignment i = j

Before:

i

2 j

2

After:

i

2 j

Object type assignment c1 = c2

Before:

c1

c2

After:

c1

c2

c2: Circle

radius = 9


Using Classes from the Java Library

• Classes from the Java Library will often be used when developing programs

• You have already done this:System.out.println(“Some text here”);

• See Example 2.4 (p. 61) for an example using System.currentTimeMillis();

• Promotes reuse of code• http://java.sun.com/j2se/1.5.0/docs/api/


Using Classes from the Java Library

Example: java.util.Date• Provides a system-independent

encapsulation of date and time• Can be used to create an instance of

the current date and time, use its toString() method to print time out

• Format: Mon Aug 15 13:19:37 EDT 2005

• See TestDate.java and TestFrame.java


Visibility Modifiers

• Java provides several modifiers to control access to data, methods, and classes– public – Accessible from any class– private – Accessible only from within its

own class– Package-private – used if public or private is not specified, accessible by any class in the same package


Visibility Modifiers

• Only used for members of a class, not local variables inside methods

public class C1 { public int x; int y; private int z; public void m1() { } void m2() { } private void m3() { } }

public class C2 { C1 o = new C1(); can access o.x; can access o.y; cannot access o.z; can invoke o.m1(); can invoke o.m2(); cannot invoke o.m3(); }

package p1; package p2;

public class C3 { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; can invoke o.m1(); cannot invoke o.m2(); cannot invoke o.m3(); }


Data Field Encapsulation

• Declare the field private• Use get (or accessor) methods to return

values of data fieldspublic returnType getProperty();public boolean isProperty();

• Use set (or mutator) methods to update data fields

public void setProperty(dataType property);

• See ExtendedCircle.java


Immutable Objects and Classes

• An object, that once created, cannot be changed is an immutable object

• The class it is derived from is an immutable class

• A class with all private data fields and no mutator methods is not necessarily immutable (p. 227)

• See Chair.java for an example of a immutable class


Passing Objects to Methods

• Objects can be passed to methods• Reference to the object is what is

actually passed• Java passes all arguments using pass-

by-value• See TestPassObject.java for a simple

example and TestPassObjectExtended.java for a more complex example


Static Variables, Constants, and Methods

• Instance variables are tied to specific instances of a class

• Static variables are shared between all instances of a class

• Static methods can be called without creating an instance of a class

• Declared using static keyword• Class constants can be declared using

the final keyword


Static Variables, Constants, and Methods

CircleWithStaticVariableAndMethod -radius: double -numberOfObjects: int +getRadius(): double +setRadius(radius: double): void +getNumberOfObjects(): int +findArea(): double

1 radius

circle1 -radius = 1 -numberOfObjects = 2

instantiate

Memory

2

5 radius

numberOfObjects

UML Notation: +: public variables or methods -: private variables or methods underline: static variables or methods

circle2 -radius = 5 -numberOfObjects = 2


Scope of Variables

• Class variables have a scope of the entire class

• Methods and variables can be declared in any order in the class except when their value is based on other variables

• Local variables take precedence over class variables although it is bad form to reuse variable names in most cases


The this keyword

• Used to refer to the invoking class• Used most often when parameter name

in a method matches a data field in a class

public void setRadius(double radius) { this.radius = radius;

}

• See ExtendedChair.java for example of constructor using this

• See TestChair.java


Array of Objects

• Object arrays can be created the same way that primitive arrays are created

Circle[] circleArray = new Circle[10];

• To initialize the array, use a for loop similar to this one

for (int i = 0; i < circleArray.length; i++) {circleArray[i] = new Circle();

}


Array of Objects

• Actually involves two levels of referencing

• circleArray is an array of reference variables

• See TestCircleArray.java

reference

Circle object 0 circleArray[0]

…

circleArray

circleArray[1]

circleArray[9]

Circle object 9

Circle object 1


Class Abstraction and Encapsulation

• Class abstraction – Separation of class implementation from the use of a class

• Class contract – Collection of methods and fields that are accessible from outside the class, along with the description of their behavior

• Class encapsulation – Details of implementation are encapsulated and hidden from the user


Inner Classes

• A class defined within the scope of another class (AKA nested class)

• Can reference the data and methods defined in the outer class

• Can be declared public, protected, or private

• Can be declared static with some restrictions on access to the outer class


Inner Classes

• Objects for inner classes are often created in the outer class

• Can be created from outside classesNon-static:OuterClass.InnerClass innerObject = outerObject.new InnerClass();

Static:OuterClass.InnerClass innerObject = new OuterClass.InnerClass();


Case Study: StackofIntegers

• Data structure that holds objects in a last-in first-out (LIFO) fashion

• Stack operations: pop(), push(), and peek()

• Array-based implementation • Stacks are used often in programming• Java provides a full featured version in java.util.Stack


Case Study: StackofIntegers

StackOfIntegers

-elements: int[] -size: int +StackOfIntegers() +StackOfIntegers(capacity: int) +empty(): boolean +peek(): int +push(element: int): int +pop(): int +getSize(): int

. . .

. . .

[0]

[1]

size - 1

capacity - 1

objects and classes chapter 6 csci 1302. csci 1302 – objects and classes2 outline introduction...

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objects data

objects class circle

outlineusing classes

typejava classes

classespassing objects

return radius

classes uml graphical

double radius