代写代考 SOFT2201 / COMP9201

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Software Design and

Construction 1
SOFT2201 / COMP9201

OO Theory in Java

School of Computer Science

Which feature of Java/OOP do you
think is the most interesting to you?
– share via Zoom chat

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Announcement

– Tutorial starts from this week (week 2)
– Please find your tutorial schedule via your timetable
– Tutorials will be either on campus or live Zoom session (no recording)

– Weekly exercise will be open at 8am Tuesday and close at
11.59pm Saturday, no late submission except SCON

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– Encapsulation
– Inheritance
– Variable Binding & Polymorphism
– Virtual Dispatch
– Abstract Classes
– Interfaces

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Types in Java

– Overview
– Java is a strongly typed language
– Primitive types: built-in types of the virtual machines

• Cannot be changed by programs
• Have same meaning across computer platforms

– Compositional types
• Interfaces

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Primitive Types in Java

– Primitive types represent basic data-types
– In-built and cannot be changed in programs
– Integral types

– byte, short, int, long
– Floating point number types

– float, double
– Character type

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Integral and Floating Types

Type Internal Smallest Value Largest Value

byte 8 bits -128 +127

short 16 bits -32,768 +32,767

int 32 bits -2,147,483,648 +2,147,483,647

long 64 bits -2.3E+18 +2.3E+18

float 32 bits -/+1.4e-45 -/+3.4e+38

double 64 bits -/+4.9e-324 -/+1.7e+308d

The University of 8

Conversion between Primitive Types

– Implicit widening of integral & floating point number types
– byte to short, int, long, float, or double
– short to int, long, float, or double
– char to int, long, float, or double
– int to long, float, or double
– long to float or double
– float to double

– Everything else requires type casts or is not possible

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Casting/Widening Matrix
From/To byte char short int long float double boole

char (byte) (short) n/a

short (byte) (char) n/a

int (byte) (char) (short) n/a

long (byte) (char) (short) (int) n/a

float (byte) (char) (short) (int) (long) n/a

double (byte) (char) (short) (int) (long) (float) n/a

boolean n/a n/a n/a n/a n/a n/a n/a

The University of 10

Example for Implicit Widening and Casting

byte x = 10;

/* implicit widening */
short y = x;
long z = x;
float f = x;

/* casting */
byte p = (byte) z;
short q = (short) f;

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Silent Failure of Types

– Example:

– Integer overflow occurs for variable i
– Type system cannot prevent this at compile-time

– weak notion of correctness

int i = Integer.MAX_VALUE;
System.out.println(i);
i = i + 1;
System.out.println(i);

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Primitive Types

– Basic data-types
– Behave the same on all platforms running Java’s virtual machine

– Primitive Types are not classes nor interfaces
– Primitive Types are building blocks for more complex types
– Beside the storing the actual value there are very little

additional overheads
– Note that classes have extra information to keep of dynamic runtime

– Operations on primitive data-types are very efficient

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Classes in Java

– Classes are types
– Composite type consisting of fields(=state) and methods(=behavior)

– Class Abstraction
– Separate class implementation from the use of a class
– User of a class does not need to know the implementation / just the

public methods
– The implementor provides the implementation of the class
– The details are encapsulated and hidden from the user

– Class creates objects (instantiate)
– Object communicates with each other via methods

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– A basic class has a name, fields, and methods:

Name of Class

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Classes (cont’d)
– The basic syntax for a class is

– Example:

class [extends ]

class Rectangle

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Adding Fields: Class Rectangle with Fields

– Add fields

– The fields (data) are also called the instance variables
– Fields can be

– Primitive types (int, bool, etc.)
– References to other class instances

public class Rectangle {
public double width, height;

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Adding Methods

– To change / retrieve state of a class, methods are necessary
– Methods are declared inside the body

– Methods are messages in an object-oriented programming sense
– A return type must be specified
– A list of arguments with their types must be specified

– The general form of a method declaration is:

type Method (argument-list) {
Method-body;

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Adding Methods to Rectangle
public class Rectangle {

public double width, height; // fields

public void setWidth(double w) {
width = w;

public void setHeight(double h) {

height = h;
public double area() {
return width * height;

Method Body

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– Methods that change the state of a class object are called mutators
– Example:

– Gives clients of the class write access to the state

public void setWidth(double w) {
width = w;

public void setHeight(double h) {

height = h;

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– Methods that read the state of a class object are called accessor
– Example:

– Gives clients of the class read access to the state

public double area() {
return width * height;

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Constructors
– Special method in class whose task is to initialize fields of the class
– The name is the same name as class
– Constructors are invoked whenever an object of that class is created
– It is called constructor because it constructs values of data members.
– A constructor cannot return a value
– There can be several constructors for a class

– Default constructor with no parameters
– Parameterized constructors

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Example: Default Constructor

– Default constructor

– Java has a synthesized default constructor when not specified

public class Rectangle {
public double width, height;
public Rectangle() { // default constructor

width = 1.0;
height = 1.0;

The University of 24

Example: Parameterized Constructor

– Parameterized constructor:
public class Rectangle {

public double width, height;

// parameterized constructor
public Rectangle(double w, double h) {

width = w;
height = h;

The University of 25

Example: Multiple Constructors
– Default and parameterized constructor (overloading)

public class Rectangle {
public double width, height;
// default constructor
public Rectangle() {

width = 1.0;
height = 1.0;

// parameterized constructor
public Rectangle(double w, double h) {

width = w;
height = h;

The University of 26

Creating Instances of a Class

– Objects are created dynamically using the new keyword.
– The new operator creates a new object on the heap

– The object will be selected as a candidate if not further used
– The object becomes a candidate for automatic garbage collection
– Garbage collection kicks in periodically and releases candidates
– developers don’t need to delete their own objects

– Example:

– Define a reference variable X of type class that points to a new

X = new ();

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Example: Creating Instances of a Class

– rec1 and rec2 refer to Rectangle objects

– First instance calls default constructor
– Second instance calls parameterized constructor

rec2 = new Rectangle(10,1);rec1 = new Rectangle();

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Copy Constructor

– Java has no copy constructor
– We can simulate copy constructor by having a constructor with

objects of same type as argument
– All classes are derived from class Object
– Object has the method clone()

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Example: Copy Constructor

– Copy constructor
public class Rectangle {

public double width, height;
// copy constructor
public Rectangle(Rectangle o) {

width = o.width;
height = o.height;

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Accessing Fields and Methods

– Access is done via the . operator
– Access to field and methods of an instance in the same style

– Access modifier is important
– public/protected/private
– Controls whether access is permitted at client or member level

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