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Introduction toJava Programming

Y. Daniel LiangEdited by Hoàng Văn Hậu – VTC Academy – THSoft co.,ltd

https://play.google.com/store/apps/developer?id=THSoft+Co.,Ltd

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Introduction

Course Objectives Organization of the Book

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Course Objectives Upon completing the course, you will understand

– Create, compile, and run Java programs

– Primitive data types

– Java control flow

– Methods– Arrays (for teaching Java in two semesters, this could be the end)

– Object-oriented programming

– Core Java classes (Swing, exception, internationalization, multithreading, multimedia, I/O, networking, Java Collections Framework)

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Course Objectives, cont.

You will be able to – Develop programs using Eclipse IDE– Write simple programs using primitive data

types, control statements, methods, and arrays.– Create and use methods– Write interesting projects

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Session 03: Method and Class

Methods Object-Oriented Programming Classes Packages Subclassing and Inheritance Polymorphism

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Introducing Methods

Method StructureA method is a collection of statements that are grouped together to perform an operation.

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Introducing Methods, cont.•parameter profile refers to the type, order, and number of the parameters of a method.

•method signature is the combination of the method name and the parameter profiles.

•The parameters defined in the method header are known as formal parameters.

•When a method is invoked, its formal parameters are replaced by variables or data, which are referred to as actual parameters.

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Declaring Methods

public static int max(int num1, int num2) {

if (num1 > num2) return num1; else return num2;}

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Calling Methods, cont.

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass i pass j

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Calling Methods, cont.

The main method i: j: k:

The max method num1: num2: result:

pass 5

5

2

5

5

2

5

pass 2 parameters

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CAUTION

A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value. public static int xMethod(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; }

To fix this problem, delete if (n<0) in the code.

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Actions on Eclipse

Open Eclipse IDE– Create project: Session3Ex– Create package: com.vtcacademy.exopp– Create java class: Ex3WithOpp.java (with main method)– Create java class: Circle.java

RunRunSourceSource

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The Math Class Class constants:

– PI– E

Class methods: – Trigonometric Methods – Exponent Methods– Rounding Methods– min, max, abs, and random Methods

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Trigonometric Methods

sin(double a)

cos(double a)

tan(double a)

acos(double a)

asin(double a)

atan(double a)

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Exponent Methods exp(double a)

Returns e raised to the power of a.

log(double a)

Returns the natural logarithm of a.

pow(double a, double b)

Returns a raised to the power of b.

sqrt(double a)

Returns the square root of a.

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Rounding Methods double ceil(double x)

x rounded up to its nearest integer. This integer is returned as a double value.

double floor(double x)x is rounded down to its nearest integer. This integer is returned as a double value.

double rint(double x)x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double.

int round(float x)Return (int)Math.floor(x+0.5).

long round(double x)Return (long)Math.floor(x+0.5).

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min, max, abs, and random

max(a, b)and min(a, b)Returns the maximum or minimum of two parameters.

abs(a)Returns the absolute value of the parameter.

random()Returns a random double valuein the range [0.0, 1.0).

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Example 3.1 Computing Mean and Standard Deviation

Generate n random numbers and compute the mean and standard deviation

n

xmean

n

ii

1

1

)(

1

2

12

nn

xx

deviation

n

i

n

ii

i

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Example 3.2 Obtaining Random Characters

Write the methods for generating random characters. The program uses these methods to generate 175 random characters between ‘!' and ‘~' and displays 25 characters per line. To find out the characters between ‘!' and ‘~', see Appendix B, “The ASCII Character Set.”

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OO Programming Concepts

data field 1

method n

data field n

method 1

An object

...

...

State

Behavior

Data Field radius = 5

Method findArea

A Circle object

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Class and Objects

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

Illustration on code

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Class Declaration

class Circle { double radius = 1.0;

double findArea(){ return radius * radius * 3.14159; }}

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Declaring Object Reference Variables

ClassName objectReference;

Example:Circle myCircle;

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Creating Objects

objectReference = new ClassName();

Example:myCircle = new Circle();

The object reference is assigned to the object reference variable.

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Declaring/Creating Objectsin a Single Step

ClassName objectReference = new ClassName();

Example:Circle myCircle = new Circle();Circle myCircle2;

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Constructors

Circle(double r) { radius = r;}

Circle() { radius = 1.0; }

myCircle = new Circle(5.0);

Constructors are a special kind of methods that are invoked to construct objects.

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Constructors

Circle(double r) { radius = r;}

Circle() { radius = 1.0; }

myCircle = new Circle(5.0);

Constructors are a special kind of methods that are invoked to construct objects.

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Constructors, cont.A constructor with no parameters is referred to as a default constructor.

       Constructors must have the same name as the class itself.

       Constructors do not have a return type—not even void.

       Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

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Example 3.3 Using Classes from the Java Library

Objective: Demonstrate using classes from the Java library. Use the JFrame class in the javax.swing package to create two frames; use the methods in the JFrame class to set the title, size and location of the frames and to display the frames.

Illustration on code

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Subclassing and Inheritance

private None protected Public

Illustration on code

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Example 3.4

Shape

TwoDimensionalShape ThreeDimensionalShape

Circle Square Triangle Sphere Cube Tetrahedron

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Example 3.4

http://blogs.unsw.edu.au/comp1400/blog/2011/09/tut-10/

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Abstract Classes and Methods

Abstract classes – Are superclasses (called abstract superclasses)– Cannot be instantiated – Incomplete

subclasses fill in "missing pieces"

Concrete classes– Can be instantiated– Implement every method they declare– Provide specifics

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Abstract Classes and Methods (Cont.)

Abstract classes not required, but reduce client code dependencies

To make a class abstract– Declare with keyword abstract– Contain one or more abstract methods

public abstract void draw();

– Abstract methods No implementation, must be overridden

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Abstract Classes and Methods (Cont.)

Application example– Abstract class Shape

Declares draw as abstract method

– Circle, Triangle, Rectangle extends Shape Each must implement draw

– Each object can draw itself

Iterators– Array, ArrayList (Chapter 22)

– Walk through list elements

– Used in polymorphic programming to traverse a collection

Illustration on code

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Case Study: Creating and Using Interfaces

Use interface Shape– Replace abstract class Shape

Interface– Declaration begins with interface keyword

– Classes implement an interface (and its methods)

– Contains public abstract methods Classes (that implement the interface) must implement

these methods

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1 // Fig. 10.18: Shape.java

2 // Shape interface declaration.

3

4 public interface Shape {

5 public double getArea(); // calculate area

6 public double getVolume(); // calculate volume

7 public String getName(); // return shape name

8

9 } // end interface Shape

Lines 5-7Classes that implement Shape must implement these methods

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1 // Fig. 10.19: Point.java

2 // Point class declaration implements interface Shape.

3

4 public class Point extends Object implements Shape {

5 private int x; // x part of coordinate pair

6 private int y; // y part of coordinate pair

7

8 // no-argument constructor; x and y default to 0

9 public Point()

10 {

11 // implicit call to Object constructor occurs here

12 }

13

14 // constructor

15 public Point( int xValue, int yValue )

16 {

17 // implicit call to Object constructor occurs here

18 x = xValue; // no need for validation

19 y = yValue; // no need for validation

20 }

21

22 // set x in coordinate pair

23 public void setX( int xValue )

24 {

25 x = xValue; // no need for validation

26 }

27

Error compile

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Action on class

Teacher – hauc2@yahoo.com– 0984380003– https://play.google.com/store/search?q=thsoft+co&c=apps

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