A Gentle Introduction Java :

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Java :

A Gentle Introduction

- presented by Fran Trees

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Java Programming Language

1991

 A group led by James Gosling and Patrick Naughton at Sun Microsystems designed “Green”, a language for consumer devices (coffee makers, VCRs, etc.).

Didn’t overly excite anyone.

1994

 Gosling – HotJava Browser

• could download “applets” from the web and run them

• applets were written in Java.

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Java Advantages

Platform independence Reuse of code

Security

Automatic garbage collection Stronger typing of objects and variables

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Java Technology:

Programming language

 Syntax and semantics

Development environment

 Compiler, interpreter, documentation generator, class packaging tool

Application environment

 Standalone programs- run on any machine where Java Runtime Environment is installed (JRE)

Deployment environment

 JRE supplied by SDK: contains class files, GUI

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Portability and Security

Java Virtual Machine

 An imaginary machine that is implemented by emulating it in software on a real machine (fake CPU that sits on top of an operating

system). Code for the JVM is stored in .class files.

 Provides hardware platform specifications

 Reads compiled byte codes that are platform

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JVM

JVM JVM JVM

A JVM has been written for every operating system.

A compiler takes Java application source code and generates bytecode (machine code instructions for JVM).

Java source code (Car.java)

compiled bytecode

Car.class

Still platform dependent not readable by humans;

not really machine code…

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JRE

Car.class

class loader

bytecode verifier

interpreter

runtime

Runtime

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JVM

JVM specification provides concrete definitions for implementation of:

 Instruction set

 Register set

 Class file format

 Runtime stack

 Garbage collecting heap

 Memory area

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SO….

Java uses a mix of compiler and interpreter

The majority of type checking is done when the code is compiled.

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Garbage Collection

Allocated memory does not need to be deallocated (delete).

Java provides tracks memory allocation

Garbage Collection

 checks and frees memory no longer needed

 done automatically

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Java promotes

Object-oriented programming

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Object-Oriented Programming

Each object has

 its own memory

 belongs to a class (instance of a class)

Class defines

 attributes (defines state)

 behavior (methods)

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BlueJ

tool for teaching object-oriented

programming and Java without much overhead (task overloading for

student)

development environment designed at Monash University, Australia.

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Shapes….

BlueJExamples

BlueJ Time

Activity_01 (#1,2)

BlueJ can be downloaded FREE

Some materials and examples are from Objects First with Java

by Barnes and Kölling goto BlueJ

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Shapes

…summary after experimentation

What we see in this BlueJ project:

 ClassesCircle, Square, Triangle, Canvas

 creation of an object : call to Constructor

 invoking (calling) methods

 passing parameters

 method signatures

 data types : int, String, and boolean

 multiple instances of the same class

 changing the state of the object

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Picture…Summary after Experimentation

How many objects?

How many constructors?

What happened when you

commented out the code for the constructor?

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Picture

"Construct" creates an instance of the Picture class

a Picture

 consists of 5 objects

• 2 squares, 1 triangle, 1 circle, 1 canvas

 Objects can create other objects!

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Constructors

If no constructor is included in the class, Java provides one.

 if values are not set, default values for instance variables of a class are:

• 0 for int

• 0.0 for double

• null for objects

• false for boolean

If a constructor is included in the class, all bets are off (no default constructor is

provided)!

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Comments

BlueJ : Interface

JavaDoc creates this document

 (more on this later).

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Comments, White space

Comments

 // comment on one line

 /*……*/ comment over several lines

 /** ……*/ javadoc comment

White space

 ignored; use white space for clarity of code

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Javadoc

some keywords :

@author*

@version*

@param*

@return*

@throws*

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JAVA

Language Basics

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Some basics:

Primitive types

 logical : boolean

•no casts between boolean and int

 texual: char

 integral: byte, short, int, long

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(not part of AP Subset)

Integral data types

 byte 8 bits -2⁷ to 2⁷ – 1

 short 16 bits -2¹⁵ to 2¹⁵ – 1

 int 32 bits -2³¹ to 2³¹ – 1

 long 64 bits -2⁶³ to 2⁶³ – 1

default is int

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(not part of AP Subset)

Floating Point data types

 float 32 bits

 double 64 bits

default is double

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Everything else

Everything that is not a primitive type is a reference type

A reference variable contains a reference to an object

Circle sun = new Circle();

 allocates memory for this new object

 initializes attributes sun

 executes constructor

 assigns reference to reference variable

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Order of Precedence

Separators

 . IMPORTANT

 []

 ()

 ;

 ,

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Operator Order of Precedence (associativity – operators)

R to L ++ -- + - ~ ! L to R * / %

L to R + -

L to R << >> >>>

L to R < > <= >= instanceof L to R == !=

L to R &

L to R ^ L to R | L to R &&

L to R ||

R to L ?:

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Conditionals – branching if –else if statements

if (boolean expression){

statement or block;

}

if (boolean expression){

statement or block;

}

else if (boolean expression) { statement or block;

} else{

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Conditionals – branching

switch statements

(not part of AP subset)

switch (expression){

case constant2:

statements;

break

case constant3:

statements;

break;

default:

statements;

break;

}

note: expression must be assignment compatible with int.

(byte, char, short are OK;

double, long, Strings not OK)

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Looping

for(startExp; booleanExp; endExp) {

statement or block;

}

while(boolean expression){

statement or block;

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Teaching Tip:

Company Rules:

 Have a permanent place in your

classroom with your Company Rules listed.

 That way everyone knows what is expected and no one will be

needlessly fired!

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Java supports

short circuit evaluation

if ((n != 0) && (x < 1/n)) evaluates first expression

if OK then evaluates second expression

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Looping

do{

statement or block;

} while(boolean expression)

(not part of AP subset)

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Looping

When do you use each loop?

 for

 while

 do

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Special Loop Flow Control--

return

is part of AP subset

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More with BlueJ

More with BlueJ – TicketMachines

 NaiveTicketMachine

• attributes (fields)

• private int price; //price of ticket

• private int balance; //how much $ put in for ticket

• private int total; //total $ in machine

• Constructor

• Methods

• insertMoney

• printTicket

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Naïve TicketMachine

Is there a default constructor?

How many constructors are there?

What do we mean by a "method signature?"

What methods are the accessors?

What methods are the modifiers (mutators)?

What's wrong with this ticket machine?

What enhancements can we make?

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Scope of variables

Fields (attributes)

 private

Formal parameters

 initialized by actual parameters

Local variables

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arrays

Part of AP CS A subset

one- and two- dimensional arrays

 more on two-dimensional arrays later (AB only)

arrays of primitive types arrays of objects

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arrays

Declaring an array variable is similar to declaring other variables.

 int[] a; //array of ints;

 double[] b; //array of doubles;

 String[] c; //array of Strings;

Does not cause Java to allocate memory (space).

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arrays

To allocate memory (space) for your array.

final int MAXNUM = 5;

int [] a = new int [10];

double[] b = new double[20];

String[] c = new String[MAXNUM];

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array length

If a is an array variable,

a.length represents the length of the array a.

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array processing

assume an array of ints

rate each of the following on a scale of 1 – 5 where 5 is very hard to implement :

 fill: value of array element filled with index of array element

 find largest value in array

 find smallest value in array

 find sum of values in array

 search for item in array

 sort items in an array

 count occurrences of a value in an array

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Initializing arrays

Initialization of named arrays:

int[] a = {1,2,3};

Declaration without initialization gets default values.

int[] a = new int[10];

filled with 0s.

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LabArrayClass class

attributes are private

• private String instructor;

• private String room;

• private String timeAndDay;

• private Student[] students;

• private int currentNumberOfStudents

 methods are public (for now)

• Constructors

• Accessors

• Mutators

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Student[] students;

private field

 each LabArrayClass object has this

array of Students

 What does this mean?

 What is an array?

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A look at the constructor

public LabArrayClass(int maxNumberOfStudents) {

instructor = "unknown";

room = "unknown";

timeAndDay = "unknown";

students = new Student[maxNumberOfStudents];

currentNumberOfStudents=0;

}

An array is a fixed-length collection of values of the same type.

Access array elements by indexing : students[3]

First element in the array has index 0.

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Comparing array elements

Comparing elements in arrays of primitives is not a problem.

 comparison operators are defined.

What about arrays of Strings?

 Strings are Objects

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Suppose we wanted to

"search?"

We need to search for a String, key…

 Look at each element in the array of Strings and check to see if it contains the same characters as key.

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Comparing Strings

Strings are objects. Do not use == to compare the values of two strings.

To test if two Objects have the same data, we use the equals method. (API)

Every Java Object supports the equals method but its implementation may be different (more on this later).

if(aString.equals(bString))…

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LabArrayClass

A peek at the code……

How can we search for a student by ID?

How can we remove a student who has a specified ID?

How can we sort the students according to ID numbers?

Work with LabArrayClass project

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What is an ArrayList???

indexed collection of objects grows automatically

AP CS A subset methods :

 int size()

 boolean add (Object x)

 Object get(int index)

 Object set(int index, Object x)

 void add(int index,Object x)

 Object remove(int index) A look at the API !

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Constructor

public LabClass(int maxNumberOfStudents) {

instructor = "unknown";

room = "unknown";

timeAndDay = "unknown";

students = new ArrayList();

capacity =

maxNumberOfStudents;

}

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…Recall Java A subset

…Recall API

public void enrolStudent(Student newStudent) {

if(students.size() == capacity) {

System.out.println("The class is full, but the student has been added.");

}

students.add(newStudent);

}

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Setters

public void setRoom(String roomNumber) {

room = roomNumber;

}

public void setTime(String timeAndDayString) {

timeAndDay = timeAndDayString;

}

public void setInstructor(String instructorName) {

instructor = instructorName;

}

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ArrayList method

public int

numberOfStudents() {

return students.size();

}

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More about objects and ArrayList

public void printList() {

…

int i;

for(i = 0; i < students.size(); i++){

Student theStudent = (Student)students.get(i);

theStudent.print();

}

…

}

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Activity_01

Complete work!!!

Projectpartners

 design

• discuss

• commit

• defend

Talk to your neighbors!