slide 1 introduction to programming instances and classes variables java virtual machine using...

Introduction to Programming

Instances and Classes

Variables

Java Virtual Machine

Using Eclipse

Introduction toObject-Oriented

Programming

Lesson 01:Introduction

Text book:

Objects First with BlueJ297

Slides that reference the text book will have this

indicator

Introductionto Programming

Programming Language Different programming languages are different

ways (encodings) that turn into (same/similar) commands for the computer

Some early definitions (1)

Program A description in a programming language of a process that

achieves some result.

Algorithm A description of a process in a step-by-step manner.

The same algorithm could be written in many languages.

Procedure Instructions or recipes, a set of commands that show how

to prepare or make something.

Algorithms often implemented in one or more procedures.

Some early definitions (2)

Aspects of a computer program that must be designed:

The logical flow of the instructions

The mathematical procedures

The layout of the programming statements

The appearance of the output

The way information is presented to the user

The program’s “user friendliness”

Manuals, help systems, and/or other forms of written documentation.

The Art of Programming (1)

Programs must be analytically correct as well.Programs rarely work the first time they are

programmed.Programmers must perform the following on a

continual basis: analyze, experiment, correct, and redesign.

Programming languages have strict rules, known as syntax, that must be carefully followed.

The Art of Programming (2)

Primitive Programming (never actually in vogue) A single set of chronologically ordered steps for the computer to take

Procedural Programming Invented in 1945 by John Von Neumann

The notion of "subroutines" or "procedures" – small blocks of code that could be jumped to in any order

Earliest major procedural language: FORTRAN (1957)

Later example: C (1972)

Object-Oriented Programming Break down a programming task into objects that expose data and behavior

Late 1970s / early 1980s

Early example: C++ (1983)

Evolution of Programming

// The shape height and widthdouble width = 3;double height = 5;

// If these dimensions are for a rectangledouble rectangleArea = width * height;System.out.println("Rectangle area: " + rectangleArea);if (rectangleArea > 10) System.out.println(" -- it is big.");else System.out.println(" -- it is small.");

// If these dimensions are for a triangle -- the width is the base!double triangleArea = 0.5 * width * height;System.out.println("Triangle area: " + triangleArea);if (triangleArea > 10) System.out.println(" -- it is big.");else System.out.println(" -- it is small.");

Primitive Example(in Java, but don't focus on syntax)

A single set of chronologically

ordered steps for the computer to

take

Drawbacks to this?

// The shape height and widthdouble width = 3;double height = 5;

printRectangleArea(width, height);printTriangleArea(width, height);

static void printRectangleArea(double width, double height) { double area = width * height; System.out.print("Rectangle area: " + area); printRelativeSize(area);}

static void printTriangleArea(double width, double height) { double area = 0.5 * width * height; System.out.print("Triangle area: " + area); printRelativeSize(area);}

static void printRelativeSize(double area) { if (area > 10) System.out.println(" -- it is big."); else System.out.println(" -- it is small.");}

Procedural Example

small blocks of code that could be jumped to in any order

very small blockof "main" code

Advantages?

Drawbacks?

public class ShapeMaker {

public static void main(String[] args) { // The shape height and width double width = 3; double height = 5;

// Make a rectangle and print its area Rectangle myRectangle = new Rectangle(width, height); myRectangle.printArea();

// Make a triangle and print its area Triangle myTriangle = new Triangle(width, height); myTriangle.printArea(); }}

public class Rectangle {

public class Triangle {

Break down a programming task into objects that expose data and

behavior

Object Oriented Example

"If you write a computer program in an object-

oriented language, you are creating in your computer a model of some part of the

world."

Instances and Classes

Textbook Example Chapter 01 - figures

Terminology Class Circle

“Create a new Circle object”“Create a new instance of the Circle class”“Create a new object of type Circle” (less formal)

Demo: Normal circle

Make visible, move around, etc.

Big black circle (diameter 100, color black)

Small red circle (diameter 30, color red)

Inspect the object (by selecting inspect or double clicking)

Terminology “Many instances of the same class”

4demo

Textbook Example Chapter 01 - figures 4-5demo

classes

instances

Class Think: generic concept

The concept of a circle

The concept of a car

The concept of a giraffe

Instance (aka Object) Think: physical object

The green circle with a 4 cm diameter on my shirt

My blue Nissan minivan with the dent on the side

Stella, the female giraffe who lives at the Philadelphia zoo

Key Concept: Class vs. Instance

DATAaka “attributes”aka “state”

For example: Circle data:

Diameter, color, (x,y) location, visibility …

Car data: Make, model, year, color …

Giraffe data: Height, weight, birthday, name …

ACTIONSaka “methods”aka “functions”

For example: Circle methods: moveDown(), changeSize(), makeVisible() …

Car methods: startEngine(), reverse(), setColor() …

Giraffe methods: eat(), walk(), sleep(),setWeight() …

Classes specify types of data object store

and types of actions the objects can do

Object

Attributes (data)

Methods(behaviors / procedures)

Write a "recipe" to solve a problem by: Defining relevant classes

Creating instances of those classes

Defining the sequence of methods that each instance should execute

So how do we program in an object-oriented way?

Objects have operations which can be invoked (Java calls them methods).

Methods may have parameters to pass additional information needed to execute.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Methods and parameters5-6

Many instances can be created from a single class.

An instance has attributes: values stored in fields.

The class defines what fields an instance has, but each instance stores its own set of values (the state of the instance ).


Other observations8


State8-9


Two circle objects10

Each class has source code (Java code) associated with it that defines its details (fields and methods).


Source code

Variables

Computer Systems: HardwareMain Memory

A section of memory is called a byte.

A section of two or four bytes is often called a word.

Main memory can be visualized as a column or row of cells.

0x0000x001

0x0030x002

0x0040x0050x0060x007

A byte is made up of 8 bits.1 0 1 0 1 0 1 0

public class HelloWorld{ public static void main(String[] args) { String message = "Hello World"; System.out.println(message); }}

Programming LanguagesSample Program

Key words in the sample program are:

Key words are lower case (Java is a case sensitive language). Key words cannot be used as a programmer-defined identifier. Semi-colons are used to end Java statements; however, not all lines

of a Java program end a statement. Part of learning Java is to learn where to properly use the

punctuation.

• public• class

• static• void

There are differences between lines and statements when discussing source code.

System.out.println(

message);This is one Java statement written using two lines. Do you

see the difference?

A statement is a complete Java instruction that causes the computer to perform an action.

Programming LanguagesLines vs Statements

Data in a Java program is stored in memory.Variable names represent a location in memory.Variables in Java are sometimes called fields.Variables are created by the programmer who assigns it a

programmer-defined identifier.ex: int hours = 40;

In this example, the variable hours is created as an integer (more on this later) and assigned the value of 40.

Programming LanguagesVariables

Variables are simply a name given to represent a place in memory.


1-28

0x0000x0010x0020x0030x0040x0050x0060x007

0x0000x0010x0020x0030x0040x0050x0060x007


The Java VirtualMachine (JVM)actually decideswhere the valuewill be placedin memory.

72

Assume that thisvariable declarationhas been made.int length = 72;

The variable lengthis a symbolic namefor the memorylocation 0x002.

The Compilerand the JVM

Source code

Each class has source code (Java code) associated with it that defines its details (fields and methods).


10

A programmer writes Java programming statements for a program.

These statements are known as source code.

A text editor is used to edit and save a Java source code file.

Source code files have a .java file extension.

A compiler is a program that translates source code into an executable form.

A compiler is run using a source code file as input.Syntax errors that may be in the program will be discovered

during compilation. Syntax errors are mistakes that the programmer has made

that violate the rules of the programming language.The compiler creates another file that holds the translated

instructions.

The Compiler and the Java Virtual Machine (1)

Most compilers translate source code into executable files containing machine code.

The Java compiler translates a Java source file into a file that contains byte code instructions.

Byte code instructions are the machine language of the Java Virtual Machine (JVM) and cannot be directly executed directly by the CPU.

The Compiler and the Java Virtual Machine (2)

Byte code files end with the .class file extension.

The JVM is a program that emulates a micro-processor.

The JVM executes instructions as they are read.

JVM is often called an interpreter.

Java is often referred to as an interpreted language.

The Compiler and the Java Virtual Machine

1-34

How Java Compiles and Interprets Code

source code(*.java files)

bytecodes(*.class files)

Java IDEOr

Text editor

Java compiler

Java interpreter

Java virtual machine (JVM)

Operating system

Java interpreter

specific to a particular operating system.

Bytecodes can run on any

platform that has a Java interpreter.

Program Development Process

Text editor Source code(.java)

Saves Java statements

Java compiler

Is read by

Byte code(.class)

Produces

JavaVirtual

Machine

Is interpreted by

ProgramExecution

Results in

Portable means that a program may be written on one type of computer and then run on a wide variety of computers, with little or no modification.

Java byte code runs on the JVM and not on any particular CPU; therefore, compiled Java programs are highly portable.

JVMs exist on many platforms:

With most programming languages, portability is achieved by compiling a program for each CPU it will run on.

Java provides an JVM for each platform so that programmers do not have to recompile for different platforms.

Portability

• Unix• BSD (Berkeley Software Distribution, UNIX derivative)• etc.

• Windows• Mac• Linux

Portability

Java VirtualMachine for Windows

Byte code(.class)

Java VirtualMachine for Linux

Java VirtualMachine for Mac

Java VirtualMachine for Unix

Using Eclipse

Not all of this will make sense at first.

But soon, it will become natural

In the early weeks of class, we will tend to use BlueJ to modify examples in the book and follow along

Eclipse to code from scratch

Eclipse stores all of your files in a special folder called a Workspace – which is just a regular folder on the file system

Let's make "Hello World" in Eclipse

Eclipse is an advanced IDE

Create a New Java Project

Using Eclipse for Java Files


Use the Java Perspective in Eclipse




Highlight the project and add a package (right click)





Name your package





Name your package

Highlight the package & add a class (right click)





Name your package

Highlight the package & add a class (right click)


Name your class

/* The classic "Hello World" program */

package hello; // Packages are just a place to store related classes

/** * The purpose of the Hello class is to say "Hello World" * * Note that these comments are in the javadoc commenting style. * @author myersjac * */public class Hello {

public static void main(String[] args) { // This is how you print to the console. System.out.println("Hello World"); }

}

Hello World

Continuing in Eclipse…


Call it InvoiceApp


Highlight the project and add a Java class – also named InvoiceApp

Add this code on page 37 to InvoiceApp.java

Run the code to get a 20% discount on your subtotal.

IOOP Glossary Terms to Know

Terms to know for tests/quizzes (see class web site)

attribute bit byte class comment

compilation compiler field instance interpreter

main method

member object* object-oriented language

single-line comment state statement statement terminator

Non-glossary terms to know

algorithm byte code JVM machine code

procedure

program variableNote: the glossary definitions are complete ones, based on a full understanding

of the material which we might not yet have. However, there are elements of the definitions that you should be able to grasp from this material.* "Object" is often defined in this fashion. While not wrong per se, the definition is incomplete and misleading as you study Java further. For our class purposes, I prefer you use the word "instance," but it is also important to know how others may use the term object.

slide 1 introduction to programming instances and classes variables java virtual machine using...

Documents

evolution of programming

introduction slide

primitive programming

procedural programming

art of programming

programming instances

programming task

programming statements