week5 1

APCS-AB: Java Intro

Introduction to Java, Classes & Objects

October 4, 2005

week5 2

The Java Programming language

• Section 1.4 of your book has a history of Java as a programming language

• Developed in the early 90s by James Gosling at Sun Microsystems

• The Java Platform has three major groups: Java 2 Platform, Standard Edition (J2SE)

• We focus on this one Java 2 Platform, Enterprise Edition (J2EE) Java 2 Platform, Micro Edition (J2ME)

week5 3

A simple Java Program

// HelloWorld.java // Author: Ms.K// Demonstrates a simple java programpublic class HelloWorld{public void sayHello(){System.out.println(“Hello”);}/* This is an example of a multi-line commentin the book it talks about the main method:public static void main(String [ ] args) we won’t be programming main until later */

}

week5 4

How Java works

public class myProgram{

sayHello();

}

Compiler(javac) Output (code)

Program in the Javalanguage

Checks for errors, makes sure your code will run Java bytecode

.class file

Hello Class!

Java Virtual Machine (JVM)Translates the bytecode to something the underlying platform (computer) understands, and runs your program

week5 5

Programming Languages

• Languages can be: High level or low level Platform dependent or independent Procedural, functional, logic, or object-oriented

week5 6

Java is…

• A high-level,• Object-oriented,• Platform independent

Programming Language

• What does that mean?

week5 7

Java as a Language

• High-level – More like a real language, can be read by a human and understood, made more for ease of use for people than for the computer

• Object-oriented – think about problems from the things (objects) that are in the problem and what those objects can do (methods) This allows you to be able to:

• design code in a natural way• easily add new capabilities without retesting all the code

• Platform independent – code that you write on one computer can be transported and run on another computer without re-compiling

week5 8

OO vs. Functional Programming

• Scheme is a functional programming language Programs are written using only operations to be

performed on inputs, without using any temporary variables to store intermediate results

• (definition from www.jsoftware.com/books/help/jforc/glossary.htm)

• Java is an object-oriented programming language Programs are written to model objects in the world, the

focus is on what objects are part of the problem, what those objects encapsulate and how they are manipulated, instead of on the procedures or functions used

week5 9

OO Programming as Modeling

• The parts of the model are objects in the problem domain

• What are the properties of objects? Objects can be organized into categories

• We will talk later about inheritance & polymorphism Objects do things

• They have methods to accomplish tasks or to communicate with other objects

Objects know things• They have internal state, variables that hold information about

themselves

week5 10

Classes

• A class describes (abstractly) a category of objects Think of a class as a blueprint for an object – it describes the

boundaries of what an object can know or can do

• Instantiation is the process of creating the object instance from the class An object must be instantiated before we can use it

week5 11

What does this all mean?

• What kinds of problems is Scheme geared to help us with?

• What kinds of problems is Java geared to help us with?

• What does this all tell us about the relative power of these two different languages? Remember, programming languages are tools.

week5 12

Lewis & Loftus: Chapter One

• Read/Skim it at your leisure - it is an overview of a lot of the ideas that we will cover (or a basic intro to some of the things that we may not have time to really discuss much) 1.1 Computer Processing (overview, binary, analog/digital) 1.2 Hardware Components (architecture, I/O devices, memory, cpu) 1.3 Networks (connections, local-area & wide-area networks, Internet,

World Wide Web, urls) 1.4 Java Programming Language (Sample program, comments,

identifiers & reserved words, white space) 1.5 Program Development (language levels, editors, compilers &

interpreters, development environments, syntax & semantics, errors) 1.6 Object-Oriented Programming (Problem-Solving, OO Software

Principles)

week5 13

Role Play

• Class Activity: Bamboozler, Acrobat & Calculator (from http://web.sbu.edu/cs/dlevine/RolePlay/roleplay.html)

week5 14

APCS-AB: Java Intro

Introduction to Java & BlueJOctober 5, 2005

week5 15

Java for Modeling

• As we mentioned yesterday, one of the primary uses of Java (or any OOP language) is to model objects in the real world

• So let’s try to model something simple, a dog

week5 16

public class Dog {

// This is a comment, used to describe code

}

Creating a Dog class in Java

So everyone can access it

Tell java that this is a class

Name the class

Curly brace starts the class

This is the start of a one-line comment

Curly brace to end the class

CODE GOES HERE: what the Dog knows and what it does

week5 17

Object/Instance• A specific description, an instance of a class

A specific dog that we are modeling

• An object knows things and does things Also called attributes and behaviors

• Things an object knows are variables Data that is unique for an object of a given type

• Things an object does are methods

• Variables and methods are defined as part of the class Variable values are set only for a specific instance

week5 18

Dog Class

public class Dog{String breed; String name; int size;

public void bark(){ }}

These are variables.Variables have a data type and a name

This is a method.Right now it doesn’t do anything

A dog object would have individual values for each of these variables

week5 19

My Dog Instance

Breed: BulldogName: BullySize: 24Color: Purple

This dog can bark (invoke the method)

week5 20

What’s a Method?

• Things an object does

void bark() {System.out.println(“Ruff! Ruff”);

}

The method doesn’t return anything

The name of the method

The empty parenthesis means there

are no parameters in the method

Brackets again!

This is a statement; this tells the computerwhat you want to do in this method

week5 21

What’s a Statement?

• A line of code that gives a specific instruction• Must end with a semicolon

int x; /* a declaration is a type of statement */x = 5; // an assignment statementSystem.out.println(“Ruff! Ruff”); /* a print

statement */

week5 22

What if you want to teach your dog to bark differently?

• Make the dog say “Grrruffff”, “Bow-wow”, or “Ruff Ruff” Could change the Dog program each time and

recompile like we did yesterday Or we could have the method take a

parameter (a String)

• {{ Explore this together in BlueJ }}

week5 23

BlueJ

week5 24

How BlueJ works

• BlueJ is a freely available program An interactive development environment (IDE) designed to help

students learn to program Makes it easy to sit down, write, and compile a program

• we’ll see later that it’s not always easy in other environments where you have to deal with classpaths, file systems, DOS commands, etc

Has visualization of class structure (very helpful for beginning and advanced programmers)

Can directly create objects and then interact with their methods

week5 25

Starting in BlueJ

BlueJ has a template that gives you some starting code

It automatically creates:

•An instance variable

•A constructor

•An example method, with example javadoc comments

week5 26

Homework

• Installing Java and BlueJ at home (handout)

• Mid-quarter comments (for the teacher) How is this class going for you?

week5 27

Lab 1: Introduction to Java and BlueJ

• Open BlueJ (and make shortcut on desktop)• Make new project called “Eyes”• Add class from file (“JavaEyes.java”) • Compile it• Run it

Call Methods on it• Look at the code (open editor)

week5 28

APCS-AB: Java

Assignments & SyntaxOctober 6, 2005

week5 29

Syntax

• Syntax is… The rules governing the structure of a language The rules that the compiler understands Defines meaning of the various symbols used in a language

• In Java if you get the syntax wrong, the compiler won’t know what you are trying to do

• So it won’t do anything (it gives you an error)

week5 30

Concatenation

• Joining together or appending one string to the end of another

• String literals cannot extend across multiple lines in your program You must use the plus sign to concatenate two

string literals togetherSystem.out.println(“This is a bad program because it extends across multiple lines without concatenating the strings together. This will not compile.”);

System.out.println(“This is a good program” + “ because it uses the + sign to extend” + “ the program across multiple lines.”);

week5 31

Variables

• A variable is a name for a location in memory that stores data

• When you declare a variable, you tell the compiler to reserve a certain size piece of memory to store data in (based on the type of data) We will talk more about the types of data tomorrow

• Example: String name; int number;

week5 32

Variables

• Variables need a data type and a identifier.• Think of a variable as a cup. A container. It

holds something. The data type tells Java the size of the container. The identifier is there so you know which container is

yours.

namenumber

week5 33

Assignments

• When you declare a variable, you just set aside the memory - but that space is “blank”

• To actually place something in the variable, you need to assign a value to it: name = “Henry”; number = 123;

number

“Henry” 123

name

week5 34

Assignment Statements

• More examples: String anotherName = “Andrew Udovic”; int num = 5 + 4*23;

• In Java, here are the basic rules: On the left side is always the variable name, the “container” in

which we are going to store data On the right side could be a single piece of data, or an

expression that has to be evaluated• The single piece of data, or the result of whatever is evaluated is

then “assigned” to the variable (ie - placed into the memory location) You always end each statement with a semi-colon

week5 35

A program example

public class Example {String name;int number; //make the containers

public void doAssignment(){name = “Nick”;number = 123;printStuff();}private void printStuff(){System.out.println(“The name in this program is: “ + name);System.out.println(“The number is” + number);}

}

week5 36

Additional assignments

• If we reassign a value to a variable, the new value is written to memory.. What happens to the old value?

int myVar = 123;myVar = 34343435345;

• If we just reference (use) a variable, the value is not changedSystem.out.print( “myVar is: “ + myVar);// after this call, myVar still has 34343435345 stored in it

week5 37

Strongly Typed

• Java doesn’t let us store the wrong kind of data in our variables The compiler will cause an error if you try to use the wrong kind of data

for a variable.

• If we declare a variable of type String:String s;• Then we can’t put an integer into that Strings = 1232; //Compile error: incompatible types - found int but expected java.lang.String

week5 38

Constants

• Sometimes we want to store a piece of data that will never change For example, we may want to say that a table at lunch can hold no more than 12

people. If we just use the number 12 in our program, people looking at our code may not know where it came from

• So we give it a name like MAX_PEOPLE to help explain what the piece of data is doing in a program

• Also, lets say that we suddenly get new lunch tables that have a maximum of 15 people… We only have to change the data in one place, instead of trying to find all the

places that the number 12 was used in our program

• Constants are not variables (the value can never change) they hold one value for the durations of the program

week5 39

Constants in Java

• To declare a constant in Java, you use the reserved word final • By convention, we use uppercase letters to name constants so that

we can tell them about from regular variablesfinal int MAX_PEOPLE = 12;• The compiler will then prevent anybody from changing the value

once it has been set to the initial value

week5 40

Naming Practices in Programming

• Class names start with a capital letter public class Dog { }

• Object names start with a lowercase letter Dog fido = new Dog();

• Method names and variables start with lowercase, but use capital letters for subsequent words public void changeMyOil() {} int myAge;

• Constants use all caps MAX_INT_SIZE

week5 41

Arithmetic Operators

• What if we want to have a counter variable? Something that starts at 0 and then counts up, one-by-one

depending on what else we may be doing in a program• int x = 0;• So we could do:• x = x + 1;

So what would x equal? How does this all work?• We do this a lot in programming, so there’s shortcut for

this (called the increment operator) x++; // is the same as x = x+1;

week5 42

Other Arithmetic Operators

• Minus x = x -1;

• Multiplication x = x * 5;

• Division: x = x/2;

• What do you think will happen if we try to divide by zero?

• Modulo x = 5%2;

• You have the special operator for increment, do you think there are any similar operators for these functions?