Lecture 2

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Lecture 2

Web page correction: a108xxx is wrong.

It should read h108xxx.

Readings

On the course Web page read:

Useful program templates:

Example programs

In the text read

Chapters 3, 4

Handouts

Assignment 1

Methods say what an object does

Why Accessing methods?

to maintain encapsulation

Setter methods

setter methods or mutator methods just set the value of a field

Getter methods

getter methods or accessor methods return the value of a field

Getter methods: getX, getY

Setter methods: setX, setY, addToX

class Point {
    private double x;
    private double y;

    public String toString() {
      return "(" + x + "," + y + ")";}
    public void setX(double value) { x = value;}
    public void setY(double value) { y = value;}
    public void addToX() { x = x + 1;}
    public void addToX(int increment) {
       x = x + increment;
    }
    public double getX() { return x;}
    public double getY() { return y;}
}
class TestPoint {
  public static void main (String[] args) {
    Point origin = new Point();
    origin.setX(0.0);
    origin.setY(0.0);
    origin.addToX(); // method polymorphism
    origin.addToX(2.0);
    System.out.println(
"The x coordinate is " + origin.getX());
  }
}

Declaring Variables

type identifier;

  int count; // defines type, allocates memory

Assignment Statements

identifier = expression;

  count = 0; // assigns value
  count = count + 1;

Writing to Standard Output

System.out.println(String);
System.out.print(String); // no newline

Reading from Standard Input

DataInputStream stdin = new DataInputStream(System.in);
String line;
line = stdin.readLine();

Constructors create new instances of a class, that is objects. Constructors are special
methods that have the same name as their class and no return type. For example,

class Point {
    private double x;
    private double y;

    Point(double xvalue, double yvalue) {
      x = xvalue;
      y = yvalue;
    }
     ...
  }

Constructors are used along with the ``new'' keyword to produce an object or instance of the class.

Point origin = new Point(0, 0);
Point point = new Point(1, 1);

Java Program Structure

A program is made up of one or more classes
A class contains one or more methods
A method contains program statements
A Java application always executes the main method

White Space

Spaces, blank lines, and tabs are collectively called white space and are used to separate words and symbols in a program
Extra white space is ignored
A valid Java program can be formatted many different ways
See Lincoln2.java and Lincoln3.java
Programs should be formatted to enhance readability, using consistent indentation

Identifiers

Identifiers are the words a programmer uses in a program
Most identifiers have no predefined meaning except as specified by the programmer
An identifier can be made up of letters, digits, the underscore character (_), and the dollar sign
They cannot begin with a digit
Java is case sensitive, therefore Total and total are different identifiers

Reserved Words

Some identifiers, called reserved words, have
specific meanings in Java and cannot be used in other ways

abstract default goto operator synchronized

boolean do if outer this

break double implements package throw

byte else import private throws

byvalue extends inner protected transient

case false instanceof public true

cast final int rest try

catch finally interface return var

char float long short void

class for native static volatile

const future new super while

continue generic null switch

Literals

A literal is an explicit data value used in a
program there are seven kinds of literals

boolean:: true or false
int:: 89, -945, 37865
long:: 89L, -945L, 5123567876L
float:: 89.5f, -32.5f,
double:: 89.5, -32.5, 87.6E45
char:: 'c', '9', 't'
String:: "This is a string literal"

The Java API

The Java
Application Programmer Interface

The Java Application Programmer Interface (API) is a collection of classes.
The println method is in an API class, not part of the language
Both methods print information to the screen; the difference is that println moves to the next line when done, but print does not
See Countdown.java
Some packages in the Java API:

import java.applet.*;

java.lang automatically imported.

String Concatenation and Addition

The + operator serves two purposes
When applied to a string and some other value (like a number), that value is converted to a string and they are concatenated

When applied to two numeric types, they are added.

Programming Languages

Four levels levels:

machine
assembly
high-level
fourth-generation

Each CPU has its own specific machine language
The other levels were created to make programming easier
compile to target language ...
interpret

Java Translation and Execution

Java compiler translates to bytecode
bytecode is not machine code
a Java interpreter translates bytecode into machine language and executes it. So machine independent (architecture neutral)

  Java source    ,-------> bytecode
     |          /           /   \
     |         /           /     \
     V        /           V       V
  Java Compiler  interpreter  bytecode compiler
                                        |
                                        |
                                        V
                                  Machine code

Syntax and Semantics

The syntax of a language defines how you can put symbols, reserved words, and identifiers together to make a valid program
Semantics is the meaning of the program construct;
A syntactically correct does not mean logically (semantically) correct

Errors

A program can have three types of errors
The compiler will find problems with syntax and other basic issues

problems occuring during program execution, such as trying to divide by zero, causing abnormal program termination

A program may run, but produce incorrect results (logical errors)

Software Engineering

We should engineer software to make it reliable and maintainable
As the complexity of a program increases, its cost to develop and revise grows exponentially.

Software Components

Programs are easier to construct and modify when they are made up of separate components
A software component can be thought of as any program element that transforms input into output

input ----> component -------> output

list of numbers --> compute average --> average
                                 
input --.   ____     input --.   _____ 
        `->|    |            `->|     |
input ---->|    |--> input ---->|     |--> output
       .-->|____|               |_____|
input--'

Object-Oriented Programming in Java

Programs made from software components called objects
An object contains data and methods
An object is defined by a class
class: cookie cutter, blueprint or template
Multiple objects can be created from the same class

Defining classes isn't easy

Diogenes Laertius. Circa 200 A. D.

Plato having defined man to be a two-legged animal without feathers, Diogenes plucked a cock and brought it into the Academy, and said, ``This is Plato's man.'' On which account this addition was made to the definition,-''With broad at nails.''

Inheritance, Classes and Objects

Bicycle --> My Touring bicycle
        --> Joan's Mountain bicycle
        --> Chiapucci's Racing bicycle
        --> Steve Robert's Recumbent bicycle

Daily Modes of Transportation
  <-- Walking
  <-- Bicycle
  <-- Roller Blades
  <-- Public Transit
  <-- Car
  <-- Truck

Java Applets

local computer

Next: Lecture 3 Up: Home

Craig MacDonald

Sat May 16 10:23:36 EDT 1998

abstract	default	goto	operator	synchronized
boolean	do	if	outer	this
break	double	implements	package	throw
byte	else	import	private	throws
byvalue	extends	inner	protected	transient
case	false	instanceof	public	true
cast	final	int	rest	try
catch	finally	interface	return	var
char	float	long	short	void
class	for	native	static	volatile
const	future	new	super	while
continue	generic	null	switch