GP2, Martin Lillholm 1 Introductory Programming (GP) Spring 2006 Lecture 2 We start at 13:00 Slides...

1GP2, Martin Lillholm

Introductory Programming (GP)Spring 2006

Lecture 2

We start at 13:00 Slides are available from the course home pageFeel free to print them now

www.itu.dk/courses/GP/F2006

Martin Lillholm


Practical Information

• News Group– it-c.courses.GP– Via http://webmail.itu.dk/

• Lab classes– www.itu.dk/courses/GP/F2006

• Questions ?


Last Week

• A basic computer and its infrastructure– CPU, main memory, storage, networks, internet, WWW

• Program Development– Problem, analysis, design, implementation (programming), testing

• (Java) programs– Classes, objects, syntax, semantics, reserved words, the simple

anatomy of Java programs

• Compilation and program execution– Source code, editors (e.g. Notepad); bytecode, compilers (javac),

execution, runtime environment (java).

• Three kinds of Java programs– ”Normal” via console, programs with GUI, Applets


Today

• More about compilation, runtime environment, classes, objects, the anatomy of Java programs

• Identifiers

• Variables

• (Primitive) types

• Operators

• Expressions

• Libraries

• Mere about Applets, GUI, and graphics


What is Programming Anyway?

• Problem / task

• Analysis

• Design (evt. OO)

• Implementation (in e.g. Java)

• Compilation

• Testing


From Source Code to Executable

Source

(Editor)

Compiler

Hello.javatext (ASCII)

Hello.class(Java bytecode)

Result

Interpreter(JVM)

Java Virtual Machine

JIT

Task

solution


Compilation Time – Run Time

Source

Compiler

Fortolker(JVM)

Syntax and type errors

Runtime and “semantic” errors


Java Components (again)

• White spaces– space, tabulator stops (\t), newline (\n)

• Comments

// Single line comment

/* Multiple linecomment ... */

• Reserved words; “core” Java.

• Identifiers

• Syntax rules …


The Anatomy of a Java Program

class ClassName {

// Attributes; Define the state of objects

// methods; The work horses of objects

public static void main (String[] args) {

// statements and today’s focus area

}

}

It’s possible to have several classes in one .java file but only one can and should have a main method.


Methods – Informally Speaking

• Methods are used for grouping and naming sequences of statements/command.

• Methods can receive input (parameters/arguments) and output results (return values).

• Methods do “the hard” work in Java programs.

• Methods are typically invoked/called by other methods. A method call is often a statement in itself.

• Program execution normally always start in the main-method.

• Methods enable task abstraction.


Using Methods

• If a class has a main method program execution starts with the first statement here and continues statement by statement.

• These statements can call other methods than the main method.

• Methods are always called using zero or more parameters in parentheses:

write() // call/invoke zero parameter method writearea(10) // call/invoke one parameter method areaadd(10,20) // call/invoke two parameter method add

• Methods can be “hidden” in other classes and/or objects.

System.out.println(“Print a line of text”);System.out.print(“Another line”);System.out.println();

• Notice the . operator


Today’s Focus

• Today we focus on code/statements in the main method.

• But any of them can and will (next time) be used in other methods.


Identifiers - Names

• Used for all program elements that must be named– Classes– Attributes / fields– Methods– Parameters– (Local) variables– Packages

• Identifier rules:– First character must be a Java letter– Then 0 or more Java letter or Java numbers– Java letter: A,...,Z, a,...z, $, _ (plus some others ...)– Java number: 0,...,9– No spaces!– Syntax diagram ... (L&L page 32)


More about Identifiers

• Examples: – Legal: HelloWorld, HelloGUI, label, i, j, ...– Illegal: 3label, !navn

• Conventions – Classes and packages has an upper case first letter and

upper case all for the first letter of all new words (no spaces): HelloApplet

– Attributes, parameters, variables, methods has a lower case first letter and upper case for the first letter of all new words (no spaces): labelColor

– (Named) constants only upper case and an _ (underscore) to separate words: MAX_HEIGHT, PI

– More conventions in appendix F


Literals – Constant Expressions

• “Atoms”

• Used for supplying specific data/information to programs

• Grouped in so-called types:

– Numbers int: 1 2 3 45 -17 (Syntax diagram L&L page 75)double: 0.1, -45.0, 243.89

– Characters/letterschar: ’a’ ’3’ ’K’ ’ ’(In reality numbers: ASCII (256), UNICODE(65536))

– Text stringsstring: ”Hello World” ”Welcome to GP!” ”h” ””Escape Sequences: L&L page 67

– Boolean values (truth values)boolean: true false


Literals – an example


(Local) variables

• A named slot in memory that can be assigned a value and read one or several times. The actual physical space is allocated by the runtime environment and the operating system.

• Variables must be declared before use!

• A variable is local/automatic when declared in a method

• A variable always has a type– Primitive types:

Integers: byte, short, int, longCharacters: charDecimal numbers: float, doubleBoolean values: boolean

• Why types? Memory usage, robustness, type safety.


Types af numbers and their capacity ...

Type

byteshortintlong

floatdouble

Storage

8 bits16 bits32 bits64 bits

32 bits64 bits

Min value

-128-32,768-2,147,483,648< -9 x 1018

+/- 3.4 x 1038 with 7 significant digits+/- 1.7 x 10308 with 15 significant digits

Max value

12732,7672,147,483,647> 9 x 1018

• Bits ?


Variable Declarations

• A variable must be declared before first use – why?

(final) type identifier;

int height;

char firstLetter;

boolean isDigit;

String firstName;

Syntax diagram L&L page 69

• Naming convention


Assignments

• Assignment statements are used for assigning values to variables. Values can be either literals or the result of an expression – more on expressions later.

• Syntax diagram L&L page 72

• Examples:

height = 23;

firstLetter = ’m’;

isDigit = true;

firstName = ”Martin”;


Simultaneous Declaration and AssignmentInitialisation

• Several variables of the same type can be declared on the same line

• Variables can have values assigned when they are declared (initialisation)

int height, weight=50, size;

char firstLetter=’M’;

double num1 = 12.23, num2, num3=-23.0;

• Syntax diagram L&L side 69

• Left- and right-hand side of assignment statements


Example 1


Eksempel 2


(Named) Constants

• Constants are used to name values that are used one or more times in a program but doesn’t change value – contrary to variables.

• Examples could be mathematical constants like PI or the maximally allowable height MAX_HEIGHT.

• Naming convention.

• Why constants? – No accidental change– A name is more descriptive than a number– Several uses only one change

• Declared and initialised. Can’t have new values assigned.

• Declared and initialised like variable but with a final prefix.

final int MAX_HEIGHT = 180;final double PI = 3.1416;


Expressions

• An expression is a combination of one or more operands and operators. Operands are values or other expressions. Mostly used for calculations – later we shall, however, see examples of String expressions.

• An arithmetic expression calculates numerical values and consists of zero or more arithmetic operators.

+ - * / % (binary and unary operators)

• Operands are the input to an expression … the result of the expression its “return” value.

• Operands can be literals, variables, and expressions.

• Variables used in expressions are (normally) only “read” not changed. Expressions make up the right-hand side of assignment statements. (Syntax diagram L&L page 72)


Expressions … Examples

1+2+3+4 102+3*4 1410/5 210/4 210.0/4, 10/4.0, 10.0/4.0 2.55%3 210%5 0

int i=4, int j=4;int k;k = i+j; 8k = k+4; 12

double x=3, double y=1;double z;z = x/y + 4; 7.0z = 1/3; 0.0 z = 1.0/3; 1/3=0.333333


Operator Precedence

• Expressions are evaluated before assignment.

• In generals expressions are evaluated from left to right

• But multiplication and division takes precedence over addition and subtraction – they have higher precedence.

• Parentheses are always evaluated first and thus can influence “normal” evaluation order

Precedence level Operator

1 Unary + -

2 * / %

3 + - +

4 = String concatenation


Example Expressions and Order of Evaluation

a + b + c + d + e1 432

a + b * c - d / e3 241

a / (b + c) - d % e2 341

a / (b * (c + (d - e)))4 123

If in doubt: Use parentheses but don’t overdo it!


Example


Specialised Operators

• Increment and decrement operators ++ and --

int a = 5, b;

a++; // same as a = a + 1;

b = a++;

b = ++a;

a = a--;

• Assignment operators +=, -=, *=, /= etc.

a += 5; // same as a = a + 5;

B *= a + 12 // same as b = b*(a+12);

• Oftentimes referred to as syntactic sugar


Type Conversion

• Java is a strongly typed language.

• It can, however, be necessary and convenient to convert between types.

• Sometimes, we e.g. want to use an integer as decimal number 5 = 5.0.

• We never change the type of a variable per se – only its use and context in expressions.

• Conversions should be and normally are lossless in terms of information.

– 5 5.0 int to double lossless– 5.5 5 double to int lossy


Types of Type Conversion

• Widening conversion: From a ”smaller” data type to a ”bigger”. short -> int, float -> double.

Usually lossless and thus safe.

• Narrowing konvertering: From a ”bigger” type to a ”smaller”.int -> short, double -> float.

Usually lossy and thus unsafe.


Assignment Conversion

• Assignment conversion occurs when a value (result of an expression) is assigned to variable of a different type.

• Only happens (automatically) when Widening is involved.

• As always the types and values of variables on the right-hand side of the assignment are unchanged.

int dollars;

float money;

money = dollars; // The value of dollars is

// automatically converted to

// float before assignment.


Promotion Conversion

• Promotion conversion happens automatically when operators promote the values of their operands in order to add, multiply etc. them.

• Again – only widening.

float sum, average;

int count;

average = sum/count;

• Also happen during String concatenation – more later.


Casting – Forced Type Conversion

• Programmer controlled and forced type conversion.

• Casting is a powerful tool and should be used judiciously.

• Both widening and narrowing is possible with casting.

• Executed by putting the desired type in parentheses before the expression that needs to be converted.

int total, count;float average;

average = (float) total/count;

• Note that casting has higher precedence than binary operators.


Strings

• String literals in quotation marks: ”Martin Lillholm”

• For now think of Strings as a primitive type – although they’re not! Notice the capital S in String.

String name = ”Martin Lillholm”;

System.out.println(name);

System.out.println(”Martin Lillholm”);

name = “Jens Hansen”;

System.out.print(name);


String concatenation – The + Operator Again

• Evaluates from left to right

• Promotes operands to String:

“A” + 5 = “A” + “5” = “A5”


An Overview of Graphics and Digital Images

• Digital images and/or graphics is made up from pixels (picture elements)

• An image is typically arranged as a rectangle of pixels

• The width and height measured as number of pixels is the image’s resolution.

• The number (again in terms of width and height) pixel a monitor/screen can display is the resolution of the monitor.


Coordinate System

• Any point in an image, in a Window, or on the screen can be identified by a pair of coordinates (x,y).

• Java and most computer systems use a coordinate system with the origin in the upper left corner :(Of the image, the Window, or the screen)

Y

X(0, 0)

(112, 40)

112

40


Black/white, Gray Scale, and Colour

• Black and white images are represented using black=0 and white=1.

• Gray scale images typically as an integer number between 0-255 (8-bit)

• Colour images typically as a triple (R,G,B) representing an additive mixture of red, green, and blue.

• R,G, and B are normally integers between 0 og 255

– (0,0,0) er sort– (255,255,255) is white – (128,128,128) is a gray ”halfway” between black and

white– (255,0,0) is red … etc.


The Color Class in Java

• Colours in Java is represented using the class Color. It’s contained in the package java.awt

• The class contains some predefined colours:

• Or can be used to mix new ones.

Object

Color.blackColor.blueColor.cyanColor.orangeColor.whiteColor.yellow

RGB values

(0, 0, 0)(0, 0, 255)(0, 255, 255)(255, 200, 0)(255, 255, 255)(255, 255, 0)


Drawing Lines using the Graphics Class

X

Y

10

20

150

45

page.drawLine (10, 20, 150, 45);

page.drawLine (150, 45, 10, 20);

eller


Drawing rectangles using the Graphics class

X

Y

page.drawRect (50, 20, 100, 40);

50

20

100

40


Drawing ovals using the Graphics Class

X

Y

page.drawOval (175, 20, 50, 80);

175

20

50

80

boundingrectangle


More about Applets

• A “normal” Java-program can be executed using the e.g. Sun runtime environment (java).

• A Java applet is designed to be downloaded via the internet and executed in a browser.

• Alternatively it can be executed using Sun’s appletviewer.

• An applet doesn’t have a main method.

• The paint method can be used instead.

• The paint-method receives a parameter of type Graphics

• Graphics objects among other things define a “drawing-context”

• Example: Einstein.java (L&L side 97)


An HTML-shell for the Einstein Applet<html>

<head> <title>The Einstein Applet</title> </head>

<body> <applet code="Einstein.class" width=350 height=175> </applet> </body>

</html>

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GP2, Martin Lillholm 1 Introductory Programming (GP) Spring 2006 Lecture 2 We start at 13:00 Slides...

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