OOP in Java : © W. Milner 2005 : Slide 1 OOP in Java

OOP in Java : © W. Milner 2005 : Slide 1

OOP in Java


Course outline

1. Getting started, primitive data types and control structures

2. Classes and objects3. Extending classes4. Using some standard packages5. OOP revisited

Sessions 1 to 3 introduce and sketch out the ideas of OOP. Session 5 deals with these ideas in closer detail.


Recommended text

The Java Programming Language Third Edition by Arnold, Gosling and Holmes – Addison Wesley

Gosling created the Java language In the spirit of The C Programming Language

by K & R, and the C++ Programming Language by Stroustrup

If you’ve never written a computer program before, it won’t make much sense


What you need to know already

What compiling and iterpreting mean Some knowledge of C

– Much Java syntax is like C, so– We’ll say ‘it’s like C except that..’


Getting started

Just follow these steps – explanations later Look at java.sun.com for what is available You need J2SE = Java 2 Standard Edition Download and install the JDK – currently 5.0

Update 5 This will install software needed in

C:\Program files\java Create a directory where you will save your

Java programs – call the folder JavaProgs


Keeping Going

Start NotePad and enter the program on the next slide.

Save it with the filename “First.java” – must be this name. Put quotes around it – make sure Notepad does not put .txt on the end.

Note CAPITAL F. Save it in your JavaProgs folder:


The first program

public class First{public static void main(String[] args)

{System.out.println("Hello world");}

}


Then..

Switch to the command prompt (Start Programs Accessories Command Prompt) and navigate to your JavaProgs folder

Use cd to change directory. Cd.. Moves ‘up’ a level.

You should have something like:


Compile it

The compiler is called javac. You’ll need to include the path to it, so type in:

After this, get this line back using the up and down arrow keys.

Then run it like this:


Explanations..

All Java source code files are called Something.java

You give this to the javac compiler This produces a file called Something.class This is in bytecode This can be interpreted by the java interpreter


More explanations

Each source code file defines a class The name of the class it defines (First in our

case) must match the filename – hence First.java

This is compiled to a bytecode file named First.class

Java is case-sensitive (like C) Classes should start with a capital letter. So the file must be called First.java and not

first.java


Exercise

Change the file First.java so it defines a class called Second – and save it as Second.java

Change it so it outputs ‘my second program’ instead of ‘Hello world’

Compile and run it.


Features of Java

Java is a general purpose high level language Core Java is well-defined and stable Versions are useful in many situations –

desktop, server, embedded, mobile etc Java is a trademark of Sun Microsystems Java is cross -platform


More features

Java is a pure object-oriented language. No functions, no global variables Unlike C++, which is C with objects Java is designed to make it hard to write

programs which will crash No pointers Compiler will not compile doubtful code Programmer must write code that ‘catches

exceptions’ Run-time checks eg array bounds exceptions Slower than C or C++ (not much), but less

chance of crash


Types

Recall data type from C – int, char double etc Java has 2 kinds of types:

Primitive typesReference types


Primitive types

These are simple types like char, double, int Similar to C Main difference – the sizes of these types are

defined eg an int is 4 bytes Each hardware platform has its own 'virtual

machine' Which all look the same So can all have the same data sizes All chars use UNICODE character set - so

characters are 2 bytes long


Reference types

Reference type variables are objects Objects belong to classes Objects made by a constructor


Primitive types first

We will look at primitive types first and control structures (loops and ifs) Then look at classes and objects


Program format

public class Testing{public static void main(String[] args)

{// find the area of a circle..double radius = 5.0;double area;area = 3.1416 * radius * radius;System.out.println("Area = " + area);}

}

Use this format to start withIn file Testing.javaCode goes hereExplain rest later


Variables - declaring and assigning

// starts a line comment double area declares a variable called area of type double double radius = 5.0; declares and initializes a variable variables can be declared anywhere in a block { } statements end in ; like C and C++

public class Testing{public static void main(String[] args)

{// find the area of a circle..double radius = 5.0;double area;area = 3.1416 * radius * radius;System.out.println("Area = " + area);}

}


Console output

System.out.println("Area = " + area); This takes a single string argument – but.. The + causes area to be converted to the

equivalent string, and concatenated with the left hand operand so we get "Area = 5.72" or whatever System.out.print stays on same line


Primitive data types - numericName Range Size

byte -27 to 27-1 (-128 to +127) 8 bits

short -215 to 215-1 ( ± 32 000 ) 16 bits

int -231 to 231-1 ( ± 2 500 million ) 32 bits

long -263 to 263-1 (very big!!) 64 bits

float about 1038, 6/ 7 sig digits 32 bits

double 10308, 14/ 15 sig digits 64 bits

Java data type sizes are platform independent

All are signed

Top four are integer, bottom two are floating point

Variables of these types declared like

short a,b,c; or initialised when declared

double x = 1.502;


Numeric data types

Can get overflow eg– int i = 64000;– int n = i * i ;

Specify type of constant like– x = 1000L; // defaults to integer– f = 1.0F; // force float - defaults to double– x = 0x1FF; // hex

Operators + - * / % is mod = remainder eg 13 % 4 is 1 Short cut – same as C

+= x+=8; same as x = x + 8;-= x-=8; same as x = x - 8;*= x*=8; same as x = x * 8;/= x/=8; same as x = x / 8;%= x%=8; same as x = x % 8;


Overflow Exercise

Use code to produce overflow as in the previous slide

Find out what happens when you compile/run it.


Two types of division

float f = 1.0 / 2.0; // floating point int i = 1 / 2; // i is 0 if both operands are integer, / is integer version - it

gives the quotient and discards the remainder So / is overloaded – different versions, same name


Increment and decrement

x++; is the same as x = x + 1; y--; is the same as y = y - 1; post-increment is like

a = b++;which first assigns b to athen increments b

pre-increment isa = ++b;which first increments bthen assigns the new value to a


Type casts

Assigning a small type to a larger type is no problem eg

int i;long x;i = 32;x = i; OK because x more bits than i

But reverse gives ‘possible loss of precision’ eg int i;

long x;x = 32;i = x; // gives compile error

Problem solved by a type casttype cast iei = (int) x;


Type cast exercise

Try out int i;

long x;x = 32;i = x;

In a program. Fix it as in the previous slide


Char type char is for a single character, like char c = ‘A’; note single quotes c++; makes c = ‘B’ Strings are different - see later Java uses UnicodeUnicode not ASCII - 16

bits per character eg

import java.applet.*;import java.awt.*;

public class TestApplet extends Applet{public void paint(Graphics g) {

char c; Font f = new Font("Arial Unicode MS",Font.PLAIN,20); g.setFont(f);

c = '\u098a'; // Unicode constant g.drawString("Some Bengali: " + c,10,30 ); }}


boolean type

If a variable is declared to be boolean, it is restricted to 2 values - true and false

boolean result; result = true;

result = ( x > y );result is true if x is greater than y

also < <= >= == != == not the same as =

&& and || or ! not result = ( x > y ) && ( y < 5 );

result is true if x is greater than y and y is less than 5

&& and || are short-cut operators


bitwise operators

& is bitwise AND (like both)| is bitwise OR (like either )^ is XOR (like not equal)~ is NOT

eg if x = 9 1001and y = 10 1010

x & y is 8 1000 x | y is 11 1011 x ^ y is 3 0011 ~ 0xFFFFFFFE is 1 inverting 11111111110


bit shift operators

>> is shift righteg if x = 7 or in binary 0000 0111x >> 1 is 0000 0011

<< is shift left sox << 1 is 0000 1110 = 14


Bit shift exercise

Try out this code:int i = 6;System.out.println(i&1);i>>=1;System.out.println(i&1);i>>=1;System.out.println(i&1); Explain what you get


precedence

2 * 3 + 4 is 10 not 142*(3+4) is 14Highest ++ --

* / %+ -< <= > >=== !=&&||

Lowest = += -= *= /= %=Use brackets when in doubt


Control - if

for exampleif ( x== 5 )

{y = 2;a++;}

elsec++;

round brackets around boolean expression indentation no then as in Visual Basic block { } around several steps to do no block if just one step -

if (x<4)a=4;

else can be omitted if not needed


if example - validation

for exampleif ( ( age>0 ) && ( age < 130 ) )

System.out.println(‘age is valid’);else

{System.out.println(‘age is invalid’);..code to deal with error..}

beware ofif ( x==5 );

y = 2;


switch - I

used where many alternative actions are possible example -

switch (y){case 5: a = 7;case 9: b = 3;case 4: a = 8;default: z = 2;}

y can be expression (like x + 4) but must be integral the 5, 9, 4 etc must be constants default is optional


switch - II

the action ‘falls through’ - when one case is triggered, all the following options execute to the end of the switch

so often combine with breakbreak - example -switch (y)

{case 5: a = 7; break;case 9: b = 3; break;case 4: a = 8; break;}

include final break - not essential but good practice, since if add further option do not need to go back and add break to previous


Conditional operator ? ;

examplex = ( y > 4 ) ? 7 : 3;if y is greater than 4, x becomes 7, and otherwise, it becomes 3

in generala ? b : cb is evaluated if a is true, c if it is not

exampleint x =9, y = 10 , a;a = (x > 9) ? x++ : y++ ;

after this a = 10, y is 11, x is still 9


loops - while

loops repeat blocks of code - called iteration example - output the numbers 3, 6, 9, ... 99

x = 3;while ( x<102 ) {

System.out.println( x ); x += 3; }

in general,while (boolean expression)

statement or block to repeat need to initialise variables may loop zero times if false first time use indentation


loops - do while

example - output the numbers 3, 6, 9, ... 99x = 3;do {

System.out.println( x ); x += 3; } while ( x<102 )

in general,do statement or block to repeat while (boolean expression)

unlike a while, it will execute the loop at least once


loops - for

example - output the numbers 3, 6, 9, ... 99for ( x = 3; x<102; x+=3 ) System.out.println(x);

in generalfor ( <initialisation> ; <loop while true>; <change every time> )

< statement or block to repeat > may loop zero times add up the integers 1 + 2 + 3 + ...100

int t = 0;int x;for ( x = 1; x<101; x++)

t += x;System.out.println( t );


loops - for - II

can use statement liststatement list, likeint t;int x;for ( x = 1, t = 0; x<101; x++)

t+=x;System.out.println(t);

can omit any part, (retain separating ; ) likeint t = 0;int x = 1;for ( ; x<101; x++)

t+=x;System.out.println(t);

for (; ; ) loops forever


can declare variable in for, likeint t = 0;for ( int x = 1; x<101; x++)

t+=x;System.out.println(t);in this case the scope of the variable is limited to the for statement

loops - for - III

do not do this -

for ( int x = 1; x<101; x++);t+=x;


Arrays - I

An array is a set of boxes (elementselements) each labelled with a number (indexindex)

Arrays are declared and created asint [ ] numbers = new int [ 100 ];which makes an array of 100 integers called numbers

or do it in 2 stepsint [ ] numbers; //declare itnumbers = new int [ 100 ]; // create it

or initialise itint [ ] numbers = { 4, 2, 1, 3, 5 };

can have arrays of anything


Arrays - II

Array elements referred to likenumbers [4] = 18;

Multi-dimensional arrays created and used likeint [ ] [ ] table = new int [5] [10];

table[3][4]=7; Array element numbering starts at 0 so

int [ ] numbers = new int [ 100 ];creates 100 elements, from numbers[0] to numbers[99]

array bounds are checked at compile time and run time


Arr

ays

- so

rtin

g int [ ] numbers = new int [5];

//.. put some numbers in the array, then...sort them

// a bubble sort.. for ( int i = 0; i < 5; i++ )

for ( int j = 0; j < 4-i; j++ ) if ( numbers[ j ] > numbers[ j+1] ) { // swap them int temp;

temp = numbers[ j ];numbers[ j ] = numbers[ j+1 ];numbers[ j+1 ] = temp;};


Array exercise

Declare an array of 100 doubles Fill the array with random numbers (use

Math.random(); ) Print them out

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OOP in Java : © W. Milner 2005 : Slide 1 OOP in Java