© Vinny Cahill 1 Writing a Program in Java. © Vinny Cahill 2 The Hello World Program l Want to...

© Vinny Cahill 1

Writing a Program in Java

© Vinny Cahill 2

The “Hello World” Program

Want to write a program to print a message on the screen.

© Vinny Cahill 3

“Hello World” – Objects and Classes

An instance of class TerminalAn instance of class String

Another instance of class String

Class String isbuilt into Java Class Terminal is part of a

collection of pre-existing classescalled tcdIO

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Functions and Parameters

Given f(x) = x2 + 4x + 13

What is f(10)?

What is f(20)?

What is f(30)?

x is the “formal parameter” of function f

10 is an “actual parameter” of function f

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Variables 1

A variable is a container for information

In our program

Terminal window; // Used to store object representing the

window

“window” is the name of a container used to store a Terminal object

Alternatively:

Terminal box;

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Variables 2

Every variable has a name, a type, and a single (current) value

window

type is Terminal

namevalue

Think of a variable as a container for a value of the specified type

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Variables 3

window is a local variablebecause it is defined within a method

public static void main (String args[]) { Terminal window; // store object representing terminal

. . . .

}

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Variables 4

The value stored in a variable can be changed using assignment

window

that why it’s called a variable!

window

window = new Terminal("Hello Window");

Assignment operator

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Variables 5

The value stored in a variable can be changed using assignment as often as we want

window

window

window = new Terminal("Another Window");

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Type int

Type int represents positive and negative whole numbers

four bytes of memory are used to store a value of type int thus, an int can contain a number in the range

-2,147,483,648 to 2,147,483,647 values of type int can be written in decimal

-123 23000 0 1000000 -7456 3990276

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Integer operators 1

There are five integer operatorsOperator Meaning ExamplesUse

+ op1 + op2 add values of op1 and op2

newSalary = oldSalary + rise;count = count +1;

- op1 - op2 subtract op2from op1

newSalary = oldSalary - 100;

* op1 * op2 multiply op1by op2

area = length * breath;minutes = hours * 60;

/ op1 / op2 divide op1by op2

length = area / breath;

% op1 % op2 remainder ondividing op1by op2

pounds = pence / 100pence = pence % 100;

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Integer operators 2

The int operators are all binary operators– they all take two operands

The int operators all return a single result of type int

For example:

int result, number1, number2;number1 = 7; number2 = 2;

result = number1 / number2;

result = number1 % number2;

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Integer expressions

We can have expressions that involve multiple operators

pay = salary + bonus - tax;

monthlyPay = salary + bonus - tax / 12;

Note that the order in which operators are evaluated is important!

*, /, % are always evaluated before +, - i.e. *, /, % have higher precedence than +, -

monthlyPay = (salary + bonus - tax) / 12;

Operators of equal precedenceare evaluated left to righti.e. they are left associative

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Average

/* A program to get the average of five numbers entered by the user */import tcdIO.*;

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

Terminal window; // Used to store object representing the windowint runningTotal, average; // used to store total

window = new Terminal("Average"); runningTotal = window.readInt("Enter first number: "); runningTotal = runningTotal + window.readInt("Enter second number: "); runningTotal = runningTotal + window.readInt("Enter third number: "); runningTotal = runningTotal + window.readInt("Enter fourth number: "); runningTotal = runningTotal + window.readInt("Enter fifth number: "); average = runningTotal / 5; window.println("The average is: " + average); }

}

integer value integer value

integer value

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Compilation

The text of any program needs to be translated into an executable form

The process of doing this translation is called “compilation”

The program that performs the process is called a “compiler”

Different programming languages use different compilers

Eclipse includes a Java compiler

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Compilation cntd.

HelloWorld.java

Java program as text

Java compiler HelloWorld.class

Executable Java program

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Flow of control

public static void main(String[] args) { Terminal window;

window = new Terminal("Address Window"); window.println("Prof. Vinny Cahill"); window.println("Discipline of Computer Systems"); window.println("School of Computer Science and Statistics"); window.println("Trinity College Dublin");}

The program always starts executing the main method

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Flow of control



On the screen

“Address Window”

In the computer

object

We execute a single statementat a time in sequence

point of control

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Flow of control



On the screen


In the computer On the screen

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Flow of control




In the computer On the screenOn the screen

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Flow of control





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Flow of control





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Flow of control





© Vinny Cahill 25

Flow of control



On the screen


In the computer

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Flow of control



On the screen


On the screenprintln! - "Prof. Vinny Cahill"

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Flow of control




On the screenOn the screenprintln! - "Discipline of Computer Systems"

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Flow of control




On the screenOn the screen

println! - "School of Computer Science and Statistics"

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Flow of control




On the screenOn the screenprintln! - “Trinity College Dublin"

© Vinny Cahill 30

Transfer of control



On the screen


In the computer

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Transfer of control



On the screen


On the screenprintln! - "Prof. Vinny Cahill"

void println (String text) { // step 1 // step 2 // step 3}

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Transfer of control




println! - "Discipline of Computer Systems"



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Transfer of control

public static void main(String[] args) { Terminal window; Square shape; int area;

window = new Terminal(“Square”); shape = new Square(10); area = shape.calculateArea(); window.println(“Area is: ” + area);}

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Transfer of control


window = new Terminal(“Square”); shape = new Square(10); area = shape.calculateArea(); window.println(“Area is: “ + area);}

“Square”

On the screen

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Transfer of control



10

calculateArea!

int calculateArea() { // step 1 // step 2 // step 3}

“Square”

On the screen100

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Transfer of control


window = new Terminal(“Square”); shape = new Square(10); area = shape.calculateArea(); window.println(“Area is: ” + area);}

“Square”


println! - “Area is: 100”

On the screen

10

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Type double

Type double represents real numbers

A value of type double occupies 8 bytes of memory A value of type double can store a number in the range

± 1.79769313486231570E+308 with 15 significant decimal digits

Values of type double can be followed by the letter ‘D’

230.6 0.0 -1.0E8 -7.4E-5 -123D -399.02E+7F and usually have either a decimal point or an exponent

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double operators

There are four double operators

Operator Meaning ExamplesUse

+ op1 + op2 add values of op1 and op2

newSalary = oldSalary + rise;distance = distance + 10.2;

- op1 - op2 subtract op2from op1

time = hours - 0.6;

* op1 * op2 multiply op1by op2

area = radius * PI;area = radius * 3.14D;

/ op1 / op2 divide op1by op2

length = area / breath;

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Real expressions

The same rules apply to writing real expressions as to integer expressions

The double operators are all binary operators that take two operands

The double operators all return a single result of type double * and / have higher precedence than + and – Operators of equal precedence are left associative Can use brackets to change the order of evaluation

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Other integer types

Java actually provides four primitive types that represent integers They differ only in their size

(i.e., the range of values that they can store)

int 32 bits -2,147,483,648 to 2,147,483,647

short 16 bits -32,768 to 32,7687

long 64 bits -9,233,372,036,854,775,808 to .....

byte 8 bits -128 to 127

Use int unless you have a really good reason not to!

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Floating-point types

Java provides two primitive types that represent real numbers Again they differ only in their size

(i.e., the range of values that they can store and their precision)

float 32 bits ± 3.40282347E+38 with 7 significant decimal digits

In general, use double rather than float

double 64 bits ± 1.79769313486231570E+308 with 15 significant decimal digits

Values of type float are followed by an ‘F’

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Assignment compatibility

The type of an expression must be the same as the type of the variable to which its value is being

assigned

“You can’t put a square peg in a round hole!”

© Vinny Cahill 1 Writing a Program in Java. © Vinny Cahill 2 The Hello World Program l Want to...

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