course overview: introduction to programming concepts · java was invented in 1995, originally for...

Course overview: Introduction to programmingconcepts

• What is a program?

• The Java programming language

• First steps in programming

• Program statements and data

• Designing programs.

This part will give an introduction to general programming

concepts, and to the Java programming language.

1

What is a program?

• A program is a set of instructions which control a computer

(laptop, desktop, tablet, etc)

• Programs can be written for huge variety of tasks: performing

complex computations; financial trading; computer graphics

and games; medical diagnosis and data processing; aircraft

autopilot, etc

• Programs can read data from computer keyboard, mouse

movements and actions, from data files, databases and any

other sensors/data sources on the computer – and from

internet if connected.

• Programs can present information graphically on computer

screen, can write to text files, or update any other device

connected to computer – if permitted to do so.

2

What is a program?

Example of simple program:

read a number X;

read a number Y;

calculate Z = (X + Y) divided by 2;

display Z;

This computes average of two numbers, eg: for X = 203, Y = 1965,

displays 1084.

3

What is a program?

• Another name for programs is software – as opposed to

hardware, the physical computer and devices.

• Programs are written in text files in the format of some

programming language

• The most widely-used programming languages are C, C++,

C# and Java.

• We’ll use Java, as it’s simplest of the popular languages.

• Java was invented in 1995, originally for web browsers and

internet software (eg., to animate web pages). Now in general

use.

4

The Java programming language

• We write Java programs in text files (eg., using WordPad or

JCreator), with .java file extension, eg.: Program1.java

public class Program1

{

public static void main(String[] args)

{ int X = 203;

int Y = 1965;

int Z = (X + Y)/2;

System.out.println("The answer is: " + Z);

}

}

Here, three integer-valued variables X, Y, Z are declared. X and Y

are given values 203 and 1965, then Z is calculated from them, and

then displayed.

5

Running Program1.java

6

First steps in programming

• Open the Windows console (Start; All Programs; Accessories;

Command Prompt)

• In the Windows console, cd to the directory cllexamples where

Program1.java is (on memory stick)

• Compile the program with javac, and run it with java:

javac Program1.java

java Program1

Alternatively, open Program1.java with JCreator, or use

https://ideone.com

7

Data in computer

8


• A Java program in a file Name.java can have the text

public class Name

{


{

// Your program code goes here

}

}

The text between the inner {} brackets can change for different

programs.

• Give programs meaningful names: Average.java would be

better name for our first program.

• Program statements: individual instructions and steps the

computer should take.

9

Eg.: int X = 203; “Introduce an integer variable called X,

and assign the value 203 to it”.

int Z = (X + Y)/2; “Introduce an integer variable called Z,

and assign (X + Y) divided by 2 to it”.

Statements end with a ;

Try changing the values assigned to X, Y, re-compile and re-run.

10


• Of course, a more useful program is one that can read inputs

from user:

import javax.swing.JOptionPane; // dialog support

public class Average

{


{ String xvalue =

JOptionPane.showInputDialog(

"Enter 1st integer value:");

int X = Integer.parseInt(xvalue);

String yvalue =


"Enter 2nd integer value:");

int Y = Integer.parseInt(yvalue);

11

int Z = (X + Y)/2;

System.out.println("Their average is: " + Z);

}

}

The dialogs prompt user for the X, Y values.

Input from user is stored in String variables xvalue, yvalue – these

store pieces of text.

Then converted to numbers by Integer.parseInt. Eg., string “334”

is converted to number 334.

12

Average.java input dialog

13


• If we enter some wrong data (not integers), our program

crashes – a professional program should be robust & continue

despite errors

• Dialogs can be used to show results:

JOptionPane.showMessageDialog(null, "The result is:

" + Z);

• The User Interface of a program consists of the visual

components used to exchange data between program and users

– in this case the input/output dialogs. Also called Graphical

User Interface (GUI).

14


Alternatively, read values from console:

import java.util.Scanner; // input support

class AverageConsole

{ public static void main(String[] args)

{ Scanner input = new Scanner(System.in);

System.out.println("Enter 1st integer value:");

int X = input.nextInt();

System.out.println("Enter 2nd integer value:");

int Y = input.nextInt();

int Z = (X + Y)/2;

System.out.println("Their average is: " + Z);

}

}

15


Average2.java:


public class Average2

{


{ String xvalue =




String yvalue =




int Z = (X + Y)/2;

16

JOptionPane.showMessageDialog(null,

"Their average is: " + Z);

}

}

Try changing program to calculate average of three input values.

17

Output from Average2.java

18

Program Statements

• Programs consist of program statements – individual processing

instructions, operating on variables

• Assignment statements assign a value to a variable:

X = 530;

Can also declare/introduce the variable:

int X = 530;

• Conditional statements enable decisions to be made: if a

condition is true, one behaviour is executed; if condition is false

another behaviour is executed.

19

Max.java:


public class Max

{


{ String xvalue =




String yvalue =




int Z;

if (X < Y)

20

{ Z = Y; }

else

{ Z = X; }


"The largest value is: " + Z);

}

}

21

The statement

if (X < Y)

{ Z = Y; }

else

{ Z = X; }

tests the condition X < Y, (‘is X less than Y?’), if this is true then

the statement Z = Y; is executed, otherwise (if X equals Y or is

larger than Y), the statement Z = X is executed.

Effect is to always set Z to be the larger of X and Y.

22

Console version MaxConsole.java:

import java.util.Scanner; // input support

public class MaxConsole

{


{ Scanner input = new Scanner(System.in);

System.out.println("Enter 1st integer value:");

int X = input.nextInt();

System.out.println("Enter 2nd integer value:");

int Y = input.nextInt();

int Z;

if (X < Y)

{ Z = Y; }

else

{ Z = X; }

23

System.out.println("The largest value is: " + Z);

}

}

24

Conditions can be as complex as needed, and multiple conditions

can be tested:


public class StudentMarks

{


{ String markvalue =


"Enter the student’s mark:");

int mark = Integer.parseInt(markvalue);

String result;

if (mark < 40)

{ result = "Fail"; }

else if (mark >= 40 && mark < 50)

{ result = "Pass -- grade D"; }

25


{ result = "Pass -- grade C"; }


{ result = "Pass -- grade B"; }

else

{ result = "Pass -- grade A"; }


"The student’s result is: " + result);

}

}

The symbols && mean “and”.

result is a String variable – it stores a piece of text.

26

Conditional Statements

Examples of different cases:

Input Output

10 “Fail”

42 “Pass – grade D”

55 “Pass – grade C”

67 “Pass – grade B”

70 “Pass – grade A”

Try executing StudentMarks with other example input values.

27

Loop Statements

• Loop statements repeat some instructions over & over again

until a task is completed

• for statements loop for a fixed number of times:

for (int i = a; i <= b; i++)

{ statements }

means ‘execute the statements with i = a, then with i = a+1, i

= a+2, ..., then with i = b’

• If a > b, does nothing

• If a equals b, just one iteration, for x = a.

28

Loop Statements


public class SumNumbers

{


{ String nvalue =


"Enter the number to sum:");

int n = Integer.parseInt(nvalue);

int sum = 0;

for (int i = 1; i <= n; i++)

{ sum = sum + i; }


"The sum is: " + sum);

}

29

}

calculates sum of numbers from 1 to n.

Results for different input values:

30

n sum

0 0

1 1

2 3

3 6

4 10

5 15

6 21

7 28

8 36

9 45

10 55

31

Graphics/drawing example:

import javax.swing.JFrame;

import java.awt.Graphics;

import java.awt.Color;

import javax.swing.JPanel; // drawing support

public class Drawings extends JPanel

{ public void paintComponent(Graphics g)

{ super.paintComponent(g);

for (int i = 0; i < 10; i++)

{ if (i % 2 == 0)

{ g.setColor(Color.PINK);

g.drawOval(10 + i*10, 10 + i*10,

50 + i*10, 50 + i*10);

}

else

32

{ g.setColor(Color.GREEN);

g.drawRect(10 + i*10, 10 + i*10,

50 + i*10, 50 + i*10);

}

}

}


{ JFrame app = new JFrame();

app.add(new Drawings());

app.setSize(300, 300);

app.setVisible(true);

}

}

drawOval(x , y ,X ,Y ) draws an oval at x, y position, of size X, Y.

33

Output from Drawings.java

34

Drawing example

Notice that y=0 is at top: y increases from top to bottom, x

increases from left to right.

The condition

if (i % 2 == 0)

is true if i is an even integer (0, 2, 4, 8) – in this case a pink oval is

drawn at position (10 + i ∗ 10, 10 + i ∗ 10).

Condition is false if i is odd (1, 3, 5, 7, 9) – in this case a green

rectangle is drawn at (10 + i ∗ 10, 10 + i ∗ 10).

Try changing the positions & sizes to see the effect.

35

Graphics/drawing example:





public class Drawings2 extends JPanel



for (int i = 0; i < 10; i++)

{ if (i % 2 == 0)

{ g.setColor(Color.PINK);

g.fillOval(10 + i*10, 10 + i*10,

50 + i*10, 50 + i*10);

}

else

36

{ g.setColor(Color.GREEN);

g.fillRect(10 + i*10, 10 + i*10,

50 + i*10, 50 + i*10);

}

}

}



app.add(new Drawings2());



}

}

fillOval(x , y ,X ,Y ) draws a filled oval at x, y position, with size X,

Y.

37

Output from Drawings2.java

38

Drawing spirals:





public class Drawings3 extends JPanel



int centerX = getWidth()/2;

int centerY = getHeight()/2;

int radius = 20;

int w = 2*radius;

int h = 2*radius;

g.drawArc(centerX, centerY, w, h, 0, 180);

int radius2 = 30;

39

g.drawArc(centerX - 20, centerY - 10,

2*radius2, 2*radius2, 0, -180);

int radius3 = 40;


2*radius3, 2*radius3, 0, 180);

int radius4 = 50;


2*radius4, 2*radius4, 0, -180);

int radius5 = 50;


2*radius5, 2*radius5, 0, 180);

}


40


app.add(new Drawings3());



}

}

drawArc(x , y ,w , h, 0, 180) draws an anticlockwise semicircle within

the box from x, y position to x+w, y+h.

drawArc(x , y ,w , h, 0,−180) draws a clockwise semicircle.

41

Output from Drawings3.java

42

Summary so far

• Simple program structure: public class Program { ... }

• Variables of integer type (int x;) – these can store values of

integers, eg.: -214, 0, 1, 55, 1000, 500000, etc.

• Variables of String type (String s;) – these can store

strings/text, eg.: “Result”, “Enter a value”

• Assignment statements var = value;

• Conditional statements if (Condition) { statement1 }else { statement2 }

• Bounded loops: for (int var = a; condition; var++) {statement }

• Simple graphics code.

43

More advanced Java

• double variables (rational numbers)

• Unbounded loops (while statements)

• What is programming?

• Multiple operations; recursion

• Multiple classes in a program

• Array variables.

44

Calculations with rational numbers

• double variables can store fractional values: 0.5, 2.25, 0.0001,

etc.

• Java provides functions for square root: Math.sqrt(d), x to

power y, etc: Math.pow(x,y)

• These functions return double values in general – but integers

can be used as doubles (1.0, -3.0, etc).

Example: calculate total amount earned if invest deposit (eg, £100)

for n years at interest rate rate (eg. 5%, or 0.05).

45

Calculating compound interest

deposit grows with interest to: total = deposit ∗ (1 + rate)n after n

years at rate rate.

public class Investment

{


{ double amount = 100.0; // total money

double deposit = 100.0; // original investment

double rate = 0.05; // interest rate

for (int year = 1; year <= 10; year++)

{ amount = deposit * Math.pow(1.0 + rate, year);

System.out.println(

"After " + year + " years, total is: " + amount);

}

}

46

}

Amounts after n years:

n amount

1 105.0

2 110.25

3 115.763

4 121.551

5 127.628

6 134.01

7 140.71

8 147.746

9 155.133

10 162.889

47

Calculating compound interest

How many years does it take for investment to double?

Need while loop:

public class Investment2

{


{ double amount = 100.0; // total money

double deposit = 100.0; // original investment

double rate = 0.05; // interest rate

int year = 1;

while (amount < 2*deposit)

{ amount = deposit * Math.pow(1.0 + rate, year);

System.out.println(

"After " + year + " years, total is: " + amount);

48

year = year + 1;

}

}

}

A while (E) { C } loop repeats C until E is false – but may run

forever!

Here, termination when year = 15.

Try changing the rate and see the effect on the result.

49

What is Programming?

• Given: a problem (“draw a spiral”, “find how many years it

takes to double an investment at 5% interest”)

• First: idea for solution

• Second: plan an algorithm to carry out the solution

• Third: code up algorithm in a programming language (Java)

• Fourth: test/correct your code.

50


• Problem to “draw a spiral”

• Idea for solution: draw semi-circles of increasing radius, joined

end-to-end

• Algorithm: draw semi-circle 1 with radius 20, centered on

panel center X, Y; draw semi-circle 2 with radius 30, displaced

to X-20, Y-10, etc.

• Code in Java: use drawArc to draw the semi-circles

• Test/correct the code.

Algorithms can be expressed in words before coding.

51


• Problem to “find number of years until investment doubles”

• Idea for solution: calculate total amount earned for successive

years, stopping when this amount is at least twice the original

deposit

• Algorithm: compute total earned to year by

amount = deposit ∗ (1.0 + rate)year

for year = 1, 2, 3, ... stopping when amount ≥ 2 ∗ deposit

• Code in Java: use while loop to repeat calculation until the

stopping condition is true

• Test with different rates and deposits.

52

Reading data from files

• Programs can read data from files (usually, text files)

• Eg.: to read student marks and classify these into grades

• Idea is that program reads lines of text from file (should be

numbers), and converts each to a grade, until end of file (eof) is

reached.

53

import java.io.*;

public class StudentMarksFile

{


{ File file = new File("marks.txt"); /* default input file */

BufferedReader br;

String s = "";

boolean eof = false;

try

{ br = new BufferedReader(new FileReader(file)); }

catch (FileNotFoundException _e)

{ System.out.println("File not found: " + file);

return;

}

while (!eof)

54

{ try { s = br.readLine(); }

catch (IOException _ex)

{ System.out.println("Reading failed.");

return;

}

if (s == null) // the file has ended

{ eof = true;

break;

}

int mark = Integer.parseInt(s);

String result;

if (mark < 0 && mark > 100)

{ result = "Invalid mark " + mark; }

else if (mark < 40)

{ result = "Fail"; }

else if (mark < 50)

55

{ result = "Pass -- grade D"; }

else if (mark < 60)

{ result = "Pass -- grade C"; }

else if (mark < 70)

{ result = "Pass -- grade B"; }

else

{ result = "Pass -- grade A"; }

System.out.println(

"The student’s result is: " + result);

} // ends the while loop over file

}

}

56

Programs with multiple operations/classes

• So far we’ve written all code in main method of one class. This

is method where execution starts.

• Also possible to write several operations in one class, called

from main method: factorial example.

• For larger programs, best to create separate classes + call their

code from main method.

• We’ll show how to do this with lottery example.

57

Calculating factorials

For integer n > 0, its factorial is product: n ∗ (n − 1) ∗ ... ∗ 1.

public class Factorial

{ public static int fact(int n)

{ if (n <= 1) { return 1; }

else

{ return n*fact(n-1); }

}


{ for (int n = 1; n <= 14; n++)

{ System.out.println(

"Factorial " + n + "! = " + fact(n));

}

}

}

58

Recursion is another way of looping: fact(5) calls fact(4), which

calls fact(3), etc. return e; ends the call and returns value of e to

the caller.

59

Calculating factorials

Notice that factorials for n > 12 don’t appear correct: the int type

is too small to represent them, need to use long integer type

instead:

public class Factorial

{ public static long fact(int n)

{ if (n <= 1) { return 1; }

else

{ return n*fact(n-1); }

}


{ for (int n = 1; n <= 21; n++)

{ System.out.println(

"Factorial " + n + "! = " + fact(n));

}

60

}

}

All data on a computer is finite, int variables can hold numbers up

to 2,147,483,647.

61

Programs with multiple classes

• Problem to “Carry out lottery with three numbers in range 1

to 30 for user to guess with five guesses”

• Idea for solution: generate random numbers, ask user to guess

these, and count correct guesses

• Algorithm:

Generate 3 random integers in 1..30;

Ask user for 5 guesses;

Count and display the correct guesses

• Code in Java: use Random class, and nextInt(30) + 1 to

generate the random numbers

• Test with different cases of correct/incorrect guesses.

62

import java.util.Random; // to generate random numbers


public class Lottery


{ Random rand = new Random();

int ball1 = 1 + rand.nextInt(30); // first lottery number

int ball2 = 1 + rand.nextInt(30); // second lottery number

int ball3 = 1 + rand.nextInt(30); // third lottery number

int correct = 0;

for (int i = 1; i <= 5; i++)

{ String sguess = JOptionPane.showInputDialog("Enter your guess:");

int guess = Integer.parseInt(sguess);

if (guess == ball1)

{ System.out.println("Correct guess of ball 1");

correct++;

63

}

else if (guess == ball2)


correct++;

}

else if (guess == ball3)


correct++;

}

else

{ System.out.println("Sorry, wrong guess!"); }

}

System.out.println("You guessed " + correct + " correct");

}

}

Random is a built-in class, rand is an instance of Random which we

use in our program.

64

Random rand = new Random();

declares rand as variable storing an instance of Random, and

assigns a new instance of Random to rand .

rand .nextInt(30) generates a random integer in the range 0 to 29.

65

Array variables

• So far, we’ve seen variables that store single items – an

individual integer, rational, string, or class instance

• Sometimes useful to have one variable storing group of items –

such as the 3 balls in lottery game

• Array variables store group of items of same kind. Eg.:

int[] balls = new int[3];

declares balls as an array of 3 integers.

• Individual items are written as balls[0], balls[1], balls[2].

66

Data in array variables

67

Lottery with array

import java.util.Random; // to generate random numbers


public class Lottery2


{ Random rand = new Random();

int[] balls = new int[3]; // 3 lottery balls

for (int i = 0; i < 3; i++)

{ balls[i] = 1 + rand.nextInt(30); }

int correct = 0;

for (int j = 1; j <= 5; j++)

{ String sguess = JOptionPane.showInputDialog("Enter your guess:");

int guess = Integer.parseInt(sguess);

68

for (int k = 0; k < 3; k++)

{ if (guess == balls[k])

{ System.out.println("Correct guess of ball " + (k+1));

correct++;

} // end of if

} // end of inner for-loop

} // end of outer for-loop

System.out.println("You guessed " + correct + " correct");

}

}

A loop of form

for (int i = 0; i < n; i++)

{ ... code for array[i] ... }

is often used to process array of size n. Notice that numbering starts

from 0!

Array lottery version has fewer lines of code. Also, easy to change

number of balls.

69

Summary

• Program structure, class definitions

• Variables of integer, rational, string, class instance types

• Array variables

• Assignment, conditional, loop statements

• Simple graphics code.

70

Further resources

We hope you have enjoyed this course. The following are useful for

further study:

• Java JDK can be downloaded from:

oracle.com/technetwork/java/javase/downloads/ Choose

the JDK option, this will provide the javac compiler and java

execution tool.

• JCreator is a visual tool for creating and running Java

programs, download from jcreator.com

• Java library information is here:

https://docs.oracle.com/javase/7/docs/api/

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course overview: introduction to programming concepts · java was invented in 1995, originally for...

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