1. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

2. Lecture 2
Java

3. Java OO This is a more practical lecture than last time
This course exists for you to learn programming principles, not programming Java.
On the following slides, these symbols mean:
Principal OO Principal Java OO

4. From code to program You write code... You compile code
My Code x = 1 ;
(raining) if {
takeUmbrella() }
You write code...
You compile code
You run the program
My Code
My Program
Compiler
This is true for most, but not all languages. It isn’t true for php or other scripts, but it is similar
My Program Me
The Computer

5. From code to program You write code... You compile code
My Code x = 1 ;
(Southampton) if {
takeUmbrella() }
You write code...
You compile code
You run the program
My Code
My Program
Javac
My Program Me
The Computer
The JVM

6. Eeeeek! What is a JVM? The JVM Java Virtual Machine
Each operating system is different on a microcode level:
Different way of putting things on the screen
Different way of making sound
Different way of taking input from keyboard
Etc
So how can java work on all these platforms?

7. Why the JVM is good Lets say Java is this shape:
Windows is this shape:
Mac is this shape:
Unix is this shape:

8. Fitting

9. What shape is the JVM?
There is a version of the JVM for each of these operating systems
So the JVM is this shape:

10. Fitting

11. That is all you need to know about the JVM for this course Stop thinking about it

12. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

13. Structure class method statement method statement OO Source file Class
Source file – has .java extention
Every object is based on a class.
A method goes in a class, between the classes curly braces.
A statement goes in a method.
method
Statement
statement

14. What does Source File mean?OO
What does Source File mean?
Source file:
With text, ends in .txt
With Java, ends in .java
The source file is simply the file that contains the source(code) for your class
Source file – has .java extention
Every object is based on a class.
A method goes in a class, between the classes curly braces.
A statement goes in a method.

15. What does Class mean? public class Dog { }OO
What does Class mean?
A class is the blueprint for an object
public class Dog {
The opening brace
This is a class
Public so everyone can see it
We’ll come back to this later in the course
The name of the class
Source file – has .java extention
Every object is based on a class.
A method goes in a class, between the classes curly braces.
A statement goes in a method. }
The closing brace

16. What does Method mean? } Methods do things, they define behaviorOO
What does Method mean?
Methods do things, they define behavior
public void bark () {
The opening brace
This is the return type. Void means we are returning nothing
Public so everyone can see it
We’ll come back to this later in the course
The brackets, these sometimes contain arguments to the methods
Source file – has .java extention
Every object is based on a class.
A method goes in a class, between the classes curly braces.
A statement goes in a method.
The name of the method }
The closing brace

17. What does Statement mean?OO
What does Statement mean?
Statements are the code that do the operations
Print “Woof”;
Every statement ends with a ;
Sometimes you write looooooooooooooooong statements that break over lines. Semicolon tells the compiler where the line ends
This statement starts with a command, Print.
Print isn’t a valid command in Java, this is an pseudo code (pretend code) example
This is the string we are outputting.
Woof should appear on screen when we run this line in the program
Source file – has .java extention
Every object is based on a class.
A method goes in a class, between the classes curly braces.
A statement goes in a method.

18. ? Hang on a sec.... public void bark(){ Print “Woof”; }
The method says:
The “return type” is void, but it’s still printing something out, that can’t be right, can it?
It is right. Printing something to the screen is very different from returning something from a method. Good question though.
public void bark(){
Print “Woof”; }

19. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

20. Before we start to think about coding...

21. Magic Incantations Programming principles is a complex study
You start learning something...
and find out you need to know something else to understand it
and something else to understand
that
and something else to understand
that

22. Magic Incantations For example
We started looking at class structure...
And then mentioned visibility modifiers (the ‘public’ word in the class and method)
And then mentioned arguments
(The brackets at the end of the method)
And then touched on return types
And we still haven’t coded anything!

23. Magic Incantations i.e. You can start going nowhere fast
These slides will give you tools to write your first programs
Go with the flow
You may not understand the ‘incantations’
don’t let it bother you, trust that it works
You will understand later

24. Magic Incantation #1 Where does a program start?
At the top of the class?
What about if you have more than one class, a cat class, a dog class and a parrot class?
At the top of a specific class, the main class?
Still not specific enough, what if it gets accidently changed, or someone writes a method on top of your top method in the class?

25. Magic Incantation #1 Where does a program start? In a special method?
public static void main(String[] args){ }

26. A post it note? Make a dog object Tell it to bark
public static void main(String[] args){ } is a post-it note With instructions for how to set your program up
Make a dog object
Tell it to bark

27. Rules It doesn’t matter in which class you put the main method in
You can put it in its own one if you like
There must be 1 and only 1 in the whole program (program is a set of classes that work together)

28. Magic Incantation #2 How do I output things to the screen?
System.out.println(“Your text here”);

29. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

30. Write a Dog Using last lecture’s ideas Let’s write a dog Source file
Class
Method
Main Method
Declare a dog
Make a dog
Call the method

31. Run a Dog Save with same name as class (Rule: Must do this)
Command line to folder
Compile - javac Dog.java
Run – java Dog

32. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

33. First Building Block
If Statements

34. Making decisions
Often we want to make the result of a program conditional on something
If the bank account doesn’t exist
Don’t allow a withdrawal
Else, if the account doesn’t have enough money
Else
Allow the withdrawal
For this we need to cover variables in more depth

35. Variables -Just like algebra?
Storing values
In variables
Eg Dog
int age;
age = 3;
Name of variable – think about ‘x’ in algebra
Data type:
Can be class or primitive
Actual value to put into variable
More on variables next lecture

36. ? What kind of decisions Conditionals are based on TRUE or FALSE x=4;
a = false;
If(x==4){ } ?

37. Comparisons #1 < less than > greater than == equal to
Yes, that is two equals signs
x = 4 means ‘assign the value 4 to x’
x == 4 means ‘does x equal 4 ? Yes (true) or No (false)

38. What kind of decisions Conditionals are based on TRUE or FALSE x=4;
a = false;
If(x==4){
Print “Elephant!”; }
If(a){
Print “Pink Elephant”;

39. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

40. Back to Banking
public class Account{ int balance; //the bank balance int amount; //the amount to withdraw boolean active; // true if the account is active //some code omitted }

41. ? Withdrawal if(!active) } public class Account{
int balance; //the bank balance
int amount; //the amount to withdraw
boolean active; // true if the account is active
//some code omitted
public void withdrawal(){
if(!active) } ?

42. Comparisons #2 ! means NOT or negate
(x!=4) means ‘TRUE if x is not equal to 4’
&& means AND
(x==4 && y==2) means:
TRUE if x equals 4 AND y equals 2
|| means OR
(x==4 || y==2)
TRUE if either x equals 4 OR y equals 2 (or both)

43. Withdrawal if(!active){
public class Account{
int balance; //the bank balance
int amount; //the amount to withdraw
boolean active; // true if the account is active
active = true; //set active to true
//some code omitted
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”); }

44. Withdrawal if(!active){
public class Account{
int balance; //the bank balance
int amount; //the amount to withdraw
boolean active; // true if the account is active
active = true; //set active to true
//some code omitted
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”);
}else if(amount>balance){
System.out.println(“Not enough money”); }

45. Withdrawal if(!active){
public class Account{
int balance; //the bank balance
int amount; //the amount to withdraw
boolean active; // true if the account is active
active = true; //set active to true
//some code omitted
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”);
}else if(amount>balance){
System.out.println(“Not enough money”);
}else{
balance = balance – amount; }

46. The flow of if
active = false
The program will only flow into one ‘slot’. Then it goes to the end
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”);
}else if(amount>balance){
System.out.println(“Not enough money”);
}else{
balance = balance – amount; }

47. The flow of if active = TRUE && amount > balance
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”);
}else if(amount>balance){
System.out.println(“Not enough money”);
}else{
balance = balance – amount; }

48. The flow of if active = TRUE && public void withdrawal{ if(!active){
amount <= balance
public void withdrawal{
if(!active){
System.out.println(“Your account isn’t active”);
}else if(amount>balance){
System.out.println(“Not enough money”);
}else{
balance = balance – amount; }

49. Content Programming Overview The JVM A brief look at Structure
Class
Method
Statement
Magic incantations
main()
output
Coding a Dog
Programming Principle(1)
If and Boolean operations
Coding a Bank Account
Quick look at ToolBox

50. A quick word about toolbox
Getting input and output can be a pain in java.
So can some other things
ECS have provided a ‘toolbox’ for you to use.
This toolbox is a class of useful methods
Like readStringfromCmd()

51. Getting input String theword; //or whatever name you choose
theword = Toolbox.readStringFromCmd();
Entering this in you code means when the program gets to that line
The command prompt will ask you for a string
You enter the string and press enter
The program keeps going, with the string you typed stored in the variable

52. Toolbox
Has some other useful functions, we’ll introduce them as we need them
But you don’t have to use them if you know what to do already.
Have fun in the lab!