1. Some basic I/O

2. Example of Output Stream
Our first program involved the output stream
// First Program HelloWorld
public class HelloWorld {
public static void main(String args []){
System.out.println(“Hello world...”): } }

3. System.out.println(“Hello world...”):
System.out is called the standard output object
This will display a line of text in the command window
In Java , any source and destination for I/O is considered a stream of bytes or characters.
To perform output we insert bytes or characters into a stream. To perform input we extract bytes or characters from a stream.
java.lang.System class contains three predefined streams
System.out
System.err for errors
System.in

4. Consider
System.in

5. System.in Standard input is this stream
Some books refer to the input method
System.in.readln();

6. Problem System.in.readln() doesn’t seem to work with system here
Solution import the scanner class

7. Consider the following program
// Second Program HelloWorld
import java.util.Scanner;
public class HelloWorld3 {
public static void main(String args []){
Scanner input = new Scanner( System.in );
String theName = input.nextLine();
System.out.println(“input was“ + theNAme);}}

8. It contains the lines // Second Program HelloWorld
import java.util.Scanner;
This imports an input Utility class called Scanner
Scanner input = new Scanner( System.in );
This creates an instance called input of the scanner utility

9. String theName = input.nextLine();
This calls a method of the class called nextLine()
This gets input from the screen
It binds this to a variable called theName which is declared to be a string type by the String Keyword

10. So now we have a basic input method which seems to work
Next we need to consider the basic components of programs in order to start building more complex examples.
We will start with basic syntax and operators

11. Primitive data types Operators Control statements
Java Syntax
Primitive data types
Operators
Control statements

12. Primitive data types
Identical across all computer platforms  portable

13. Primitive data types char (16 bits) a Unicode character byte (8 bits)
int (32 bits) a signed integer
short (16 bits) a short integer
long (64 bits) a long integer

14. Primitive data types float (32 bits) a real number
double (64 bits) a large real number
boolean (8 bits)
values are true or false (keywords)
cannot be converted to and from other data types e.g. while (i!=0) not while(i)

15. Primitive Data Types

16. Operators Addition Subtraction + -
+ -
Increment operators (postfix and prefix)
Multiplication Division Modulus * / %
Equality == !=
Assignment operators = += -= *= /= %=

17. Arithmetic Operators

18. Back to our simple input program
Remember the last night we had a program to input an integer, manipulate it and output it.

19. We used input.nextLine(); Returns a String
It would need to be converted if we are to use this string as an integer using the following parseInt method

20. Parsing Strings for integers
Integer.parseInt(String s);
The parseInt() function parses a string and returns an integer.

21. Using our Scanner input method
// Second Program HelloWorld
import java.util.Scanner;
public class HelloWorld4 {
public static void main(String args []){
Scanner input = new Scanner( System.in );
String theName = input.nextLine();
int num1 = Integer.parseInt(theName);
num1 = num1 + 1;
System.out.println("result" + num1);}}

22. Some anomalies
Note the String declaration starts with an Uppercase Letter
And the int declaration starts with a lowercase letter

23. String Contatenation anomaly
Note we use the arithmetic operator + to increment num1 by 1
We use the same notation + to concatenate the result num1 to the string result an example of operator overloading which in turn is part of polymorphism

24. Use of parseInt int num1 = Integer.parseInt(theName);
We use the method parseInt on the string theName to produce our integer num1

25. Exercise
Write code to input 2 integers , add them together and output the result.

26. Towards more complicated programming
Next Conditionals and Loops
Now all Conditionals and Loops are characterised by Conditions (called guards in the case of Loops)
Conditions are boolean expressions largely based on relational operators
Next we look at these operators

27. Operators Relational operators < <= > >=
Conditional operator ?:

28. Logical Operators Not ! Logical AND && Logical OR ||
Boolean logical AND &
Boolean logical inclusive OR |
Boolean logical exclusive OR ^ (true if only one operand is true)

29. Relational Operators

30. If Statements Similar to C/C++ syntax: if statement
if (x != n){ … } else if { … } else { … }

31. Example of Classic Conditional Program
Consider our Leap Year program

32. // Second Program HelloWorld
import java.util.Scanner;
public class Leapyear {
public static void main(String args []){
Scanner input = new Scanner( System.in );
String theName = input.nextLine();
int num1 = Integer.parseInt(theName);
if(num1 % 4 ==0 && (num1 % 100 != 0 ||num1 % 400 == 0))
System.out.println("leap");
else
System.out.println("not leap"); }}

33. Exercises Write javaclasses which will
1: Input two Integers and output the largest
2 :Input 3 sides of a triangle and determine whether the triangle is scalene, isoceles or equilateral.
3 Input a month and a year and calculate how many days are in that month

34. Exercises continued Remember
30 days hath September , April , June and November,
All the Rest have 31,
Except February which has 28,
save for one year in four
When it has one day more

35. More Exercises
An insurance company offers different classes of insurance based on age
Over 20 and less than 40 is Category A
Between 40 and 60 is Category B
Over 60 is Category C
Write a program to accept in someones age and tell them the class of insurance they are entitled to.

36. Control Statements switch statement
switch (n){ case 1: … break; case 2: case 3: … break; default: break; };

37. Loop Statements for statement while statement do statement
for (int i=0; i<max; i++){ … };
while statement
while (x==n ){ … };
do statement
do { … } while( x<=y && x!=0);

38. General Points to Note Case sensitive Use lower case
objects have capitalised first letter

39. Reserved Words