JAVA

1. JAVA Variables Declaring Variables Over writing Variables Data types Displaying contents of a Variable Naming a Variable Strings

2. What is a Variable? • When a Java program is running, it would need memory storage to store values • RAM is used to store these temporary values • These small memory locations are called variables.

3. How do Variables work? • As programmers we can create variables that have a particular data type to store values • All the different variables have to have unique names • If we imagine the RAM as a set of pigeon holes, to put a pigeon in its pigeon hole we would need to give the hole a name and then the pigeon/data which it can home must be set/declared.

4. Example • Lets say we need to store three pigeons we need to create two pigeon holes (variables) called P1which is set to hole Billy, P2 set to hold Henry and P3 to hold Milly . Billy Henry Milly NOTE: A variable can only hold one value, you cannot have two pigeons in one pigeon hole P3 P2 P1

5. Declaring a Variable • Before using a we have to declare/set it • When declaring a variable we must include two aspects; • Variable name • Variable data type typevariable_name;

6. Example • Lets say we want to declare to variables to hole numbers N2 N1 NOTE: Type int can only hold integers. We will learn different data types soon. intN1; intN2;

7. class VariablesExample { public static void main (String args[]){ //variables are declared intN1; intN2; int tot; //assign values to variables N1 = 50; N2 = 13; //the total of variables N1 and N2 //is stored in tot tot = N1 + N2; } } The memory locations for three variables are created. These can only store whole numbers. Numbers are stored in the variables Instead of storing a particular value in tot, the sum of N1 and N2 is stored. Copy this program

8. What we have … N1 N2 tot

9. Overtyping • If we take the example pf before where we have the following variables • N1 = 50 • N2 = 13 • If we change N1 to hold the number 33 (N1 = 33) then the integer 50 will be overwritten but 33 and the 50 will be lost

10. Direct initialisation • We can decide to set a value to the variables as soon as we create them • This is call Direct initialisation • The program would work the same it would just be shorter

11. class VariablesExample { public static void main (String args[]){ //variables are declared are assigned intN1 = 50; intN2 = 13; inttot; //the total of variables N1 and N2 //is stored in tot tot = N1 + N2; } } N1 and N2 are declared as int and assigned a value immediately. Copy this program

12. Data Types – Integers • The following are examples of integer data types you need to know

13. Data Types – Real (numbers) • The following are examples of real data types you need to know

14. Data Types – • The following are examples of some other data types you need to know

15. Important information • When assigning a value to a character variable, the character to be assigned must be enclosed in single quotes. class CharEx { public static void main (String args[]){ char label; label = ‘B’; } } Copy this program

16. Important information • When assigning a value to a boolean variable, we can only store true or false; these are not to be placed in single quotes. class BooleanEx { public static void main (String args[]){ boolean canDrive; canDrive=false; } } Copy this program

17. Displaying Contents of a Variable • We print the contents of a variable using System.out.ptintln() • In order to print the contents of a variable it is important that the variable name is not placed inside quotes • If we put the variable name in quotes the name of the variable will appear System.out.println(variable_name)

18. Example char letter; letter = ‘C’; System.out.println(“letter”); Output = letter char letter; letter = ‘C’; System.out.println(letter); Output = C

19. Activity • Go back to the programs we have writing and output the contents of the variables

20. Giving Variables a Name These are some of the things to remember: • It must start with a lowercase letter, • It cannot be a keyword used in Java, • It must be unique, • It cannot contain blank spaces (use CamelCase) • It must reflect the contents that it will store

21. Strings • In some programs we would want to store text • The char data type cannot be used since it can only store one character • We use what is called a String which stores characters, numbers, a mixture of both and also symbols

22. Example • Lets say we want to store out name • To output a string we use System.out.print() • To combine a String with more text, the + operator is used String fullName = “Ruby Borg”; System.out.println(fullName); System.out.println (“The next person is “+fullName);

23. Activity • Create a program to hold and output your name and class. • Out put should be as follows My name is Ruth Borg I am in form 4 One