Understanding Java Variables: A Tale of 4 Variables

1. intX intY char c double d A Tale of 4 Variables

2. Once upon a time… • There was a program • And in that program there were methods • A click method • And the • main method public void click() public static void main()

3. intX intY And in the methods… There were variables • And the variables were declared • Creating space in memory to hold information public void click() { int intX; int intY; This is a declaration -> • But the variables were empty • There was nobody home.

4. And different kinds of variables hold different kinds of information • Are you happy? • That’s true or false • boolean • What’s your middle initial? • That’s a single letter • char • How far away do you live? • That’s a number, it could include a fraction or decimal • double These are different types of variables

5. intX intY And the assignment statementwas executed • And it gave values to the variables • intX = 8; • intY = 5; 8 5 These are assignment statements

6. Just be sure to declare the right kind of variable to hold the right kind of information • Which ones are correct types? • int distance = 45.5; • boolean happy = false; • char initial = “Tom”; • double distance = “far away”; • char letterGrade = ‘A’; • boolean b = “true”; This is also called a floating point value, it has a decimal point. An integer does not. These assignment statements initialize and declare the variables

7. Of course, assignment is not the same as equals No number can equal itself plus 3 • intX = intX + 3; • The algebra teacher said • The computer teacher said = means Always do the equation on the right first, then assign the result to the variable on the left

8. intX intX This is a literal value, it’s the same every time the program runs, intX is a variable, it can change So… intX = intX + 3 • Means… • First take the value of intX • Add 3 • Store the result in intX 8 8 + 3 11 =11

9. And the students were happy • Because they understood

10. intX intX intY And the students could assign values to variables • They knew that after the assignment statement • intX = intY + intX * 2 • intX = 5 11 27

11. intY So whenever you want to remember information in a program • Or if you want to leave a fill-in-the-blank space to be filled in later (like a Mad Lib) • Use a variable to store the information 5

12. Rules for making up your own variable names: • Must start with a letter • Can use numbers, letters and underscores (_) • Cannot use a work that already has a special meaning in Java like: • main • void • system

13. length println happyCamper best friend ready? good4u 2many tax_amount total! b45 Which identifiers do NOT follow the rules for Java variables? Identifiers must follow the rules