Variables and Data Types

What is a variable?: 1.3.1

• A variable is a name associated with a memory location
  • Stores a value which can vary

How to create a variable in Java

1. Identify the type
   • 3 Main types you need to know for the CSA Exam
   1. int
      • Whole number
   2. double
      • Decimal value
   3. boolean
      • true or false
2. identify the name
3. identify the initial value
   • You don’t have to do this in all cases

int myVariable = 0;  // Declaring a variable without a value
boolean foo; // Declaring a variable without a value
foo = true; // Assigning a value to an existing variable

Data Types: 1.3.2

• Two types of variables in Java
  • Primitive variables
  • Object or reference variables
    • Hold a reference to an object of a class
      • A reference is a way to find the object
• 3 Primitive types are on the AP CSA Exam
  1. int
     • Stores integers (whole numbers)
     • stores values from -2³¹ to 2³¹-1 (32 bit)
  2. double
     • stores floating point numbers (decimal numbers)
     • 64 bit
  3. boolean
     • Stores boolean values (true or false)
• String is an object type on the exam
  • The name of a class
  • A sequence of characters enclosed in double quotes
    • “Hi there!! :)”
• A type is a set of values (a domain; think math class!!) and a set of operations which can be done on them
  • You can add together integers, but can’t add booleans

Declaring variables: 1.3.3

• To create a variable, Java needs to know the data type and the name
  • called “declaring a variable”
  • Type is a keyword like int, double, or boolean
  • You make up a name
  • When you declare a primitive variable…
    • Java allocates (sets aside) enough bits of memory for that type
    • Associates the memory address with the name you chose
• Computers store all values as bits (binary digits)
  • Each bit has two values
    • Conventionally called either 1 or 0
  • Each type requires a different number of bits, and each type gets represented differently
    • integer has 32 bits of memory
    • double has 64 bits of memory
    • boolean only needs 1 bit of memory

How to declare a variable

1. Specify the type
2. Specify the name

int score;
double health;
boolean isRunning;

After a variable has been declared, it can be assigned a value using =

double health;
health = 0.88;

An initial value can also be set during declaration

double health = 0.88;

How to print a variable

• Use the string concatenation operator +
• You can’t put a variable inside quotes
  • Will just print the variable letter

String myName = "Nathan";
System.out.println("myName"); //Outputs: myName
System.out.println(myName); //Outputs: Nathan

Assignment Operator

• In Java, = is the assignment operator
  • Doesn’t mean both sides are equal
  • Sets the value on the left with a copy of the value on the right

Constants

• The keyword final can be used in front of a variable declaration to make a value constant
  • It cannot be reassigned
  • These variable names are conventionally capitalized

final double STANDARD_GRAVITY = 9.81; // m/s^2

Naming Variables: 1.3.4

• Variables should start with an alphabetic character
• Can include letters, numbers, and underscores
• Must be all 1 word (no spaces)
• Cannot be a keyword/reserved word
• The variable should be meaningful
  • When keeping track of a score, score makes more sense than x
• Avoid really long names
• Use camelCase
  • This is convention in Java
  • first word is lower case, and every word after has the first letter capitalized
  • Looks like camel humps :)
• Can also use underscore to seperate words
  • Can’t use spaces
• Variables are case sensitive!
  • myvar is not the same as myVar

Summary: 1.3.6

• A variable is a name for a memory location where you can store a value
• A variable is declared like so:

  int score;
  

• And initialized:

  double gpa = 3.5;
  

• Data types can be primitive (int/boolean/double) or reference types (String, etc.)
• Variables have associated memory locations which hold their values
  • Memory associated with primitive types hold actual values
• When a variable is declared with final, it is constant and cannot be changed

AP Practice: 1.3.7

Nothing to note here, just complete the task.

Closing thoughts

Overall I think the section was a good introduction to variables and data types. In particular, I really liked the comparison of calling the possible set of values of a type the domain, because I think it creates a valid link between Mathematics and Computer Science. It definitely wasn’t an important part of the lesson, I just have always found that link to be cool :). In spite of that, I don’t know why every other primitive type was ignored completely. While I understand that AP doesn’t require a student to know about them, I find it odd that they weren’t at least acknowledged as basic features of the language.