Variables and Data Types: Revision Notes (Computer Science A 1.2)

Short on time? This page is the recap. For explanations and worked examples, use the full study guide.

Recap

A data type is a set of values and a set of operations on those values. Types are primitive or reference.
The primitive types used in this course are int (an integer), double (a real number) and boolean (true or false).
A reference type defines objects. String is a reference type: a String variable holds a reference to a String object.
A variable is a storage location with a name, a data type and a value. The value can change while the program runs; the type cannot.
A variable of a primitive type holds a primitive value of that type.
The compiler checks that the value stored in a variable is of a compatible type.

Idea	How to use it
Declare	`int count;` gives the type and name
Declare and initialize	`int count = 12;`
Change the value later	`count = 20;` (no type on the left)
int in a double variable	allowed: `double rate = 4;` stores `4.0`
double in an int variable	not allowed: `int size = 3.0;` fails to compile
`String` in a boolean or int variable	not allowed
Valid name	letters, digits, `_` or `$`; cannot start with a digit; cannot be a keyword
Case	`count` and `Count` are different names
Choosing a type	count with `int`, measure with `double`, yes/no with `boolean`, text with `String`

The other five Java primitive types (long, short, byte, float, char) are outside the course and exam.
Variable names start with a lower-case letter by convention, with later words capitalised (totalPrice).
A value written with a decimal point (such as 4.0) is a double; one without (such as 4) is an int.
Text values are called string literals and are written in double quotes.

Treating 4 and 4.0 as identical. They have different types and print differently.
Storing a double in an int. The compiler rejects it.
Leaving out the quotes around a String value, or putting quotes around a number or true.
Declaring the same name twice in the same method, or mixing up count and Count.
Starting a name with a digit or using a keyword as a name.
Expecting double x = 9 / 2; to give 4.5. The integer division happens first, so x holds 4.0.

Which type is best for the number of pages in a book? (int)
Which type is best for a mass of 62.4 kg? (double)
Does boolean open = "true"; compile? (No: "true" is a String, not a boolean.)
What do a and b hold after int a = 6; double b = a; a = 10;? (a holds 10 and b holds 6.0; b has its own copy.)
Is int total = 5; int Total = 9; allowed? (Yes: they are two different variables.)