Variables and declarations
1
A variable is a name with an associated value. Throughout this course, w e draw a variab le as a named box
with the value in the box. For example, here is a variable x associated with the valu e 5. Som etimes, we draw a
line instead of a box, as shown to the right below:
Java is strongly typed, meaning that it has properties like th e following.
1. Each variable must have a type. When we want to make the type of a vari-
able explicit, we put the type to the right of the box.
2. Only values of that type may be associated with the variable. So, variable
x, declared above, cannot contain a boolean value true or fa lse.
3. Each expression has a type, which depends on the typ es of its operands, and
4. An operation can be applied only to operands of the appropriate type.
For example, an int variable may contain only an int value. A boolean variable may contain only a
boolean value, and so on. Also, operation b && c is legal only if b and c are of type boolean.
Matlab is not strongly typed —one second, a variable may contain a complex number, the next s econd, an
array of characters. The programm ing language C is very weakly typed —nothing prev ents one from treating a
string (character array) like “whatever ” as a struct, an array of 32-bit integers, or anything else.
A later short essay tells you the advant ages and disadvantages of strong typing.
Because of the strong typing principal, each variable must be declared before it is used. A variab le declara-
tion has the basic form
<type> <variable-name>
For example, here are two declarations and the corresponding variables, where we have annotated the variable
itself with its type:
int x
boolean isEven
We make two points about declarations .
First, depending on where the declaration appears in a program, it may be followed by a semicolon or a
comma, and it may also be preceded by an “access modifier”, which indicates wh at p art of the program can
reference it, and other things. But the two basic parts —type and variable name— will always be required.
Second, the declaration is not a statement to be executed. It is simply an announcement that a variable is
needed in a program. A variable declaration is never executed.
Declarations in th e interactions pan e
Let’s put a declaration in the interactions pane and see what the value of the var iable is:
int k;
k // the initial value of k is 0
Note that if we try to declare k again, Java complains. Ea ch variable can be declared only once.
Java conventions for variable names
Here are rules for identifiers —names of variables. They are :
(1) A variable contains only letters, numbers, and the char acters '_' and '$'.
x
5
x ___5_____
x
5
int
isEven
false
boolean
x
5
int
