Variables: main properties

Variables are used to denote the data inside a program.

A variable is characterized by the following properties:

1. Name: it is necessary in order to identify the variable; for example: line. Such a name must be a Java identifier, i.e.:
   • a sequence of letters, digits, or the '_' character, starting with a letter or with '_' (but not with a digit)
   • it can be of any length
   • lowercase and uppercase letters are considered to be different
   • some identifiers, called keywords, are reserved: class, public, if, while, etc.

2. Type: specifies the type of the data that the variable can store; for example: a variable of type String can store a reference to a string.

3. Address of the memory location containing the stored data:
   • each variable has an associated memory location
   • the size of the memory location depends on the type of the variable
   • in Java there is no way to know the address of a memory location!!! This solves several problems related to security, such as attacks by viruses, etc.

4. Value: the data denoted by the variable at a certain point during the execution of the program; for example: the reference to the object "JAVA".

During program execution, the name, type, and address of a variable cannot change, while the value can.

Note: even if a variable contains the reference to an object, often we abuse terminology and say that the value of the variable "is the object" which the variable refers to.