- There are 2 kinds of
variables in Java:
- variables of a primitive type
Examples:
double a; int b; ...
- variables of a user-defined type
Examples:
String a; BankAccount b; ...
- variables of a primitive type
- The type information are used
differently in the 2 kinds
of variables:
- The type information of
primitive types tells the computer
which encoding schmeme it
needs to use to encode/decode information.
Example: int uses the binary number system as code
Content value encoded ========================================= 00000000 0 00000001 1 00000010 2 00000011 3 ... ...
- The type information of
user-defined types (= objects !) tells the computer
how to find the variables
inside the user-defined object
Example: description of the BankAccount object tells the Java compiler how to find the various variables in a BankAccount object
Type information (= description) of a BankAccount object: public class BankAccount { private int accNum; // an int variable is 4 bytes long private String name; // a String variable is 4 bytes long private double balance; // a double variable is 8 bytes long }
BankAccount a = new BankAccount( .... ); The variable a contains the location of a BankAccount object:
(BTW, when the Java compiler has located the variable a.accNum, it also knows that a.accNum is a primitive type variable (int) and will use the binary number system to decode the value)
- The type information of
primitive types tells the computer
which encoding schmeme it
needs to use to encode/decode information.
- The type information of
user-defined objects (classes) can
not only
tells the Java compiler
how to locate
the variables inside a
user-defined object,
but also:
- which variable name is a valid variable name of the user-defined object
- Example:
Type information (= description) of a BankAccount object: public class BankAccount { private int accNum; private String name; private double balance; }
BankAccount a = new BankAccount( .... ); The type information (= description) of BankAccount tells the Java compiler that: The variable a.accNum is valid The variable a.name is valid The variable a.balance is valid The variable a.bal is invalid
- We will next examine what will happen if we
make a variable of
user-defined type 1
point to an
object of
user-defined type 2....
- Important facts to remember and apply
to the discussion in the next few webpages:
- Every variable in a
Java program has a (unique)
type
- The type of a variable is
determined by the
definition clause of that
variable and the type
will remain unchanged
after its definition
Example:
BankAccount a; CheckingAccoung b;Then:
- The type of a is always BankAccount, no matter where a is pointing to.
- The type of
b is
always CheckingAccount,
no matter where b is
pointing to.
- The type of a variable does not depend on which object it is pointing to !
- The type of a variable determines what variable names are valid !!!
- Every variable in a
Java program has a (unique)
type