- We want to
store the following
information
on a bank account
- account number (an int)
- owner name (a String)
- balance (an double)
- Example: storing information on
2 bank accounts
of students
public class Class01 { public static void main(String[] args) { int stu1_accNum = 12345, stu2_accNum = 23456; String stu1_name = "Mary", stu2_name = "John"; double stu1_balance = 1000, stu2_balance = 1500; System.out.println( stu1_accNum + " " + stu1_name + " " + stu1_balance); System.out.println( stu2_accNum + " " + stu2_name + " " + stu2_balance); } }
- Comments on the solution:
- The various pieces of
information belonging to the
same bank account
are stored in different variables
- The organization of the information is very impractical.
- The various pieces of
information belonging to the
same bank account
are stored in different variables
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac Class01.java
- To run: java Class01
- Java has 4 different kinds
of variables
(See:
click here)
- Class variables
public static class SomeClassName { public static double a; // Class variable ... } - Instance variables
- Not yet discussed
- But we will discuss this soon !
- Local variables
public static class SomeClassName { public static void main(String[] args) { double a; // Local variable ... } } - Parameter variables
public static class SomeClassName { public static void main(String[] args) { ... } }
- Class variables
- Recall that a
class is used to
organize methods
Example:
- Methods that compute Mathematical functions (such as sin, cos, sqrt, etc) are store in the class Math inside the Java library
- Using a class to
organize information:
- A class can also be used to specify the structure of the information for some entity
- Example:
specifying
the structure
of the information for one
bank account
public class BankAccount { public int accNum; // Account number public String name; // Name of the account holder public double balance; // Balance in the account }Explanation:
- The class BankAccount contains
3 instance variables
(accNum,
name and
balance)
- When a class contains
instance variables,
it defines a
group of
related information that
belong to the
same entity
- Important note:
- The class definition
only
defines the
structure of
information
(I.e., the class only specifies the name and the type of each variable that make up the complete information structure)
- The class definition
does not
create
the
variables themselves !!!
The creation of the variables happens elsewhere (see next topic !)
- The class definition
only
defines the
structure of
information
- The class BankAccount contains
3 instance variables
(accNum,
name and
balance)
- Instance variable:
- Instance variables are
defined
inside some class
- Instance variables are
defined
outside every method
of that class.
- Instance variables are defined without using the keyword static
public class BankAccount { public int accNum; // instance variables public String name; // public double balance; // } - Instance variables are
defined
inside some class
- Java lingo:
object
and
instance
- A variable of a
user-defined type (class) in
Java is called an
object.
-
Object
=
a group of instance variables
specified in the definition of a
class
Java programmers often calls an object of a class C an instance of class C
- A variable of a
user-defined type (class) in
Java is called an
object.
- In most programming languages
(e.g., C++),
defining objects
is a one-step process
In Java however ---- (just like the case of arrays - see: click here) ---- defining objects is a two-step process
- Creating an object (making a new object) in Java:
- Step 1:
- Define a variable to
store the
location
of the object.
- Recall that: Object = the group of instance variables that was specified in the definition of the class
This variable is called an object reference variable
(Recall that in Computer Science, a reference is an address of a variable or method)
- Define a variable to
store the
location
of the object.
- Step 2:
- Create the object (= the group of instance variables that was specified in the definition of the class) and store the location of the first variable of the array in the object reference variable
Example:
Step 1: BankAccount stu1; // The type "BankAccount" is the // "object reference" type to a BankAccount object // The variable stu1 contains an address of // a BankAccount object
Step 2: stu1 = new BankAccount() ; // new BankAccount() creates a BankAccount object // - it creates the instance variables specified // inside the BankAccount class // new BankAccount() returns the address // of the first variable of the BankAccount object // which is stored in variable stu1
- Step 1:
- What exactly happens when we
define
an object:
- "BankAccount stu1;" creates
a local variable that
contains a reference:
- "stu1 = new BankAccount();" creates
the BankAccount object and
make stu1 "point"
to the newly created group of variables:
(This process is exactly the same as that of defining an array in Java)
- "BankAccount stu1;" creates
a local variable that
contains a reference:
- Accessing the information stored
inside an object:
- The individual variables
of the object can be
accessed
through the following
variable names:
- stu1.accNum
- stu1.name
- stu1.balance
Illustrated:
- The variables in the object are accessed through the local (object reference) variable stu1
- The individual variables
of the object can be
accessed
through the following
variable names:
- Example Java program:
public class BankAccount { public int accNum; // instance variabe: a NEW kind of variables !!! public String name; public double balance; }
public class Class02 { public static void main(String[] args) { BankAccount stu1, stu2; // Define 2 object variables stu1 = new BankAccount(); // Create one BankAccount object (with 3 data field // stu1.accNum, stu1.name, stu1.balance) stu2 = new BankAccount(); // Create another BankAccount object (with // stu2.accNum, stu2.name, stu2.balance) stu1.accNum = 12345; stu1.name = "Mary"; stu1.balance = 1000; stu2.accNum = 23456; stu2.name = "John"; stu2.balance = 1500; System.out.println( stu1.accNum + " " + stu1.name + " " + stu1.balance); System.out.println( stu2.accNum + " " + stu2.name + " " + stu2.balance); } }Output:
12345 Mary 1000.0 23456 John 1500.0
- Example Program:
(Demo above code)
- The BankAccount.java Prog file: click here
- The Class02.java Prog file: click here
How to run the program:
- Right click on links and
save in a scratch directory
- To compile: javac Class02.java
- To run: java Class02