Pj01 x-classes and objects

Basic Java Programming
Sasidhara Marrapu

 Classes
 Properties and Methods
 Constructors
 Objects
 Java Access Modifiers
 Lab Problem
 Home Work
Java Access Modifiers

3
Classes and Objects
• A Java program consists of one or more classes
• A class is an abstract description/Blueprint of objects
• Every class is Subclass (child) of Object
• A class is used as a template or pattern to create new
objects (instances of the class)

4
Classes and Objects
• A class defines the characteristics that are common to a
group of similar objects
• defines the data/variables/properties that hold the object’s state
• defines the methods/behaviors that describe the object’s behaviors
Lamp is an object
• It can be in on or off state.
• You can turn on and turn off lamp (behavior).
Bicycle is an object
• It has current gear, two wheels, number of gear etc. states.
• It has braking, accelerating, changing gears etc. behavior.

5
Class Structure
• A program consists of one or more
classes
• Typically, each class is in a separate
.java file
Program
File File
File
File
Class
Variables
Constructors
Methods
Variables
Variables
Statements
Statements

6
Classes and Objects
public class Person {
// data or variables that hold object state
String name;
int age;
String gender;
// Behaviors
public void canRead() {
}
public void canWrite() {
}

7
Classes and Objects
• A class is used as a template or pattern to create new
objects (instances of the class)
• Every object is an instance of a class
• each object has its own name (identity)
• each object has its own variables (state)
• all instances of the class share the methods defined by the
class (behaviors)

Methods
• A method in a class performs specific behaviors by using
statements
• A statement causes the object to do something
• Class can have many methods
• Two types of methods
• Class level – static methods – can be accessible directly
• Instance/Member/Object level methods – can only be
accessible after creating object/instance.

Constructor
• A method in a class that initialize an instance of an
object before it's used.
•The same name as the class and have no return type
• Multiple Constructors: the same name but a different
number of arguments or different typed arguments
• Java Provides default constructors
• The special variable, this, can be used inside a method
to refer to the object instance.

11
Constructors
public class Person {
// data or variables that hold
object state
String name;
int age;
String gender;
Person(){
}
Person(String personName){
this.name = personName;
}

12
Creating objects of a class
• Objects are created dynamically using the new keyword.
• student and employee refer to Person objects
employee = new Person(“John”) ;
student = new Person() ;

13
Creating objects of a class
student = new Person();
employee = new Person(“John”) ;
employee = student;
P
student
Q
employee
Before Assignment
P
student
Q
employee
After Assignment

14
Automatic garbage collection
• The object does not have a reference and cannot be
used in future.
• The object becomes a candidate for automatic garbage
collection.
• Java automatically collects garbage periodically and releases
the memory used to be used in the future.
Q

15
Accessing Object Data
Person person = new Person();
person.age = 17
person.name = “Mary”
person.gender = “Female”
ObjectName.VariableName
ObjectName.MethodName(parameter-list)

16
Executing Methods in Object
• Using Object Methods:
sent ‘message’ to aCircle
Person person = new Person();
person.canRead()

Passing Information into a Method
• Argument types
•primitive and reference data type: Yes
•method: No
• Argument Names
•Can have the same name as one of the class's member
variable
• Use this to refer to the member variable
• Primitive arguments are passed by value.
• Reference arguments are passed by reference.

Using
package,
import, and
CLASSPATH
Java
Access
Specifiers

class DoFoo {
public static void main(String[] args) {
Foo f = new Foo();
f.fun();
f.i = f.i + 1;
}
}
public class Foo {
// . . .
int i;
// . . .
fun(){
i = i + 1;
}
}
new
Foo.class
javac
Access
to
What?
Access
to
Everything

Java Access Specifiers
• public
• protected
• "friendly“/default
• private

Packages
• Assume the files are class
files
• Assume all the files in the
same folder are declared to
be in the appropriate
package relative to the
CLASSPATH

public class Foo {
// . . .
public int i;
// . . .
fun(){
i++;
}
}
public class Foo {
// . . .
public int i;
// . . .
fun(){
i++;
}
}
Foo f = new Foo();
f.i++;
Can this
reference
access this
member?
Where Can We Access
"i" in Foo From?

public class Foo {
// . . .
public int i;
// . . .
fun(){
i++;
}
}
Y
Y
Y
Y
Y
Y
Y
public class Foo {
// . . .
public int i;
// . . .
fun(){
i++;
}
}
Foo f = new Foo();
f.i++;
public int i;

public class Foo {
// . . .
int i;
// . . .
fun(){
i++;
}
}
N
Y
Y
N
N
N
N
public class Foo {
// . . .
int i;
// . . .
fun(){
i++;
}
}
Foo f = new Foo();
f.i++;
int i;

public class Foo {
// . . .
private int i;
// . . .
fun(){
i++;
}
}
N
N
N
N
N
N
N
public class Foo {
// . . .
private int i;
// . . .
fun(){
i++;
}
}
Foo f = new Foo();
f.i++;
private int i;

obj
Class Bar extends Foo {
// Stuff
}
Class Foo . . . {
// Stuff
}
Inheritance &
Access

public class Foo ext. . . {
// . . .
protected int i;
// . . .
fun(){
i++;
}
}
obj
Foo f = new Foo();
f.i++;
Inheritance &
Access

// . . .
protected int i;
// . . .
fun(){
i++;
}
}
obj
N
Y
Y
Y
N
N
N
// . . .
protected int i;
// . . .
fun(){
i++;
}
}
obj
Foo f = new Foo();
f.i++;
Inheritance &
Access

The Member Accessed
from anywhere
from same package
from same class
public "friendly" private

The Member Accessed
from anywhere
from same package
from a child class
from same class
protected

You May Wish to Order Declarations
• public
• protected
• "friendly"
• private

Summary
• Access is based on library structure (packages) and inheritance.
• The less you expose the more flexibility is preserved

Pj01 x-classes and objects

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