Java OOP Programming language (Part 3) - Class and Object

Java Programming –
Class/Object
Oum Saokosal
Master’s Degree in information systems,Jeonju
University,South Korea
012 252 752 / 070 252 752
oumsaokosal@gmail.com

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3
Class/Object
•“Class” means a category of things
• A class name can be used in Java as the
type of a field or local variable or as the
return type of a function (method)
•“Object” means a particular item that
belongs to a class
• Also called an “instance”

4
(“Fields” or “Data
Members”)
class Ship1 {
public double x, y, speed, direction;
public String name;
}
public class Test1 {
public static void main(String[] args) {
Ship1 s1 = new Ship1();
s1.x = 0.0;
s1.y = 0.0;
s1.speed = 1.0;
s1.direction = 0.0;
s1.name = "Ship1";
}
}

5
Class/Object
•Java naming convention
•Format of class definitions
•Creating classes with new
•Accessing fields with
variableName.fieldName

6
Java NamingConventions
• Leading uppercase letter in class name
public class MyClass {
...
}
• Leading lowercase letter in field, local
variable, and method (function) names
• myField,myVar,myMethod

7
The general form of a simple class is
modifier class Classname {
modifier data-type field1;
modifier data-type field2;
...
modifier data-type fieldN;
modifier Return-Type methodName1(parameters) {
//statements
}
...
modifier Return-Type methodName2(parameters) {
//statements
}
}

8
Objects and References
• Once a class is defined,you can easily declare a variable (object
reference) of theclass
Ship s1, s2;
Point start;
Color blue;
• Object references are initially null
• The null value is a distinct type in Java and should not be
considered equal to zero
• A primitive data type cannot be cast to an object (use wrapper
classes)
• The new operatoris required to explicitly create the object that
is referenced
ClassName variableName = new ClassName();

9
Accessing InstanceVariables
• Use a dot between the variable name and the field
name, as follows:
variableName.fieldName
• For example,Java has a built-in class called Point that
has x and y fields
Point p = new Point(2, 3); // Build a Point object
int xSquared = p.x * p.x; // xSquared is 4
p.x = 7;
• One major exception applies to the “access fields
through varName.fieldName”rule
• Methodscan accessfields of current object without
varName
• This will be explainedwhen methodsare discussed

10
Example 2: Methods
class Ship2 {
public double x=0.0, y=0.0, speed=1.0,
direction=0.0;
public String name = "UnnamedShip";
private double degreesToRadians(double degrees) {
return(degrees * Math.PI / 180.0);
}
public void move() {
double angle = degreesToRadians(direction);
x = x + speed * Math.cos(angle);
y = y + speed * Math.sin(angle);
}
public void printLocation() {
System.out.println(name + " is at ("
+ x + "," + y + ").");
}
}

11
Methods (Continued)
s1.name = "Ship1";
s2.direction = 135.0; // Northwest
s2.speed = 2.0;
s2.name = "Ship2";
s1.move();
s2.move();
s1.printLocation();
s2.printLocation();
}
}
• Compiling and Running:
javac Test2.java
java Test2
• Output:
Ship1 is at (1,0).
Ship2 is at (-1.41421,1.41421).

12
Example 2: Major Points
•Format of method definitions
•Methods that access local fields
•Calling methods
•Static methods
•Default values for fields
•public/private distinction

13
Defining Methods
(Functions Inside Classes)
• Basic method declaration:
public ReturnType methodName(type1 arg1,
type2 arg2, ...) {
...
return(something of ReturnType);
}
• Exception to this format: if you declare the return type
as void
• This special syntaxthat means “thismethodisn’t going to
return a value
• In such a caseyou do not need (in fact,are not permitted),
a return statement that includes a value tobe returned.

14
Examples of Defining Methods
• Here are twoexamples:
• The first squares an integer
• The second returns the faster of two Ship objects, assuming that a
class called Ship has been defined that has a field named speed
// Example function call:
// int val = square(7);
public int square(int x) {
return(x*x);
}
// Example function call:
// Ship faster = fasterShip(someShip, someOtherShip);
public Ship fasterShip(Ship ship1, Ship ship2) {
if (ship1.speed > ship2.speed) {
return(ship1);
} else {
return(ship2);
}
}

15
Static Methods
• Static functions are like global functions in other
languages
• You can call a static method through the class name
ClassName.functionName(arguments);
• For example, the Math class has a static method
called cos that expects a double precision
number as an argument
• So you can call Math.cos(3.5) without ever
having any object (instance) of the Math class

16
MethodVisibility
• public/private distinction
• A declaration of private meansthat “outside”
methods can’t call it -- only methods within the same
class can
• Thus, for example, the main methodof theTest2 class
could not have done
double x = s1.degreesToRadians(2.2);
•Attempting to do so would have resulted in an errorat
compile time
• Only say public for methods that you want to
guarantee your class willmake available to users
• You are free to change or eliminate private methods
without telling users of your class about

17
Example 3: Constructors
class Ship3 {
public double x, y, speed, direction;
public String name;
public Ship3(double x, double y,
double speed, double direction,
String name) {
this.x = x; // "this" differentiates instance vars
this.y = y; // from local vars.
this.speed = speed;
this.direction = direction;
this.name = name;
}
private double degreesToRadians(double degrees) {
return(degrees * Math.PI / 180.0);
}
...

18
Constructors (Continued)
public void move() {
double angle = degreesToRadians(direction);
x = x + speed * Math.cos(angle);
y = y + speed * Math.sin(angle);
}
public void printLocation() {
System.out.println(name + " is at ("
+ x + "," + y + ").");
}
}
Ship3 s1 = new Ship3(0.0, 0.0, 1.0, 0.0, "Ship1");
Ship3 s2 = new Ship3(0.0, 0.0, 2.0, 135.0, "Ship2");
s1.move();
s2.move();
s1.printLocation();
s2.printLocation();
}
}

19
Constructor Example: Results
• Compiling and Running:
javac Test3.java
java Test3
•Output:
Ship1 is at (1,0).
Ship2 is at (-1.41421,1.41421).

20
Example 3: Major Points
•Format of constructor definitions
•The “this” reference

21
Constructors
• Constructors are special functions called when a class
is created with new
• Constructorsare especially useful for supplying values of
fields
• Constructorsare declared through:
public ClassName(args) {
...
}
• Notice that the constructorname must exactly match the
class name
• Constructorshave no return type(not even void), unlike a
regular method
• Java automatically provides a zero-argument constructorif
and only if the class doesn’t define it’s own constructor
• That’s why you could say
in the first example,even though a constructor wasnever
defined

22
The thisVariable
• The this object reference can be used inside any
non-staticmethod to refer to the current object
• The common uses of the thisreference are:
1. To passa reference to thecurrent object as a parameter
to other methods
someMethod(this);
2. To resolve name conflicts
• Using this permitsthe use of instance variablesin
methodsthat have local variableswith the same name
• Note that it is only necessary to say this.fieldName
when you have a localvariable anda class field with the
same name; otherwise just use fieldName with no
this

Java OOP Programming language (Part 3) - Class and Object

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