Lecture 4

Lecture Topic:

Classes, Methods, Members, Inheritance and Encapsulation

class_type

members

field

methods

data variable

code

Example

class Point {: public double x, y;
}
Point p1 = new Point( );: p1.x = 10; // malloc not available
P1.y = 11; // in Java code.

Example

// what is the print out of the following code
import java.awt.*;
import java.applet.Applet;
class apple {: int i = 1; // a.i = 1;
}
class inc {: static int add_one( int i ) {
i = i + 1;
return i;
}
static void add_one ( apple a ) {: a.i = a.i + 1;
}
}
public class : ref extends Applet {: public static void main ( string args [] ) {
apple a = new apple ( );
system.out.println ( a.i + "\t" + inc.add_one(a.i) + "\t" + a.i);
inc.add_one( a );
system.out.println ( a.i );
}
}

Answer:

Class & Overloaded methods

Object and Classes

An object is a combination of code ( functions ) and data ( variables ) joined together into a single entity.
A class is essentially a description of how to make an object.
Every object has a class which is used to determine how to create the object, what variables the object will contain, and what messages the object will respond to.

Object have type

Class Definition

class <class name> [extends <class name>] [implements <interface name>] {: [<variable_declaration>;]
[<method_declaration>;]
}

Example

class Lamp {: boolean on;
}
Lamp l = new Lamp ( );: l.on = true;

Overloaded methods

Default constructor takes null.

Example

class Lamp {: boolean on;
int Wattage;
Lamp ( ) { // if, Lamp ( int w ) here: Wattage = 100; // then, Wattage = w; or Lamp bulb = new Lamp ( 100 ) here
} // otherwise, Lamp bulb = new Lamp ( ) , syntax error
}

Class don't need methods!

Example

Class CS410 {: student[] sa;
int number_of_student;
final string name = "programming in Java"; // cant be changed
CS410( int i ) {: number_of_student = i;
}
}
void Print ( ) {: system.out.println(" I have" + number_of_student +" in my class.");
}

Class instance

CS410 c.i = new CS410(30);

Static variables propagate to all instances of classes.

Example

class Student{: static int student_count = 0;
void Student ( ) {: student_count++;
}
}

Static method a good for grouping utility functions.

Example

class Trig {: static float area ( float r ) {
return Math.PI * r * r;
}
float A = Trig.area(10);

Default for numeric fields not explicitly initialized to a different value is 0.

system.out class has a static method called println.

A static method cannot make reference to a non-static method.

Example

class foo {: static int inc( int i ) {
return i++;
}
int wow( int i ) { // OK: return inc( i );
}
static int wee( int i ) {: return wow( i );
}
}

Example

double negative ( ) {: return Math.SQRT ( x * x + y * y );
}
void Print ( ) {: system.out.println ( this.getclass( ).getname( ) + " = " );
system.out.println ( x + " " + y );
}

Sample Quiz

1. A static method cannot make reference to a non-static method.

True

False
2. Default constructor does not takes null in overloaded method.

True

False