Java Basics - Part2

19Z305OBJECT
ORIENTED
PROGRAMMING
Unit 2

Overview
PART
-
1
• Characteristics
of Java
• Data types,
Variables,
Operators
• Control
Statements,
Arrays
PART
-
2
• Classes
• Methods,
Constructors
• Inheritance
• Abstract class
12/11/2020 Vani Kandhasamy,PSG Tech 2

Methods
 Method definition:
TYPE NAME (PARAMETER LIST) {
STATEMENTS
return VALUE;
}
 Method call:
NAME (ARGUMENT LIST);

Output:
4
9
16

Classes
Part 1Chapter 6

Class
Class <class name> {
}
Data Members
Methods
How to represent Box as Class?

Class
Class Box {
}
Class <class name> {
}
Data Members
Methods
double width;
double length;
double height;
double volume() {
return width*length*height;
}
Box as Class

Class Box {
}
Box mybox1; // declare reference to object
mybox1 = new Box(); // allocate a Box object
(or)
Box mybox2 = new Box();
double width;
double length;
double height;
double volume() {
}
Objects

Statement Effect
Box mybox;
mybox
mybox = new Box()
mybox width
height
length

Statement Effect
Box mybox2 = mybox1;
mybox2
width
height
length
mybox1
width
height
length
mybox1

Class Box {
}
class BoxDemo1 {
public static void main(String args[]) {
double vol;
// assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.length = 15;
// assign values to mybox2's instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.length = 9;
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
}
}
double width;
double length;
double height;
double volume() {
}
Objects

Statement Effect
mybox1.width = 10;
mybox1.height = 20;
mybox1.length = 15;
mybox2.width = 3;
mybox2.height = 6;
mybox2.length = 9;
mybox2
width = 3
height = 6
length = 9
mybox1
width =10
height =20
length = 15

class BoxDemo1 {
double vol;
// assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.length = 15;
// assign values to mybox2's instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.length = 9;
// get volume of first box
// get volume of second box
}
}

Methods &
Constructors
Part 1Chapter 6 & 7

Practice7
Open repl.it
Update Box() class – add methods
for initializing member/instance
variables

class Box {
// sets dimensions of box
void setDim(double w, double h, double l) {
width = w;
height = h;
length = l;
}
}
class BoxDemo2 {
// initialize each box
mybox1.setDim(10, 20, 15);
}
}
Methods -
a fix for the
encapsulation violation

class Box {
// sets dimensions of box
void setDim(double w, double h, double l) {
width = w;
height = h;
length = l;
}
}
class BoxDemo2 {
// initialize each box
}
}
Automatic
Initialization:
Constructor

Constructor

class Box {
// This is the constructor for Box.
Box() {
System.out.println("Constructing Box");
width = 10;
height = 20;
length = 15;
}
Box(double w, double h, double l) {
width = w;
height = h;
length = l;
}
}
class BoxDemo6 {
// declare, allocate, and initialize Box objects
Box mybox2 = new Box(3,6,9);
}
}
DefaultConstructor
Parameterized Constructor

class Box {
Box() {
width = 10;
height = 20;
length = 15;
}
width = w;
height = h;
length = l;
}
}
class BoxDemo6 {
}
}

class Box {
Box() {
width = 10;
height = 20;
length = 15;
}
Box(double width, double height, double length) {
width = width;
height = height;
length = length;
}
}
class BoxDemo6 {
}
}
Output:
3000
0

class Box {
Box() {
width = 10;
height = 20;
length = 15;
}
this.width = width;
this.height = height;
this.length = length;
}
}
class BoxDemo6 {
}
}
Keyword: this

Statement Effect
Box mybox2 =new Box(10,15,20); mybox2
width=10
height=15
length=20
mybox1
width=10
height=15
length=20

Statement Effect
Mybox1.width = 5 mybox1
width=5
height=15
length=20
mybox1
width=10
height=15
length=20

Statement Effect
Mybox1.width = 5 mybox1
width=5
height=15
length=20
mybox1
width=10
height=15
length=20
Its still possible to
access Data
members outside
the class –
violating OOP

Access
Specifiers

class Box {
private double width;
private double height;
private double length;
this.width = width;
this.height = height;
this.length = length;
}
}
class BoxDemo6 {
mybox2.width = 10; // ERROR!!!
}
}
Access Specifiers -
a fix for the abstraction
violation

Static
members
public class UseStatic {
static int a = 3;
static int b;
static void meth(int x) {
b = a * 4;
System.out.println("x = " + x);
System.out.println("a = " + a);
System.out.println("b = " + b);
}
meth(42);
}
}
a, b - StaticVariables
meth - Static Method

class StaticDemo {
static int a = 42;
static int b = 99;
static void callme() {
System.out.println("a = " + a);
}
}
class StaticByName {
StaticDemo.callme();
System.out.println("b = " + StaticDemo.b);
}
}
Class name is used to
access static methods

Method
Overloading
 Polymorphism
 Same method name but type and/or number of their
parameters differs
 Different return types alone insufficient to overload

class OverloadDemo {
void test() {
System.out.println("No parameters");
}
// Overload test for one integer parameter.
void test(int a) {
System.out.println("a: " + a);
}
// Overload test for two integer parameters.
void test(int a, int b) {
System.out.println("a and b: " + a + " " + b);
}
// overload test for a double parameter
double test(double a) {
System.out.println("double a: " + a);
return a*a;
}
}
class Overload {
OverloadDemo ob = new OverloadDemo();
double result;
// call all versions of test()
ob.test();
ob.test(10);
ob.test(10, 20);
result = ob.test(123.25);
System.out.println("Result of ob.test(123.25): " + result);
}
}

void test() {
}
}
return a*a;
}
}
class Overload {
double result;
int a = 88;
ob.test();
ob.test(a);
ob.test(10, 20);
}
}

Constructor
Overloading
class Box {
// This is the constructor for cube.
Box(double side) {
width = height = length = side;
}
// This constructor is used when all dimensions specified
width = w;
height = h;
length = l;
}
}
class BoxDemo6 {
Box mycube1 = new Box(4);
}
}
Differs only by
#parameters

Argument
Passing
Address of
argument
Value of
argument

class Test {
void meth(int a, int b) {
a *= 2;
b /= 2;
}
}
class CallByValue {
Test ob = new Test();
int a = 15, b = 20;
System.out.println("a and b before call: " +
a + " " + b);
ob.meth(a, b);
System.out.println("a and b after call: " +
a + " " + b);
}
}
class Test {
int a, b;
Test(int i, int j) {
a = i;
b = j;
}
// pass an object
void meth(Test o) {
o.a *= 2;
o.b /= 2;
}
}
class CallByRef {
Test ob = new Test(15, 20);
System.out.println("ob.a and ob.b before call: " +
ob.a + " " + ob.b);
ob.meth(ob);
System.out.println("ob.a and ob.b after call: " +
ob.a + " " + ob.b);
}
}

Constructor +
Object
class Box {
// This is the constructor for cube.
Box(double side) {
width = height = length = side;
}
// This constructor is used when all dimensions specified
Box(Box ob) {
width = ob.width;
height = ob.height;
length = ob.length;
}
}
class BoxDemo6 {
Box mycube1 = new Box(4);
Box mycube2 = new Box(mycube1);
}
}

Returning
Objects
class Test {
int a;
Test(int i) {
a = i;
}
Test incrByTen() {
Test temp = new Test(a+10);
return temp;
}
}
class RetOb {
Test ob1 = new Test(2);
Test ob2;
ob2 = ob1.incrByTen();
System.out.println("ob1.a: " + ob1.a);
System.out.println("ob2.a: " + ob2.a);
ob2 = ob2.incrByTen();
System.out.println("ob2.a after second increase: "
+ ob2.a);
}
}
Returning Objects

Practice9
Read the problem sheet2 provided
in the class room
Open repl.it
Complete the code stub
IntegerSetTest.java

References
 "Java:The Complete Reference” by Schildt H - Part 1 - Chapter 6 & 7
 https://www.javatpoint.com/java-tutorial

Java Basics - Part2

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