Session 3 Constructors, Types, Overloading, Static MethodsNotes.pdf

Constructors, Overloading and
Static Members
1

Constructors
 Java allows objects to initialize themselves when
they are created through the use of a constructor.
 Constructors play the role of initializing objects
 Constructors are a special kind of methods that are
invoked using the new operator when an object is
created.

Constructors, cont.
 Constructors have following properties:
 Constructors must have the same name as the class itself.
 Constructors do not have a return type— not even void.
 The implicit return type is class itself
 A constructor with no parameters is referred to as a
default constructor and constructor with parameters is
called as parameterized constructor
3

Program for constructor
class Box {
double width;
double height;
double depth;
// This is the constructor for Box.
Box() {
System.out.println("Constructing Box");
width = 10;
height = 10;
depth = 10;
}
// compute and return volume
double volume() {
return width * height * depth;
}
}
class BoxDemo1 {
public static void main(String args[]) {
// declare, allocate, and initialize Box objects
Box mybox1 = new Box(); //constructor invoked
Box mybox2 = new Box(); //constructor invoked
double vol;
// get volume of first box
vol = mybox1.volume();
System.out.println("Volume is " + vol);
// get volume of second box
}
}
4

Parameterized Constructors
class Box {
double width;
double height;
double depth;
// This is the constructor for Box.
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
double volume() {
}
}
class BoxDemo2 {
// declare, allocate, and initialize Box objects
Box mybox1 = new Box(10, 20, 15);
double vol;
}
}
5

Exercise
 WAP to create class rectangle and calculate
area and perimeter of it by using constructor.
 WAP to create class Employee. Add methods
to read and display employee information by
using constructor such as id and salary.
6

Method Overloading
 Two or more methods within the same class that have the
same name but methods must differ in the type and/or
number of their parameters.
 Method overloading is one of the ways that Java implements
polymorphism
 When an overloaded method is invoked, Java uses the type
and/or number of arguments to determine which version of
the overloaded method to actually call.
7

Example
class OverloadDemo {
void test()
{
System.out.println("No parameters");
}
void test(int a)
{
System.out.println("a: " + a);
}
void test(int a, int b)
{
System.out.println("a and b: " + a + " " + b);
}
double test(double a)
{
System.out.println("double a: " + a);
return a*a;
}
}
class Overload {
public static void main(String args[])
{
OverloadDemo ob = new OverloadDemo();
double result;
// call all versions of test()
ob.test();
ob.test(10);
ob.test(10, 20);
result = ob.test(123.2);
System.out.println("Result of ob.test(123.2): " + result);
}
}
8

Constructor Overloading
 Similar to method overloading we can also overload
constructor .
 Overloaded constructors have the same name but must differ
in the type and/or number of their parameters.
9

Example
class Box {
double width;
double height;
double depth;
// constructor used when all dimensions specified
Box(double w, double h, double d) {
width = w;
height = h;
depth = d;
}
// constructor used when no dimensions specified
Box() {
width = -1; // use -1 to indicate
height = -1; // an uninitialized
depth = -1; // box
}
// constructor used when cube is created
Box(double len) {
width = height = depth = len;
}
double volume() {
}
class OverloadCons {
// create boxes using the various constructors
Box mybox2 = new Box();
Box mycube = new Box(7);
double vol;
System.out.println("Volume of mybox1 is " + vol);
System.out.println("Volume of mybox2 is " + vol);
// get volume of cube
vol = mycube.volume();
System.out.println("Volume of mycube is " + vol);
}
}
10

Exercise
 WAP to overload add method to perform
addition of two integers, addition of three
integers and addition of two float numbers.
 WAP to create overloaded constructor to
calculate area of triangle ,rectangle and circle.
11

Using Objects as Parameters
class Test {
int a, b;
Test(int i, int j) {
a = i;
b = j;
}
void add(Test o) {
if(o.a == a && o.b == b)
System.out.println(" Equal”);
else
System.out.println(" Not Equal”);
}
}
class PassOb
{
public static void main(String args[])
{
Test ob1 = new Test(100, 22);
Test ob2 = new Test(100, 22);
Test ob3 = new Test(-1, -1);
ob1.equals(ob2));
ob1.equals(ob3));
}
}
12

Returning Object
class Test {
int a ,b;
Test() {
a = 0;
b=0;
}
Test(int i ,int j) {
a = i;
b=j;
}
Test add()
{
Test temp = new Test();
temp.a=a+10;
temp.b=b+10;
return temp;
}}
class RetOb {
Test ob1 = new Test(2 ,4);
Test ob2;
ob2 = ob1.add();
System.out.println("ob1.a: " + ob1.a);
System.out.println("ob1.b: " + ob1.b);
System.out.println("After Addition :");
System.out.println("ob2.a: " + ob2.a);
System.out.println("ob2.b: " + ob2.b);
}
}
13

static Members
 Normally a class member must be accessed only with an
object.
 When a member is declared static, it can be accessed before
any objects of its class are created, and without reference to
any object.
 There can be a static method, static variable and a static block
 When objects of class are declared, no copy of a static
variable is made.
 Instead, all instances of the class share the same static
variable.
 Static methods can only call other static methods
 Static methods must only access static data
 Static block gets executed exactly once, when the class is first
loaded.
14

class UseStatic {
static int a = 3;
static int b;
static void meth(int x) {
System.out.println("x = " + x);
System.out.println("a = " + a);
System.out.println("b = " + b);
}
static {
System.out.println("Static block
initialized.");
b = a * 4;
}
meth(42);
}}
OUTPUT
Static block initialized.
x = 42
a = 3
b = 12
15

 Outside of the class in which they are defined, static methods and
variables can be used independently of any object.
 To call a static method and variable from outside its class, class name is
used .
General form:
classname.method( );
Classname.variable1;
16

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);
}
}
 output :
a = 42
b = 99
17

Session 3 Constructors, Types, Overloading, Static MethodsNotes.pdf

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