Classes
The introduction to object-oriented
concepts in the lesson titled Object-oriented Programming Concepts used a Test class as
an example, with num1, num2 fields. Here is sample code for a possible
implementation of a
Test class, to give you an overview of a
class declarationDeclaring Classes
You've seen
classes defined in the following way:
class MyClass
{
// field, constructor, and
// method declarations
}
This is a class declaration. The class body (the area between the braces) contains
all the code that provides for the life cycle of the objects created from the
class: constructors for initializing new objects, declarations for the fields
that provide the state of the class and its objects, and methods to implement
the behaviour of the class and its objects.
Declaring Member Variables
There are several kinds of
variables:
·
Member variables in a class—these
are called fields.
·
Variables in a method or block of
code—these are called local variables.
·
Variables in method
declarations—these are called parameters.
class
Test
{
// the Test class
has two fields
public
int num1;
public
int num2; // num1 and num2 are fields
public
void method1(int a) // variable a is
called parameters
{
Int
a; // local variables
.........
.........
}
} // end of class Test
The Test class uses the
following lines of code to define its fields:
public int num1;
public int num2;
Field declarations are composed of three
components, in order:
- Zero or more modifiers, such as public or private.
- The field's type.
- The field's name.
The fields of Test are
named num1 and num2 and are all of data type integer (int). The public keyword
identifies these fields as public members, accessible by any object that can
access the class.
Naming
a Method
Although a
method name can be any legal identifier, code conventions restrict method
names. By convention, method names should be a verb in lowercase or a
multi-word name that begins with a verb in lowercase, followed by adjectives,
nouns, etc. In multi-word names, the first letter of each of the second and
following words should be capitalized. Here are some
Examples:
run
runFast
getBackground
getFinalData
compareTo
setX
isEmpty
Note:-
Typically,
a method has a unique name within its class. However, a method might have the
same name as other methods due to method overloading.
Overloading
Methods
The
Java programming language supports overloading methods, and Java can distinguish
between methods with different method
signatures. This means that methods within a class can have the same name
if they have different parameter lists
public class DataArtist
{ ...
public void draw(String s) {
...
}
public void draw(int i) {
...
}
public void draw(double f) {
...
}
public void draw(int i, double f) {
...
}
}
Constructors for Your Classes
A
class contains constructors that are invoked to create objects from the class
blueprint. Constructor declarations look like method declarations—except that
they use the name of the class and have no return type.
For example,
Test has one constructor: public Test(int a, int b) // two parameter constructor
{
num1 = a;
num2 = b;
}
To
create a new
Test object called obj,
a constructor is called by the new operator: Test obj = new Test(30,8);
new
Test(30,8) creates
space in memory for the object and initializes its fields.
Although
Test only has one constructor, it could
have others, including a no-argument constructor.public Test()
{
num1 = 1;
num2 = 10;
}
Test
obj = new Test(); invokes
the no-argument constructor to create a new Test object called obj1.Creating Objects
A class provides the blueprint
for objects; you create an object from a class. The new operator instantiates a class by
allocating memory for a new object and returning a reference to that memory.
The new operator
also invokes the object constructor.
ClassName
objectName=new ClassConstructor();Example:-Test
obj = new Test();
LOOPS in JAVA
1. for
2. while
3. do -while
4. for-each
Java
for loop used to repeat execution of statement(s) until a certain condition
holds true. for is a keyword in Java
programming language.
for (/* Initialization of variables */ ; /*Conditions to test*/ ; /* Increment(s) or decrement(s) of variables */) {
// Statements to execute i.e. Body of for loop
}
Example:-
class ForLoop
{
public static void main(String[] args)
{
int c;
for (c = 1; c <= 10; c++)
{
System.out.println(c);
}
}
}
Example for Infinite loop
// Infinite for loop
for (;;)
{
System.out.println("Java programmer");
}
Java while loop
Java
while loop is used to execute statement(s) until a condition holds true. In
this tutorial we will learn looping using Java while loop examples. First of
all lets discuss while loop syntax:
{
// Body of loop
}
// Body of loop
}
Example:-
import java.util.Scanner;
class BreakWhileLoop {
public static void main(String[] args) {
int n;
Scanner input = new Scanner(System.in);
while (true) {
System.out.println("Input an integer");
n = input.nextInt();
if (n == 0) {
break;
}
System.out.println("You entered " + n);
}
}
}
do-while loop in JAVA
The Java
programming language also provides a
do-while statement, which can
be expressed as follows:
do {
statement(s)
} while (expression);
The difference
between
do-while and while is that do-while evaluates its
expression at the bottom of the loop instead of the top. Therefore, the
statements within the do block are always
executed at least once, as shown in the following DoWhileDemo program:class DoWhileDemo {
public static void main(String[] args){
int count = 1;
do {
System.out.println("Count is: " + count);
count++;
} while (count < 11);
}
}
For-each loop
(Advanced or Enhanced For loop):
The for-each loop introduced in Java5. It is mainly used to
traverse array or collection elements. The advantage of for-each loop is that
it eliminates the possibility of bugs and makes the code more readable.
Advantage of for-each loop:
- It
makes the code more readable.
- It
eliminates the possibility of programming errors.
Syntax of for-each loop:
for(data_type variable : array ){ .........}
class ForEachExample1
{
public static void main(String args[])
{
int arr[]={12,13,14,44};
for(int i:arr)
{
System.out.println(i);
}
}
}
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