Java Programs

Display message on computer screen.
class First
{
public static void main(String args[])
{
System.out.println("Let's do something using
Java technology.");
}
}

Print integers
class Integers
{
public static void main(String[] arguments)
{
int c; //declaring a variable /* Using for loop to repeat
instruction execution */
for (c = 1; c <= 10; c++)
{
System.out.println(c);
}
}
}

If else control instructions:
class Condition
{
public static void main(String[] args)
{
boolean learning = true;
if (learning)
{
System.out.println("Java programmer");
}
Else
{ System.out.println("What are you doing here?");
}
}
}

Command line arguments:
class Arguments
{
{
for (String t: args)
{
System.out.println(t);
}
}
}

Java if else program
import java.util.Scanner;
class IfElse
{
{
int marksObtained, passingMarks;
passingMarks = 40;
Scanner input = new Scanner(System.in);
System.out.println("Input marks scored by you");
marksObtained = input.nextInt();
if (marksObtained >= passingMarks)
{
System.out.println("You passed the exam.");
}
else { System.out.println("Unfortunately, you failed to pass the exam.");
}
}
}

Nested If Else statements
class NestedIfElse
{
{
int marksObtained, passingMarks;
char grade;
passingMarks = 40;
System.out.println("Input marks scored by you");
marksObtained = input.nextInt();
if (marksObtained >= passingMarks)
{
if (marksObtained > 90) grade = 'A';
else if (marksObtained > 75) grade = 'B';

else if (marksObtained > 60)
grade = 'C';
else grade = 'D';
System.out.println("You passed the exam and your grade is " +
grade);
}
else
{
grade = 'F'; System.out.println("You failed and your grade
is " + grade);
}
}
}

Infinite for loop
for ( ; ; )
{
System.out.println("Java programming");
}

Simple for loop example in Java
class ForLoop
{
{
int c;
for (c = 1; c <= 10; c++)
{
System.out.println(c);
}
}
}

Java while loop example
class WhileLoop
{
public static void main(String args [])
{
int n;
System.out.println("Input an integer");
while ((n = input.nextInt()) != 0)
{
System.out.println("You entered " + n);
System.out.println("Input an integer");
}
System.out.println("Out of loop");
}
}

Java program to print multiplication
table
class MultiplicationTable
{
{
int n, c;
System.out.println("Enter an integer to print it's multiplication
table");
Scanner in = new Scanner(System.in);
n = in.nextInt(); System.out.println("Multiplication table of " + n + " is : ");
for (c = 1; c <= 10; c++)
System.out.println(n + "*" + c + " = " + (n*c));
}
}

• A constructor in Java is a method which is
used used to initialize objects.
• Constructor method of a class has the same
name as that of the class,
• Constructor can't be called explicitly
Constructor

class Programming
{
Programming()
{
System.out.println("Constructor method called.");
}
public static void main(String[] args)
{
Programming object = new Programming();
}
}

Creating class and object
public class Puppy
{
public Puppy(String name)
{
System.out.println("Passed Name is :" + name );
}
{
myPuppy = new Puppy( "tommy" );
}
}

Types of java constructors
• There are two types of constructors in java:
• 1.Default constructor (no-arg constructor)
• 2. Parameterized constructor

Java Default Constructor
A constructor is called "Default Constructor" when it
doesn't have any parameter.
Example of default constructor
class Bike1
{
Bike1()
{
System.out.println("Bike is created");
}
{
Bike1 b=new Bike1();
}
}
OUTPUT
Bike is created

Example of default constructor that
displays the default values
class Student3{
int id;
String name;
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student3 s1=new Student3();
Student3 s2=new Student3();
s1.display();
s2.display();
}
}
OUTPUT
0 null
0 null

parameterized constructor
• Parameterized constructor is used to provide different values to the distinct objects.
class Student4{
int id;
String name;
Student4(int i,String n){
id = i;
name = n;
}
void display(){System.out.println(id+" "+name);}
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
s1.display();
s2.display();
}
}

Java - Methods
• A Java method is a collection of statements
that are grouped together to perform an
operation.
• When you call the System.out.println()
method.
• For example, the system actually executes
several statements in order to display a
message on the console.

import java.io.*;
class Addition
{
int sum = 0;
public int addTwoInt(int a, int b)
{
// adding two integer value.
sum = a + b;
//returning summation of two values.
return sum;
}
class GFG
{
public static void main (String[] args)
{
// creating an instance of Addition class
Addition add = new Addition();
// calling addTwoInt() method to add two integer using instance created in above step.
int s = add.addTwoInt(1,2);
System.out.println("Sum of two integer values :"+ s);
}
}
OUTPUT:
Sum of two integer values :3

Access Specifiers
• In java we have four Access Specifiers and
they are listed below.
1. public
2. private
3. protected
4. default(no specifier)

Real Time Example for Access Specifier

private:
• Private members of class in not accessible
anywhere in program these are only
accessible within the class. Private are also
called class level access modifiers.

class Hello
{
private int a=20;
private void show()
{
System.out.println("Hello java");
}
}
public class Demo
{
{
Hello obj=new Hello();
System.out.println(obj.a);
//Compile Time Error, you can't access private data
obj.show();
//Compile Time Error, you can't access private methods
}
}

public:
Public members of any class are accessible
anywhere in the program in the same class
and outside of class, within the same package
and outside of the package. Public are also
called universal access modifiers.

class Hello
{
public int a=20;
public void show()
{
System.out.println("Hello java");
}
}
public class Demo
{
{
Hello obj=new Hello();
System.out.println(obj.a); obj.show();
}
}

• Output
• 20
• Hello Java

protected:
• Protected members of the class are accessible
within the same class and another class of the
same package and also accessible in inherited
class of another package. Protected are also
called derived level access modifiers.

//save A.java
package pack1;
public class A
{
protected void show()
{
System.out.println("Hello Java");
}
}
//save B.java
package pack2;
import pack1.*;
class B extends A
{
{
B obj = new B(); obj.show();
}
}
Output
Hello Java

default:
• Default members of the class are accessible only within the same class and another class of the same package. The default
are also called package level access modifiers.
//save by A.java
package pack;
class A
{
void show()
{
System.out.println("Hello Java");
}
}
//save by B.java package pack2; import pack1.*; c
lass B
{
{
A obj = new A(); //Compile Time Error, can't access outside the package
obj.show();
//Compile Time Error, can't access outside the package
}
}

Java static variable
• f you declare any variable as static, it is known
static variable.
• The static variable can be used to refer the
common property of all objects (that is not
unique for each object) e.g. company name of
employees,college name of students etc.
• The static variable gets memory only once in class
area at the time of class loading.
• Advantage of static variable
• It makes your program memory efficient (i.e it
saves memory).

Understanding problem without static
variable
class Student{
int rollno;
String name;
String college="ITS";
}
Suppose there are 500 students in my college, now all instance data
members will get memory each time when object is created.All
student have its unique rollno and name so instance data member
is good.Here, college refers to the common property of all objects.If
we make it static,this field will get memory only once.

Example of static variable
/Program of static variable
class Student8{
int rollno;
String name;
static String college ="ITS";
Student8(int r,String n){
rollno = r;
name = n;
}
void display (){System.out.println(rollno+" "+name+" "+college);}
Student8 s1 = new Student8(111,"Karan");
Student8 s2 = new Student8(222,"Aryan");
s1.display();
s2.display();
}
}
Output:111 Karan ITS
222 Aryan ITS

Java Comments
There are 3 types of comments in java.
Single Line Comment
Multi Line Comment
Documentation Comment
• Java Single Line Comment
• The single line comment is used to comment only one line.
• Syntax:
• //This is single line comment
• Example
• public class CommentExample1 {
• public static void main(String[] args) {
• int i=10;//Here, i is a variable
• System.out.println(i);
• }
• }

Java Multi Line Comment
The multi line comment is used to comment multiple lines of code.
/*
This
is
multi line
comment
*/
Example:
public class CommentExample2 {
public static void main(String[] args) {
/* Let's declare and
print variable in java. */
int i=10;
System.out.println(i);
}
}
Output:
10

Java Documentation Comment
The documentation comment is used to create
documentation
Syntax:
/**
This
is
documentation
comment
*/

Data Types in Java
• Data types classify the different values to be stored in the variable. In java,
there are two types of data types:
• Primitive Data Types
• Non-primitive Data Types
• byte (1 byte)
• short (2 bytes)
• int (4 bytes)
• long (8 bytes)
• float (4 bytes)
• double (8 bytes)
• char (2 bytes)
• boolean (1 byte) (true/false)

Variables in Java
A variable is the name given to a memory location. It is the basic unit of
storage in a program.
The value stored in a variable can be changed during program execution.
A variable is only a name given to a memory location, all the operations done
on the variable effects that memory location.
In Java, all the variables must be declared before they can be used.
How to declare variables?
We can declare variables in java as follow
datatype: Type of data that can be stored in this variable.
variable_name: Name given to the variable.
value: It is the initial value stored in the variable.

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