List of Java Programs
1. Program1 – WAP to display the List of even numbers
2. Program2 - Factorial of a number
3. Program3 - Compare Two Numbers using else-if
4. Program4 - Determine If Year Is Leap Year
5. Program5 - Fibonacci Series
6. Program6 - Palindrome Number
7. Program7- Generate prime numbers between 1 &
given number
8. Program8- Pyramid of stars using nested for loops
9. Program9 - Reversed pyramid using for loops &
decrement operator.
10. Program10 - Nested Switch
11. Program11 - Calculate Circle Area using radius
12. Program12 - Factorial of a number using
recursion
13. Program13 - Pyramid of numbers using for loops
14. Program14 - To Find Maximum of Two Numbers.
15. Program15 - To Find Minimum of Two Numbers
using conditional operator
16. Program 16 - Write a program that will read a
float type value from the keyboard and print the
following output.
->Small Integer not less than the number.
->Given Number.
->Largest Integer not greater than the number.
17. Program 17 - Write a program to generate 5
Random nos. between 1 to 100, and it should not
follow with decimal point.
18. Program 18 - Write a program to display a greet
message according to Marks obtained by student
19. Program 19 - Write a program to find SUM AND
PRODUCT of a given Digit.
20. Program 20 - Write a program to find sum of all
integers greater than 100 and less than 200 that are
divisible by 7
21. Program 21 - Write a program to concatenate string
using for Loop
22. Program 22 - Program to Display Multiplication
Table
23. Program 23 - Write a program to Swap the values
24. Program 24 - Write a program to convert given
no. of days into months and days.(Assume that each
month is of 30 days)
25. Program 25 - Write a program to Display Invert
Triangle using while loop.
26. Program 26 - Write a program to find whether
given no. is Armstrong or not.
27. Program 27 - switch case demo
28. Program 28 - Write a program to generate
Harmonic Series.
29. Program 29 - Write a program to find average of
consecutive N Odd numbers and even numbers.
30. Program 30 - Display Triangle as follow: (using
for loops)
1
2 3
4 5 6
7 8 9 10 ... N */
Programs to work out
1.WAP to display a color name depending on color value
using switch.
2. Accepting single character, int, float, string and
double value from the keyboard.
3. To grade the students using switch and if-else.
4. To compute the power of 2 using for loop
5. To find the sum of the digits of a given integer
number.
6. Given the month , identify the season using switch.

Program1 – List of even numbers
/*
List Even Numbers Java Example
This List Even Numbers Java Example shows how to find and list even
numbers between 1 and any given number.
*/
public class ListEvenNumbers {
public static void main(String[] args) {
//define limit
int limit = 50;
System.out.println("Printing Even numbers between 1 and " +
limit);
for(int i=1; i <= limit; i++){
// if the number is divisible by 2 then it is even
if( i % 2 == 0){
System.out.print(i + " ");
}
}
}
}
/*
Output of List Even Numbers Java Example would be
Printing Even numbers between 1 and 50
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
*/
Program2 - Factorial of a number
/*
This program shows how to calculate
Factorial of a number.
*/
public class NumberFactorial {
public static void main(String[] args) {
int number = 5;
/*
* Factorial of any number is! n.
* For example, factorial of 4 is 4*3*2*1.
*/
int factorial = number;
for(int i =(number - 1); i > 1; i--)
4
{
factorial = factorial * i;
}
System.out.println("Factorial of a number is " + factorial);
}
}
/*
Output of the Factorial program would be
Factorial of a number is 120
*/
Program3 - Compare Two Numbers using else-if
/*
Compare Two Numbers Java Example
This Compare Two Numbers Java Example shows how to compare two numbers
using if else if statements.
*/
public class CompareTwoNumbers {
public static void main(String[] args) {
//declare two numbers to compare
int num1 = 324;
int num2 = 234;
if(num1 > num2){
System.out.println(num1 + " is greater than " + num2);
}
else if(num1 < num2){
5
System.out.println(num1 + " is less than " + num2);
}
else{
System.out.println(num1 + " is equal to " + num2);
}
}
}
/*
Output of Compare Two Numbers Java Example would be
324 is greater than 234
*/
Program4 - Determine If Year Is Leap Year
/*
Determine If Year Is Leap Year Java Example
This Determine If Year Is Leap Year Java Example shows how to
determine whether the given year is leap year or not.
*/
public class DetermineLeapYearExample {
public static void main(String[] args) {
6
//year we want to check
int year = 2004;
//if year is divisible by 4, it is a leap year
if(year % 400 == 0) || ((year % 4 == 0) && (year % 100 != 0))
System.out.println("Year " + year + " is a leap year");
else
System.out.println("Year " + year + " is not a leap year");
}
}
/*
Output of the example would be
Year 2004 is a leap year
*/
Program5 - Fibonacci Series
/* Fibonacci Series Java Example
This Fibonacci Series Java Example shows how to create and print
Fibonacci Series using Java.
*/
public class JavaFibonacciSeriesExample {
public static void main(String[] args) {
//number of elements to generate in a series
int limit = 20;
long[] series = new long[limit];
//create first 2 series elements
7
series[0] = 0;
series[1] = 1;
//create the Fibonacci series and store it in an array
for(int i=2; i < limit; i++){
series[i] = series[i-1] + series[i-2];
}
//print the Fibonacci series numbers
System.out.println("Fibonacci Series upto " + limit);
for(int i=0; i< limit; i++){
System.out.print(series[i] + " ");
}
}
}
/*
Output of the Fibonacci Series Java Example would be
Fibonacci Series upto 20
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
*/
Program6 - Palindrome Number
/*
This program shows how to check for in the given list of numbers
whether each number is palindrome or not
*/
public class JavaPalindromeNumberExample {
public static void main(String[] args) {
//array of numbers to be checked
int numbers[] = new int[]{121,13,34,11,22,54};
//iterate through the numbers
for(int i=0; i < numbers.length; i++){
int number = numbers[i];
int reversedNumber = 0;
int temp=0;
/*
* If the number is equal to it's reversed number, then
* the given number is a palindrome number.
*
* For ex,121 is a palindrome number while 12 is not.
*/
//reverse the number
while(number > 0){
temp = number % 10;
number = number / 10;
reversedNumber = reversedNumber * 10 + temp;
8
}
if(numbers[i] == reversedNumber)
System.out.println(numbers[i] + " is a palindrome");
else
System.out.println(numbers[i] + " not a palindrome ");
}
}
}
/*
Output of Java Palindrome Number Example would be
121 is a palindrome number
13 is not a palindrome number
34 is not a palindrome number
11 is a palindrome number
22 is a palindrome number
54 is not a palindrome number
*/
Program7- Generate prime numbers between 1 & given
number
/*
Prime Numbers Java Example
This Prime Numbers Java example shows how to generate prime numbers
between 1 and given number using for loop.
*/
public class GeneratePrimeNumbersExample {
public static void main(String[] args) {
//define limit
int limit = 100;
System.out.println("Prime numbers between 1 and " + limit);
//loop through the numbers one by one
for(int i=1; i < 100; i++){
boolean isPrime = true;
//check to see if the number is prime
for(int j=2; j < i ; j++){
if(i % j == 0){
isPrime = false;
break;
}
}
// print the number
if(isPrime)
System.out.print(i + " ");
}
9
}
}
/*
Output of Prime Numbers example would be
Prime numbers between 1 and 100
1 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
*/
Program8- Pyramid of stars using nested for loops
/*
Java Pyramid 1 Example
This Java Pyramid example shows how to generate pyramid or triangle
like given below using for loop.
*
**
***
****
*****
*/
public class JavaPyramid1 {
public static void main(String[] args) {
for(int i=1; i<= 5 ;i++){
for(int j=0; j < i; j++){
System.out.print("*");
}
//generate a new line
System.out.println("");
}
}
}
/*
Output of the above program would be
*
**
***
****
*****
*/
Program9 – Reversed pyramid using for loops &
decrement operator.
/*
10
Java Pyramid 5 Example
This Java Pyramid example shows how to generate pyramid or triangle
like given below using for loop.
12345
1234
123
12
1
*/
public class JavaPyramid5 {
public static void main(String[] args) {
for(int i=5; i>0 ;i--){
for(int j=0; j < i; j++){
System.out.print(j+1);
}
System.out.println("");
}
}
}
/*
Output of the example would be
12345
1234
123
12
1
*/
Program10 - Nested Switch
/*
Statements Example
This example shows how to use nested switch statements in a
java program.
*/
11
public class NestedSwitchExample {
public static void main(String[] args) {
/*
* Like any other Java statements, switch statements
* can also be nested in each other as given in
* below example.
*/
int i = 0;
int j = 1;
switch(i)
{
case 0:
switch(j)
{
case 0:
System.out.println("i is 0, j is 0");
break;
case 1:
System.out.println("i is 0, j is 1");
break;
12
default:
System.out.println("nested default
case!!");
}
break;
default:
System.out.println("No matching case found!!");
}
}
}
/*
Output would be,
i is 0, j is 1
*/
13
Program11 - Calculate Circle Area using radius
/*
This program shows how to calculate
area of circle using it's radius.
*/
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class CalculateCircleAreaExample {
public static void main(String[] args) {
int radius = 0;
System.out.println("Please enter radius of a circle");
try
{
//get the radius from console
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
radius = Integer.parseInt(br.readLine());
}
//if invalid value was entered
catch(NumberFormatException ne)
{
System.out.println("Invalid radius value" + ne);
System.exit(0);
}
catch(IOException ioe)
{
System.out.println("IO Error :" + ioe);
System.exit(0);
}
/*
* Area of a circle is
* pi * r * r
* where r is a radius of a circle.
*/
//NOTE : use Math.PI constant to get value of pi
double area = Math.PI * radius * radius;
System.out.println("Area of a circle is " + area);
}
}
/*
Output of Calculate Circle Area using Java Example would be
14
Please enter radius of a circle
19
Area of a circle is 1134.1149479459152
*/
Program12 - Factorial of a number using recursion
/*
This program shows how to calculate
Factorial of a number using recursion function.
*/
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class JavaFactorialUsingRecursion {
public static void main(String args[]) throws NumberFormatException,
IOException{
System.out.println("Enter the number: ");
//get input from the user
BufferedReader br=new BufferedReader(new
InputStreamReader(System.in));
int a = Integer.parseInt(br.readLine());
//call the recursive function to generate factorial
int result= fact(a);
System.out.println("Factorial of the number is: " + result);
}
static int fact(int b)
{
if(b <= 1)
//if the number is 1 then return 1
return 1;
else
//else call the same function with the value - 1
return b * fact(b-1);
}
}
/*
Output of this Java example would be
Enter the number:
5
Factorial of the number is: 120
*/
15
Program13 – pyramid of numbers using for loops
/*
Generate Pyramid For a Given Number Example
This Java example shows how to generate a pyramid of numbers for given
number using for loop example.
*/
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class GeneratePyramidExample {
public static void main (String[] args) throws Exception{
BufferedReader keyboard = new BufferedReader (new
InputStreamReader(System.in));
System.out.println("Enter Number:");
int as= Integer.parseInt (keyboard.readLine());
System.out.println("Enter X:");
int x= Integer.parseInt (keyboard.readLine());
int y = 0;
for(int i=0; i<= as ;i++){
16
for(int j=1; j <= i ; j++){
System.out.print(y + "\t");
y = y + x;
}
System.out.println("");
}
}
}
/*
Output of this example would be
Enter Number:
5
Enter X:
1
0
1 2
3 4 5
6 7 8 9
10 11 12 13 14
----------------------------------------------
17
Enter Number:
5
Enter X:
2
0
2 4
6 8 10
12 14 16 18
20 22 24 26 28
----------------------------------------------
Enter Number:
5
Enter X:
3
0
3 6
9 12 15
18 21 24 27
30 33 36 39 42
*/
18
Program14 – To Find Maximum of Two Numbers.
/*
To Find Maximum of 2 Numbers using if else
*/
class Maxoftwo{
public static void main(String args[]){
//taking value as command line argument.
//Converting String format to Integer value
int i = Integer.parseInt(args[0]);
int j = Integer.parseInt(args[1]);
if(i > j)
System.out.println(i+" is greater than "+j);
else
System.out.println(j+" is greater than "+i);
}
}
Program15 – To Find Minimum of Two Numbers using
conditional operator
/*
To find minimum of 2 Numbers using ternary operator
*/
class Minoftwo{
public static void main(String args[]){
//taking value as command line argument.
//Converting String format to Integer value
int i = Integer.parseInt(args[0]);
19
int j = Integer.parseInt(args[1]);
int result = (i<j)?i:j;
System.out.println(result+" is a minimum value");
}
}
20
Program 16
/* Write a program that will read a float type value from the keyboard and
print the following output.
->Small Integer not less than the number.
->Given Number.
->Largest Integer not greater than the number.
*/
class ValueFormat{
public static void main(String args[]){
double i = 34.32; //given number
System.out.println("Small Integer not greater than the number :
"+Math.ceil(i));
System.out.println("Given Number : "+i);
System.out.println("Largest Integer not greater than the number :
"+Math.floor(i));
}
Program 17 - Write a program to generate 5 Random nos.
between 1 to 100, and it should not follow with decimal
point.
class RandomDemo{
public static void main(String args[]){
for(int i=1;i<=5;i++){
System.out.println((int)(Math.random()*100));
}
}
21
}
Program 18 - Write a program to display a greet message
according to Marks obtained by student.
class SwitchDemo{
public static void main(String args[]){
int marks = Integer.parseInt(args[0]); //take marks
as command line argument.
switch(marks/10){
case 10:
case 9:
case 8:
System.out.println("Excellent");
break;
case 7:
System.out.println("Very Good");
break;
case 6:
System.out.println("Good");
break;
case 5:
System.out.println("Work Hard");
break;
case 4:
System.out.println("Poor");
break;
case 3:
22
case 2:
case 1:
case 0:
System.out.println("Very Poor");
break;
default:
System.out.println("Invalid value Entered");
}
}
}
Program 19 - Write a program to find SUM AND PRODUCT of
a given Digit.
class Sum_Product_ofDigit{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
//taking value as command line argument.
int temp = num,result=0;
//Logic for sum of digit
while(temp>0){
result = result + temp;
temp--;
}
System.out.println("Sum of Digit for "+num+" is : "+result);
//Logic for product of digit
temp = num;
23
result = 1;
while(temp > 0){
result = result * temp;
temp--;
}
System.out.println("Product of Digit for "+num+" is : "+result);
}
}
Program 20 - Write a program to find sum of all
integers greater than 100 and less than 200 that are
divisible by 7
class SumOfDigit{
public static void main(String args[]){
int result=0;
for(int i=100;i<=200;i++){
if(i%7==0)
result+=i;
}
System.out.println("Output of Program is : "+result);
}
}
Program 21 - Write a program to concatenate string
using for Loop
Example:
Input - 5
24
Output - 1 2 3 4 5 */
class Join{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
String result = " ";
for(int i=1;i<=num;i++){
result = result + i + " ";
}
System.out.println(result);
}
}
Program 22 - Program to Display Multiplication Table
class MultiplicationTable{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
System.out.println("*****MULTIPLICATION TABLE*****");
for(int i=1;i<=num;i++){
for(int j=1;j<=num;j++){
System.out.print(" "+i*j+" ");
}
System.out.print("\n");
}
}
}
25
Program 23 - Write a program to Swap the values
class Swap{
public static void main(String args[]){
int num1 = Integer.parseInt(args[0]);
int num2 = Integer.parseInt(args[1]);
System.out.println("\n***Before Swapping***");
System.out.println("Number 1 : "+num1);
System.out.println("Number 2 : "+num2);
//Swap logic
num1 = num1 + num2;
num2 = num1 - num2;
num1 = num1 - num2;
System.out.println("\n***After Swapping***");
System.out.println("Number 1 : "+num1);
System.out.println("Number 2 : "+num2);
}
}
26
Program 24 - Write a program to convert given no. of
days into months and days.(Assume that each month is of
30 days)
Example :
Input - 69
Output - 69 days = 2 Month and 9 days */
class DayMonthDemo{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
int days = num%30;
int month = num/30;
System.out.println(num+" days = "+month+" Month and "+days+" days");
}
}
Program 25 - Write a program to Display Invert Triangle
using while loop.
Example:
Input - 5
Output :
5 5 5 5 5
4 4 4 4
3 3 3
2 2
1
27
*/
class InvertTriangle{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
while(num > 0){
for(int j=1;j<=num;j++){
System.out.print(" "+num+" ");
}
System.out.print("\n");
num--;
}
}
}
Program 26 - Write a program to find whether given no.
is Armstrong or not.
Example :
Input - 153
Output - 1^3 + 5^3 + 3^3 = 153, so it is Armstrong no. */
class Armstrong{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
int n = num; //use to check at last time
int check=0,remainder;
while(num > 0){
remainder = num % 10;
28
check = check + (int)Math.pow(remainder,3);
num = num / 10;
}
if(check == n)
System.out.println(n+" is an Armstrong Number");
else
System.out.println(n+" is not a Armstrong Number");
}
}
Program 27 - switch case demo
Example :
Input - 124
Output - One Two Four */
class SwitchCaseDemo{
public static void main(String args[]){
try{
int num = Integer.parseInt(args[0]);
int n = num; //used at last time check
int reverse=0,remainder;
while(num > 0){
remainder = num % 10;
reverse = reverse * 10 + remainder;
num = num / 10;
}
29
String result=""; //contains the actual output
while(reverse > 0){
remainder = reverse % 10;
reverse = reverse / 10;
switch(remainder){
case 0 :
result = result + "Zero ";
break;
case 1 :
result = result + "One ";
break;
case 2 :
result = result + "Two ";
break;
case 3 :
result = result + "Three ";
break;
case 4 :
result = result + "Four ";
break;
case 5 :
result = result + "Five ";
break;
case 6 :
result = result + "Six ";
break;
30
case 7 :
result = result + "Seven ";
break;
case 8 :
result = result + "Eight ";
break;
case 9 :
result = result + "Nine ";
break;
default:
result="";
}
}
System.out.println(result);
}catch(Exception e){
System.out.println("Invalid Number Format");
}
}
}
Program 28 - Write a program to generate Harmonic
Series.
Example :
Input - 5
Output - 1 + 1/2 + 1/3 + 1/4 + 1/5 = 2.28 (Approximately) */
class HarmonicSeries{
31
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
double result = 0.0;
while(num > 0){
result = result + (double) 1 / num;
num--;
}
System.out.println("Output of Harmonic Series is "+result);
}
}
32
Program 29 - Write a program to find average of
consecutive N Odd no. and Even no.
class EvenOdd_Avg{
public static void main(String args[]){
int n = Integer.parseInt(args[0]);
int cntEven=0,cntOdd=0,sumEven=0,sumOdd=0;
while(n > 0){
if(n%2==0){
cntEven++;
sumEven = sumEven + n;
}
else{
cntOdd++;
sumOdd = sumOdd + n;
}
n--;
}
int evenAvg,oddAvg;
evenAvg = sumEven/cntEven;
oddAvg = sumOdd/cntOdd;
System.out.println("Average of first N Even no is "+evenAvg);
System.out.println("Average of first N Odd no is "+oddAvg);
}
}
33
Program 30 - Display Triangle as follow.
1
2 3
4 5 6
7 8 9 10 ... N */
class Output1{
public static void main(String args[]){
int c=0;
int n = Integer.parseInt(args[0]);
loop1: for(int i=1;i<=n;i++){
loop2: for(int j=1;j<=i;j++){
if(c!=n){
c++;
System.out.print(c+" ");
}
else
break loop1;
}
System.out.print("\n");
}
}
}
34
Extra Programs 1 - Write a program to Find whether
number is Prime or Not.
class PrimeNo{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
int flag=0;
for(int i=2;i<num;i++){
if(num%i==0)
{
System.out.println(num+" is not a Prime Number");
flag = 1;
break;
}
}
if(flag==0)
System.out.println(num+" is a Prime Number");
}
}
35
Program 2 - Write a program to find whether no. is
palindrome or not.
Example :
Input - 12521 is a palindrome no.
Input - 12345 is not a palindrome no. */
class Palindrome{
public static void main(String args[]){
int num = Integer.parseInt(args[0]);
int n = num; //used at last time check
int reverse=0,remainder;
while(num > 0){
remainder = num % 10;
reverse = reverse * 10 + remainder;
num = num / 10;
}
if(reverse == n)
System.out.println(n+" is a Palindrome Number");
else
System.out.println(n+" is not a Palindrome Number");
}
}
36
Program 3 - Display Triangle as follow
0
1 0
1 0 1
0 1 0 1 */
class Output2{
public static void main(String args[]){
for(int i=1;i<=4;i++){
for(int j=1;j<=i;j++){
System.out.print(((i+j)%2)+" ");
}
System.out.print("\n");
}
}
}

Some useful data validation code for NetBeans IDE

What is Data Validation?

Data Validation is the process of ensuring that a program operates on clean, correct and useful data. It uses routines, often called "validation rules" or "check routines", that check for correctness, meaningfulness, and security of data that are input to the system.
Example - A name text field should not allow user to enter numbers or characters such as '+','=',''&', etc. Mobile numbers should only include digits and '+' sign.

Some Useful Data Validation Code

1. Allowing only Numeric values in a text field

For this purpose we should use KeyTyped event handler of textField. Every KeyTyped event handler receives a KeyEvent object by default.

Char kc=evt.getKeyChar();

If(!(kc>=’0’ && kc<=’9’))

Evt.consume();

private void jTextField4KeyTyped(java.awt.event.KeyEvent evt) {                                    

        char kc = evt.getKeyChar();

        if (!(((kc >= '0') && (kc <= '9')) || kc == KeyEvent.VK_BACKSPACE)) {

            evt.consume();

        }

2. Checking for 'No Input'
   Users often miss out to enter data in some fields or sometimes they assume a necessary field to be an optional one. So, a program must check if the user has entered the required data or not. If the user has not provided the required data the program must not run further.
   Checking for empty textfields - You can check if the textfield is empty or not by the following coding:
   if (jTextField1.getText().isEmpty() || jTextField2.getText().isEmpty())
           JOptionPane.showMessageDialog(null, "One of the required field is empty!", "Error", JOptionPane.ERROR_MESSAGE);

or you can use the following method:

if (jTextField1.getText().length()==0 || jTextField2.getText().length()==0)

        JOptionPane.showMessageDialog(null, "One of the required field is empty!", "Error", JOptionPane.ERROR_MESSAGE);

Here, jTextField1 and jTextField2 are the fields which cannot be left empty. You can also notify user about the field that has been left empty by using if..else construct. The same coding, will work for jTextArea and jPasswordField.

3. Checking for 'No selection from list' 
Another run-time error occurs if the user does not select any value from the list component. User must be notified for the same.
if (jList1.getSelectedIndex()==-1)
       JOptionPane.showMessageDialog(null, "Please select a value from list");

This will show a message to user for a null value from list i.e no value selected by user.

4. Checking for 'No selection from Radio buttons' 
Checking if user has not selected any choice from a group of radio buttons.
Remember, always put radio buttons in a button group, otherwise all radio buttons could be selected at a particular time.

if (jRadioButton1.isSelected()==false && jRadioButton2.isSelected()==false)
        JOptionPane.showMessageDialog(null, "Please Select your Gender!");

or you can try the following code:

 if (jRadioButton1.isSelected())

        gender="Male";

    //Instead do your functioning!

    else if (jRadioButton2.isSelected())

        gender="Female";

    //Instead do your functioning!

    else

        JOptionPane.showMessageDialog(null, "Please Select your Gender!");

You can use the same piece of code for checkbox.

5. Prohibiting characters in numbers field
This code will help you to prohibit users from entering characters other than digits in numbers field like Moblie Number.
Write the following code on the KeyTyped event of a text field.

char k=evt.getKeyChar();
if (!(k>='0' && k<='9'))
        evt.consume();

Now, if a user types characters other than digits, the event will be consumed.
The below coding will prevent user from typing digits in a field placed for characters(eg- Name field)

char k=evt.getKeyChar();
if (k>='0' && k<='9')
        evt.consume();

6. Prohibiting multiple selection in a list
Set the selectionMode property of list to Single_Selection by the following code to prevent user from selecting multiple items from list. Default isMultiple_Interval_Selection.

jList1.setSelectionMode(javax.swing.ListSelectionModel.SINGLE_SELECTION);

Also, each GUI form must have a Exit and Clear button.

Exit Button -
System.exit(0);

Clear Button -

Clearing TextFields

jTextField1.setText("");
jTextField2.setText("");

Clearing selection from list

jList1.clearSelection();

Clearing selection from Combo Box

jComboBox1.setSelectedIndex(-1);

Clearing selection from Radio Buttons

buttonGroup1.clearSelection();

Clearing the contents of a table

DefaultTableModel model=(DefaultTableModel) jTable1.getModel();
    int rows=model.getRowCount();
    if (rows>0){
       for(int j=0; j<rows; j++){
           model.removeRow(0);
       }
   }