This example shows how to check if a number is a prime number in Java. This example also shows how to find prime numbers between 1 and 100 or between any two numbers.
What is a prime number?
Definition of a prime number from Wikipedia,
A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.
How to check if a number is a prime number in Java?
If we cannot divide a number (greater than 1) by any positive number other than 1 or itself, then the number is a prime number. We will use the modulus operator (%) to check if we can divide the number as given below.
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package com.javacodeexamples.basic; public class CheckIfNumberIsPrimeExample { public static void main(String[] args) { //an int array of input numbers to check int[] numbers = { 2, 15, 17, 55, 63, 76, 83, 91, 99 }; //call the method to check if a number is prime number for(int number : numbers){ System.out.println( number + ": " + isPrimeNumber(number) ); } } private static boolean isPrimeNumber(int number) { //number should be greater than 1 if(number < 2) return false; for(int i = 2; i < number / 2; i++){ //if number is perfectly divisible, it is not prime if(number % i == 0) return false; } return true; } } |
Output
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1 2 3 4 5 6 7 8 9 |
2: true 15: false 17: true 55: false 63: false 76: false 83: true 91: false 99: false |
As you may have observed from the above code, the for loop inside the isPrimeNumber method goes only up to number/2. That is because any number cannot be perfectly divided by anything above half of that number.
How to find the prime numbers between 1 and 100?
We will do a slight modification to the above program to find prime numbers between 1 and 100 as given below.
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package com.javacodeexamples.basic; public class CheckIfNumberIsPrimeExample { public static void main(String[] args) { for( int i = 2; i <= 100 ; i++){ boolean isPrime = true; for( int j = 2; j <= i/2; j++){ if(i % j == 0){ isPrime = false; break; } } if(isPrime) System.out.print(i + " "); } } } |
Output
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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 |
Here, we have introduced a new loop that goes through 2 to 100 and checks the number one by one. The inner loop goes from 2 to half of the number and tries to divide the number. If a reminder of any of the divisions is zero then the number is not a prime number.
How to find all prime numbers between 1 to n?
In the above example, we printed prime numbers between 1 and 100. Now, we are going to take input from the user using the Scanner and print the prime numbers between 2 and the input number as given below.
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package com.javacodeexamples.basic; import java.util.Scanner; public class CheckIfNumberIsPrimeExample { public static void main(String[] args) { //create a scanner Scanner s = new Scanner(System.in); System.out.println("Input a number"); //get the int value from the console int n = s.nextInt(); //loop up to user input value for( int i = 2; i <= n ; i++){ boolean isPrime = true; for( int j = 2; j <= i/2; j++){ if(i % j == 0){ isPrime = false; break; } } //print if it is a prime number if(isPrime) System.out.print(i + " "); } //close the scanner s.close(); } } |
Output
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1 2 3 |
Input a number 50 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 |
This example is a part of the Java Basic Examples. Please also visit the Java String tutorial to learn more about strings in Java.
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