# Java Challenge - Prime Number Finder

This community-built FAQ covers the “Prime Number Finder” code challenge in Java. You can find that challenge here, or pick any challenge you like from our list.

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``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
if(n<2) {
}
for (int i=2; i<=n; i++){
for(int j=2; j<=i; j++){
if(i%j==0 && i!=j){
break;
}else if(i==j){
}
}
}

}
}
``````
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList<Integer> arr = new ArrayList<>();

if(n == 1)
{
return arr;
}

for(int i = 1; i <= n; i++)
{
int counter = 0;
for(int j = 2; j <= i/2; j++)
{
if(i % j == 0)
{
counter++;
}
}

if(counter == 0 && i != 1)
{
}
}

return arr;
}
}
``````
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList<Integer> integerArray = new ArrayList<Integer>();

for (int i = 1; i <= n; i++) {
int count = 0;
for (int j = i; j >= 1; j--) {
if (i % j == 0) {
count++;
}
}
if (count == 2) {
}

}
return integerArray;
}
}
``````
``````import java.util.ArrayList;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList <Integer> out = new ArrayList<>();
for (int i = 2; i <= n; i++){
boolean isPrime = true;
for (int j = 2; j < i; j++){
if (i % j == 0) {
isPrime = false;
break;
}
}
}
return out;
}
}
``````
1 Like
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList result = new ArrayList();
for(int i=2;i<=n;i++){
if(isPrime(i)){
}
}
return result;
}

public static boolean isPrime(int n) {
int count = 0;
boolean result;
for(int i=1;i<=n;i++){
if(n%i==0){
count++;
}
}
System.out.printf("%d%d\n", count, n);
result = count == 2?true:false;
return result;
}
}
``````
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList<Integer> primeList = new ArrayList<Integer>();
for (int i=2;i<=n; i++) {
if (i==2) {
} else {
boolean divisible = false;
for (int j : primeList) {
if (i % j == 0) {
divisible = true;
break;
}
}
if (!divisible) {
}
}
}
return primeList;
}
}
``````
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList<Integer> intArrayList = new ArrayList<Integer>();
if(n < 2){
return intArrayList;
}
for(int i = 2; i <= n; i++){
boolean isPrime = true;
for(int j = 2; j < n/2; j++){
if(i == j) continue;
if(i % j == 0){
isPrime = false;
break;
}
}
if(isPrime){
}
}
return intArrayList;
}
}
``````
``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {

ArrayList<Integer> primeNo = new ArrayList<Integer>();

for(int i=2; i <= n; i++){
if((i%2!=0 && i%3!=0 && i%5!=0 && i%7!=0) || i == 2 || i == 3 || i == 5 || i == 7){
}
}
return primeNo;
}
}
``````

I tried the Sieve of Erastothenes algorithm for this challenge.

``````  public static boolean[] SieveOfErastothenes(int limit) {
if (limit < 0) {
return new boolean[0];
}
else if (limit == 0) {
return new boolean[]{false};
}
else if (limit == 1) {
return new boolean[]{false, false};
}
boolean[] isPrimes = new boolean[limit + 1];
Arrays.fill(isPrimes, true); // set all values to true
isPrimes[0] = false;
isPrimes[1] = false;
double maxFactor = Math.sqrt(limit);
for (int i = 2; i <= maxFactor; i++) {
if (isPrimes[i]) {
for (int j = i * i; j <= limit; j += i) {
isPrimes[j] = false;
}
}
}
return isPrimes;
}

public static ArrayList<Integer> primeFinder(int n) {
ArrayList<Integer> primes = new ArrayList<Integer>();
boolean[] sieve = SieveOfErastothenes(n);
for (int i = 2; i <= n; i++) {
if (sieve[i]) {
}
}
return primes;
}
``````

import java.util.*;

public class PrimeFinder {
public static void main(String args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
int flag=0;
ArrayList res = new ArrayList<>();
for(int i = 2;i<=n;i++){
for(int j=2;j<=i/2;j++){
if(i%j==0){
flag=1;
break;
}
}
if(flag==0)
flag=0;
}
return res;
}
}

``````import java.util.*;

public class PrimeFinder {
public static void main(String[] args) {
System.out.println(primeFinder(13));
}

public static ArrayList primeFinder(int n) {
ArrayList<Integer> primes = new ArrayList<>();

if (n < 2) {
return primes;
}
for (int i = 2; i <= n; i++) {
if (isPrime(i)) {
}
}
return primes;
}
public static boolean isPrime(int number) {
for (int i = 2; i < number; i++){
if (number % i == 0) {
return false;
}
}
return true;
}
}
``````