Sieve of Eratosthenes Solution Added #295

Java/SieveOfEratosthenes.java

@@ -0,0 +1,60 @@
+public class SieveOfEratosthenes {
+		//Driver Code
+	public static void main(String[] args) {
+
+		Scanner input= new Scanner(System.in);
+		System.out.println("Enter a number : ");
+		int a= input.nextInt();
+		System.out.println("The number of primes =");
+		int primes = countPrimes(a);
+		System.out.println(primes);
+
+
+
+	}
+	public static int countPrimes(int n) {
+
+		if(n<2) return 0;
+		//checking for 0 & 1
+		boolean[] composites = new boolean[n];
+		//Array of composites --> true represents composites and false represents prime
+		int limit = (int)Math.sqrt(n);
+		for(int i=2; i<=limit;i++) {
+			if(composites[i]==false) {
+				//mark all the multiples of i as true.
+				//The first index to be flipped to true is i*i
+				for(int j= i*i; j<n; j+=i) {
+					composites[j]=true;
+				}
+
+			}
+		}
+		int count=0;
+		for (int i=2;i<n;i++) {
+			if(composites[i]==false) {
+				count++;
+			}
+		}
+
+
+		return count;
+	}
+
+}
+
+// TestCases
+//
+// #1
+//Input = 12
+//Expected Output  = 5
+//Actual Output = 5
+
+//#2
+//Input = 45
+//Expected Output  = 14
+//Actual Output = 14
+
+//#3
+//Input = 5446
+//Expected Output  = 720
+//Actual Output = 720