A.java

class A {

	public static boolean isPrime(int i) {
		if (i < 2) return false;
		for (int j = 2; j <= (int) Math.sqrt(i); ++j) {
			if (i % j == 0) return false;
		}
		return true;
	}

	public static void main(String... args) {
		int count = 0;
		for (int i = 0; i < 10_000_000; ++i) {
			if (isPrime(i)) ++count;
		}
		System.out.println(count);
	}
}

B.java

class B {

	public static boolean isPrime(int i) {
		if (i < 2) return false;
		final int limit = (int)Math.sqrt(i);
		for (int j = 2; j <= limit; ++j) {
			if (i % j == 0) return false;
		}
		return true;
	}

	public static void main(String... args) {
		int count = 0;
		for (int i = 0; i < 10_000_000; ++i) {
			if (isPrime(i)) ++count;
		}
		System.out.println(count);
	}
}

C.java

class C {

	public static boolean isPrime(int i) {
		if (i < 2) return false;
		for (int j = 2; j*j <= i; ++j) {
			if (i % j == 0) return false;
		}
		return true;
	}

	public static void main(String... args) {
		int count = 0;
		for (int i = 0; i < 10_000_000; ++i) {
			if (isPrime(i)) ++count;
		}
		System.out.println(count);
	}
}