Boolean return value not accepted in prime check method

I am a java beginner but I have a bit of experience in python. To learn more, I try to recreate some of the things I did in python in java to see how it is different. My idea: create a method that checks whether a given integer parameter is a prime number. Again, this can probably be done easier/more efficient etc., but I see it as a training exercise. This is why I do it with two different for loops (so just for the sake of using for loops ).
However, I want the method to return true or false.
When I declare it as in my code below, I get an exception (unsresolved compilation problem), because my method should return a boolean value.
If I change the method to void, I get an error saying my method isn't supposed to return anything.

So what is it... is it returning something or not? And why does this not count as returning a boolean?

I use Win10 and the eclipse IDE.

Thank you!

PS: This is also to try out how to properly set up arrays, as I have zero experience with them.

The Code:

public class AB4Funk {
    public static boolean ab40(int n) {
	    int x = n/2+1;
	    int[] array = new int[x];
	    for(int i=2; i<x; i++) {
		    array[i-2]=i;
	    }
	    for(int zahl : array) {
		    if(n%zahl==0) {
			    return true;
		    }
		    else {
		    	return false;
		    }
	    }
	
    }
}

After some very helpful comments, my new code is:

public class AB4Funk {
	public static boolean ab40(int n) {
		//int x = n/2+1;
		int[] array = new int[n];
		for(int i=2; i<n; i++) {
			array[i-2]=i;
		}
		for(int zahl : array) {
			if(n%zahl==0) {
				return false;
			}
		}
		return true;
		
	}
}

Which gives me a division by zero error, which I cannot locate. (for debugging I commented out the x (which I don't really need anyways) to see if it is the source of my error)

There are two main problems with your method:

• it is possible that the loop is never executed, e.g. if the input number is smaller than 2, thus resulting in the method never returning anything
• if the loop is executed, the method will return true or false after the very first iteration, i.e. after testing the first potential divisor

You can fix both problem by moving the return false after the loop, i.e. if the number is not evenly divisible by any of those numbers.

Also, you may consider inverting true and false, since you want to test whether a number is prime, and also call your method accordingly. Finally, you do not have to store all the potential divisors in an array first but can just test them directly.

public static boolean prime(int n) {
    int x = n/2+1; // actually, testing until x = sqrt(n) would be enough
    for (int zahl = 2; zahl < x, zahl++) {
        if (n % zahl == 0) {
            return false; // not prime
        }
    }
    return true; // is prime
}

As mentioned in comments, you may also want to change the upper bound x for numbers to test from n/2 to sqrt(n) (or, conversely, change the check in the loop to zahl * zahl <= n). After all, if the number is evenly divisible by a number > sqrt(n), it also has to be evenly divisible by a number < sqrt(n). This may seem like a minor improvement, like skipping even divisors after having tested for 2, but it's actually equivalent to changing an O(n²) algorithms to O(n) and quite significant for testing larger numbers.