Java类无法编译

Question

Im quite the novice when it comes to programming and im trying to translate this PHP algorithm to Java.

function isPrime($n)
{
$i = 2;

if ($n == 2) {
    return true;    
}

while ($i < $n) {
    if ($n % $i == 0) {
        return false;
    }
    $i++;
}

return true;
}
for ($i = 3; $i < 100; $i++) {
if (isPrime($i)) {
    echo $i;
}
}

The only thing i've come up with so far is this.

public class Primtal {
public static boolean isPrime(int n) 
{
    int i = 2;

    if (n == 2) {
        return true;    
    } 
    while (i < n) {
        if ( n % i == 0) {
            return false;
        }
        i++;
    }

    return true;
}

for(int i = 3; i < 1000; i++){
    if (isPrime(i)) {
        System.out.print(i);
    }
}
}

I realize this look really stupid but i really need to get this to work. I think the problem lies mostly with the for loop as im currently getting the error illegal start of type there. Im not really sure how to translate this to Java and i would appreciate any help i can get.

Answer 1

I believe the problem with your code is that you've put a for loop in the middle of class declaration, which is incorrect - it needs to be inside some method. It seems logical in this case to put it in main() , so it's executed when you run your program. Maybe something like this:

public class Primtal
{
    public static boolean isPrime(int n) 
    {
        int i = 2;

        if(n == 2)
        {
            return true;    
        } 

        while(i < n)
        {
            if(n % i == 0)
            {
                return false;
            }

            i++;
        }

        return true;
    }

    public static void main(String[] args)
    {
        for(int i = 3; i < 1000; i++)
        {
            if(isPrime(i))
            {
                System.out.print(i);
            }
        }
    }
}

(Note the addition of public static void main(String[] args) in the second half of the code.)

Oracle has official tutorials on how Java programs need to be structured, and other basics of the language. You can find the one related to the main method here . Or, to start from the beginning, the full tutorial starts here .

Answer 2

you can't write the for loop

for(int i = 3; i < 1000; i++){
    if (isPrime(i)) {
        System.out.print(i);
    }
}

directly inside a class .

i believe what you wish to do is to have a main method, in which you can have the for loop

Answer 3

Your for loop needs to be within a method of some sort,so you can put it in the main method:

public class Primtal {

    public static void main(String [] args)
    {                
        for(int i = 3; i < 1000; i++)
        {
            if (isPrime(i)) {
            System.out.print(i);
        }
    }

    public static boolean isPrime(int n) 
    {
        int i = 2;

        if (n == 2) {
            return true;    
        } 
        while (i < n) {
            if ( n % i == 0) {
                return false;
            }
            i++;
        }

        return true;
    }

}

Answer 4

The problem is that your for loop isn't in a method. Enclose it in a main method.

public static void main(String[] args) {
   // Your for loop here
}

Also, change print to println , or else all the numbers will appear concatenated together on one line.