Java syntax issues

Question

I'm trying to do a Josephus problem, but I'm not allowed to use code snippets from other people. With this in mind, I have 27 errors in my code, but can't figure out how to fix it. Would you wonderful people explain to me why it won't compile. I want to see if my logic is flawed, but I can't test it because it won't compile. Any other tips are more than welcome! Here is my code:

import java.util.*;

public class Josephus
{
    public class Link
    {
        public int num;
        public Link next;

        public Link (int d)
        {
            num = d;
            next = null;
        }
    }

    public class Main
    {

        Scanner in = new Scanner(System.in);
        System.out.println("How many players");

        int numPlayers = in.nextInt();
        Link first, last;
        first = last = new Link (1);

        for(int k=2; k<=numPlayers; k++)
        {
            last.next = new Link(k);
            last = last.next;
        }
        last.next = first;

        System.out.println("How many skips");
        int m = in.nextInt();

        for (int g=0; g<numPlayers; g++)
        {
            for (int k=0; k<=m; k++);
            {
                last = last.next;
            }
            last.next;
            last = last.next;
        }
    }
}

Answer 1

I cleaned up a bit your code so it compiles, you might be better off with two classes

Josephus.java

import java.util.Scanner;

public class Josephus {

public static void main(String[] args) {

    Scanner in = new Scanner(System.in);
    System.out.println("How many players");

    int numPlayers = in.nextInt();
    Link first = new Link(1);
    Link last = new Link(1);

    for (int k = 2; k <= numPlayers; k++) {
        last.next = new Link(k);
        last = last.next;
    }
    last.next = first;
    first.next = last;

    System.out.println("How many skips");
    int m = in.nextInt();

    for (int g = 0; g < numPlayers; g++) {
        for (int k = 0; k <= m; k++)
        {
            last = last.next;
        }
        // last.next;
        last = last.next;
    }

    in.close();
}
}

and Link.java

public class Link {

    public int num;
    public Link next;

    public Link(int d) {
        num = d;
        next = null;
    }
}

This compiles and accepts input, then throws an error. I haven't fixed that since I don't know exactly what you're trying to achieve.

Changes:

1. Changed `"class main"` to `public static void main(){...)` The Java runtime will look for this method when it's called.
2. Extracted class `Link` to it's own file other solutions are possible, like some other answers say, you could declare it as static or instantiate `Josephus` (and probably some other ways)
3. Commented out the line `last.next`, this doesn't actually solve any problem, it just eliminates the compilation error and allows you to compile, since I don't see what you're trying to do here I couldn't think of a better solution.
4. It's not necessary to compile but I added the line `in.close();` at the end of the main method to free allocated resources, without this line it'll throw a compile warning.
5. Added `first.next = last;` to avoid the null pointer exception that was happening.

All that you should need now is to implement the Josephus logic inside the nested loops and output your results (if desired) all of which should be quite language agnostic.

Answer 2

Instead of

public class Main

You want

public static void main(String [] args)

Answer 3

One other problem of with your code is that you have an non static inner class Link and you try to instantiate it without an enclosing instance of the containing class Josephus.

So either make is static

public static class Link

or instantiate it using an instance of enclosing class

Josephus outer = new Josephus();
first = last = outer.new Link (1);
// ...
last.next = outer.new Link(k);

Better yet, move the class into a own compilation unit, i.e a file Link.java until you learn about inner classes.

And the line

last.next;

is not correct. What are you trying to do with last.next? You have to assign it to some other variable. I don't think you need it anyways, just remove it.