How to use goto statement correctly

Question

I am taking my high school AP Computer Science class.

I decided to throw a goto statement into a one of our labs just to play around, but I got this error.

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Syntax error on token "goto", assert expected
    restart cannot be resolved to a variable
at Chapter_3.Lab03_Chapter3.Factorial.main(Factorial.java:28)

I went to a goto question on Stackoverflow to find out how to do it properly, and I did exactly as was demonstrated in one of the answers. I really don't understand why the compiler wants an assert statement (at least that's what I assume it wants), nor do I have any idea how to use assert. It seems to want the restart part of goto restart; to be a variable, but restart is just a label that pulls the program back up to line 10 so that the user can enter a valid int. If it wants restart to be a variable, how do I do that?

import java.util.*;

public class Factorial 
{
    public static void main(String[] args) 
    {
        int x = 1;
        int factValue = 1;
        Scanner userInput = new Scanner(System.in);
        restart:
        System.out.println("Please enter a nonzero, nonnegative value to be factorialized.");
        int factInput = userInput.nextInt();

        while(factInput<=0)
        {
            System.out.println("Enter a nonzero, nonnegative value to be factorialized.");
            factInput = userInput.nextInt();
        }

        if(x<1)//This is another way of doing what the above while loop does, I just wanted to have some fun.
        {
            System.out.println("The number you entered is not valid. Please try again.");
            goto restart;
        }
        while(x<=factInput)
        {
            factValue*=x;
            x++;
        }
        System.out.println(factInput+"! = "+factValue);
        userInput.close();
    }
}

Answer 1

As already pointed out by all the answers goto - a reserved word in Java and is not used in the language.

restart: is called an identifier followed by a colon.

Here are a few things you need to take care of if you wish to achieve similar behavior -

outer:                  // Should be placed exactly before the loop
loopingConstructOne  {  // We can have statements before the outer but not inbetween the label and the loop          
    inner:
    loopingConstructTwo {
        continue;       // This goes to the top of loopingConstructTwo and continue.
        break;          // This breaks out of loopingConstructTwo.
        continue outer; // This goes to the outer label and reenters loopingConstructOne.
        break outer;    // This breaks out of the loopingConstructOne.
        continue inner; // This will behave similar to continue.
        break inner;    // This will behave similar to break.
    }
}

I'm not sure of whether should I say similar as I already have.

Answer 2

The Java keyword list specifies the goto keyword, but it is marked as "not used".

This was probably done in case it were to be added to a later version of Java.

If goto weren't on the list, and it were added to the language later on, existing code that used the word goto as an identifier (variable name, method name, etcetera) would break. But because goto is a keyword, such code will not even compile in the present, and it remains possible to make it actually do something later on, without breaking existing code.