NullPointerException but compiling?

问题:

I'm writing a simple command line game. I've got many functions and all, and will only post the essential here.

Problem: The program compiles but when levelup() is called and a number is chosen, I get this:

        You have 5 skill points to spend.
        What would you like to upgrade?
        [1:]    STR                     [2:]    DEF
1
Exception in thread "main" java.lang.NullPointerException
        at Game.levelup(cmdquest.java:300)
        at Game.start(cmdquest.java:336)
        at Menu.show_menu(cmdquest.java:195)
        at cmdquest.main(cmdquest.java:263)

Here is my code:

class Player{
    String name;
    int hp;
    int str;
    int def;
    String  eff;

    Player(String n) {
    name = n;
    hp = 100;
    str = 1;
    def = 1;
    eff = "none";
    }
}


class Game{

    static Player p;

    static void levelup(){
        while (points > 0){
            System.out.println("\t[1:]\tSTR\t\t\t[2:]\tDEF");
            int lvlup = kb.nextInt();

            switch (lvlup){
                case 1: p.str++;
                    break;
                case 2: p.def++;
                    break;
            }

            points--;
        }

    //variables
    static Scanner kb = new Scanner(System.in);
    static int points = 5;

    }

static void start(){

        System.out.print("\t\t\t\tAnd so our adventure starts.....");

        System.out.print("\tWhat's your name: ");
        String nome = kb.next();
        Player p = new Player(nome);

        System.out.println("\tHello " + p.name);
        System.out.println("\tYou have 5 skill points to spend.\n\tWhat would you like to upgrade?");
        levelup();

    }



class cmdquest{

public static void main(String args[]) throws Exception{

    Scanner kb = new Scanner(System.in);

    //Importing foes.txt to create objects of foes
    java.io.File file = new java.io.File("foes.txt");
    Scanner imp = new Scanner(file);  

    for(int i =0; i<3; i++){
        foes[i]=foe.leDados(imp);
    }
    //____________________________________________

    Game.start();

}
}

Can anyone point me in the right direction here? What am I doing wrong? I sense it's a class problem with the class "Player" and the object being created in the "Game" class.

回答1:

You get a NullPointerException because p is null. What you've done here:

Player p = new Player(nome);

is declare a local variable p. The static class variable p is untouched, so it remains null.

This is called shadowing (JLS, Section 6.4.1):

Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

...

A declaration d of a type named n shadows the declarations of any other types named n that are in scope at the point where d occurs throughout the scope of d.

Remove Player, so the reference to the static class variable p is what you want:

p = new Player(nome);