java - NullPointerException 但编译？

Question

我正在编写一个简单的命令行游戏。 我已经有很多功能了，这里只发布必要的功能。

问题:程序可以编译，但是当调用 levelup() 并选择一个数字时，我得到:

        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)

这是我的代码:

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();

}
}

有人能指出我正确的方向吗？ 我究竟做错了什么？我感觉这是“Player”类和“Game”类中创建的对象的类问题。

Answer 1

您会得到一个 NullPointerException，因为 p 是 null。您在这里所做的事情:

Player p = new Player(nome);

声明一个本地变量p。静态类变量 p 未受影响，因此它仍为 null。

这称为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.

删除Player，因此对静态类变量p的引用就是您想要的:

p = new Player(nome);