Java 空指针异常

Question

我尝试了 printStackTrace 并将所有内容都隐藏为静态(我认为)...但是，第 17 行和第 38 行是问题...因为这个错误:

You picked up: Pickaxe
java.lang.NullPointerException
        at item.addInv(item.java:38)
        at item.main(item.java:17)
Description: Can be used to mine with.
Press any key to continue . . .

第 17 行:anItem.addInv(1);

第 38 行:arr.add("Dan");

这是我的代码:

import java.io.*;
import java.util.*;
import javax.swing.*;

public class item
{
    public static int attack, defense;
    public static ArrayList<String> arr;
    public static String name, desc, typeOf, attackAdd, defenseAdd, canSell, canEat,earnedCoins,canEquip;

    String stats[];

    public static void main(String args[])
    {
        item anItem = new item();
        ArrayList<String> arr = new ArrayList<String>();
        anItem.addInv(1);
    }

    public static void addInv(int e) {
        String iname = getItem(1)[0];
        String idesc = getItem(1)[1];
        int itypeOf = Integer.parseInt(getItem(1)[2]);
        int iattackAdd = Integer.parseInt(getItem(1)[3]);
        int idefenseAdd = Integer.parseInt(getItem(1)[4]);
        boolean icanSell = Boolean.parseBoolean(getItem(1)[5]);
        boolean icanEat = Boolean.parseBoolean(getItem(1)[6]);
        int iearnedCoins = Integer.parseInt(getItem(1)[7]);

        attack = attack + iattackAdd;
        defense = defense + idefenseAdd;
        System.out.println("You picked up: " + iname);
        try {
            arr.add("Dan");
        } catch(NullPointerException ex) {
            ex.printStackTrace();
        }

        System.out.println("Description: " + idesc);
    }

    public static String[] getItem(int e) {

        String[] stats = new String[7];

        String name = "Null";
        String desc = "None";
        String typeOf = "0";
        String attackAdd = "0";
        String defenseAdd = "0";
        String canSell = "true";
        String canEat = "false";
        String earnedCoins = "0";

        if (e == 1) {
            name = "Pickaxe";
            desc = "Can be used to mine with.";
            typeOf = "2";
            attackAdd = "2";
            earnedCoins = "5";
        }

        return new String[] { name, desc, typeOf, attackAdd, defenseAdd, canSell, canEat, earnedCoins};
    }
}

如您所见，就是那些行，我不知道该怎么做...:\

Answer 1

当您在 arr 上调用 add() 方法时，它尚未初始化，因此会出现 NullPointerException。

由于您可能还会在其他方法中使用 ArrayList，因此您应该在构造函数中对其进行初始化；即:

public item() {
   arr = new ArrayList<String>();
}