我的问题在于updateItem函数，我想通过setup函数将ItemStack添加到创建的库存'inv'中。但是这会抛出一个NullPointerException。
这是我的代码：
public class Shop {
public static enum ShopMode { OPEN,CLOSED,FILLING }
private int shopID;
private Area area;
private ShopMode mode;
private boolean disabled;
private ArrayList<ItemStack> items = new ArrayList<ItemStack>();
private Inventory inv;
private Connection conn = null;
private PreparedStatement ps = null;
private Statement st = null;
private ResultSet rs = null;
private String name;
public Shop(int no) {
shopID = no;
updateItems();
setup();
}
private void setup() {
inv = Bukkit.createInventory(null, 36, "Shop "+name);
updateItems();
}
private void updateItems() {
try {
Class.forName("com.mysql.jdbc.Driver");
conn = DriverManager.getConnection("jdbc:mysql://localhost/odd","odd","odd");
String sql = "SELECT * FROM `items` WHERE shopid = ?";
ps = conn.prepareStatement(sql);
ps.setInt(1, shopID);
rs = ps.executeQuery();
while (rs.next()) {
String name = "";
if (rs.getString("name") != "") {
name = rs.getString("name");
System.out.println("Name: "+name);
}
Material material = Material.valueOf(rs.getString("type"));
int damage = rs.getInt("damage");
System.out.println("Damage: "+damage);
int count = rs.getInt("count");
System.out.println("Count: "+count);
//String[] ecs = null;
if (rs.getString("enchantment") != "") {
//ecs = rs.getString("enchantment").split(";");
}
ItemStack is = new ItemStack(material,count,(byte) damage);
//System.out.println(ecs.length);
System.out.println("Enchants: "+rs.getString("enchantment"));
/*if (ecs.length >= 1){
for (String enn : ecs) {
String[] enc = enn.split(enn);
is.addEnchantment(Enchantment.getByName(enc[0]),Integer.parseInt(enc[1]));
}
}*/
ItemMeta im = is.getItemMeta();
if (rs.getString("lore") != "") {
List<String> list = new ArrayList<String>();
String[] t = rs.getString("lore").split(";");
for (int i = 0;i<t.length;i++) {
list.add(t[i]);
}
if (rs.getDouble("sell") <= 0) {
list.add("Sorry, you can't sell this material to me.");
} else {
list.add("Sell: "+rs.getDouble("sell"));
}
if (rs.getDouble("buy") <= 0) {
list.add("Sorry, you can't buy this item from me.");
} else {
list.add("Buy: "+rs.getDouble("buy"));
}
im.setLore(list);
System.out.println("Lore: "+list.toString());
}
im.setDisplayName(name);
System.out.println("DisplayName: "+name);
is.setItemMeta(im);
System.out.println("ItemStack: "+is.toString());
inv.addItem(is); // Line 176
}
} catch (Exception e) {
e.printStackTrace();
} finally { close(); }
}
}
这是我的错误日志：
[00:32:42 INFO]: Name: LOG
[00:32:42 INFO]: Damage: 2
[00:32:42 INFO]: Count: 1
[00:32:42 INFO]: Enchants:
[00:32:42 INFO]: Lore: [, Sorry, you can't sell this material to me., Sorry, you
can't buy this item from me.]
[00:32:42 INFO]: DisplayName: LOG
[00:32:42 INFO]: ItemStack: ItemStack{LOG x 1, UNSPECIFIC_META:{meta-type=UNSPEC
IFIC, display-name=LOG, lore=[, Sorry, you can't sell this material to me., Sorr
y, you can't buy this item from me.]}}
[00:32:42 WARN]: java.lang.NullPointerException
[00:32:42 WARN]: at no.jovang.odd.Shop.updateItems(Shop.java:176)
[00:32:42 WARN]: at no.jovang.odd.Shop.<init>(Shop.java:39)
[00:32:42 WARN]: at no.jovang.odd.manager.ShopManager.loadShops(ShopManag
er.java:106)
[00:32:42 WARN]: at no.jovang.odd.manager.ShopManager.setup(ShopManager.j
ava:118)
[00:32:42 WARN]: at no.jovang.odd.Odd$Startup.run(Odd.java:120)
[00:32:42 WARN]: at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftTask.ru
n(CraftTask.java:71)
[00:32:42 WARN]: at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftSchedul
er.mainThreadHeartbeat(CraftScheduler.java:350)
[00:32:42 WARN]: at net.minecraft.server.v1_8_R1.MinecraftServer.z(Minecr
aftServer.java:709)
[00:32:42 WARN]: at net.minecraft.server.v1_8_R1.DedicatedServer.z(Dedica
tedServer.java:316)
[00:32:42 WARN]: at net.minecraft.server.v1_8_R1.MinecraftServer.y(Minecr
aftServer.java:634)
[00:32:42 WARN]: at net.minecraft.server.v1_8_R1.MinecraftServer.run(Mine
craftServer.java:537)
[00:32:42 WARN]: at java.lang.Thread.run(Unknown Source)
[00:32:42 INFO]: Name: LOG
[00:32:42 INFO]: Damage: 2
[00:32:42 INFO]: Count: 1
[00:32:42 INFO]: Enchants:
[00:32:42 INFO]: Lore: [, Sorry, you can't sell this material to me., Sorry, you
can't buy this item from me.]
[00:32:42 INFO]: DisplayName: LOG
[00:32:42 INFO]: ItemStack: ItemStack{LOG x 1, UNSPECIFIC_META:{meta-type=UNSPEC
IFIC, display-name=LOG, lore=[, Sorry, you can't sell this material to me., Sorr
y, you can't buy this item from me.]}}
答案 0 :(得分：0)
在Shop
的构造函数中public Shop(int no) {
shopID = no;
updateItems();
setup();
}
你应该删除对updateItems（）的调用。这是因为首先需要调用setup（）方法才能初始化 inv 成员变量。此外，由于setup（）调用updateItems（）本身，因此无需在构造函数中调用updateItems（）。
如果您进行此更改，那么当您的代码到达此行时
inv.addItem(is);
inv不会为null，这就是您所看到的异常的原因。
答案 1 :(得分：0)
当您尝试在值为null的对象上调用方法时，抛出NullPointerException。错误日志表明错误位于 176 行，即此行：
inv.addItem(is);
inv被抛出，因为null等于inv.addItem(is)。
当您的代码到达inv时，null没有值（它是.addItem(is)），因此，编译器不知道要调用inv的内容，导致你的插件抛出NullPointerException
您在setup()函数中初始化public Shop(int no){
shopID = no; //this works fine
updateItems(); //this causes the NPE - inv is not yet initialized
setup(); //inv is initialized here, so it is now safe to call updateItems()
}
，因此，而不是使用：
updateItems()您应该切换位置setup()和public Shop(int no){
shopID = no; //this works fine
setup(); //inv is initialized here, so it is now safe to call updateItems()
updateItems(); //it is safe to call this because 'inv' was just initialized
}
，然后使用：
setup()在初始化updateItems() 后，您的inv方法看起来似乎public Shop(int no){
shopID = no;
setup();
}
，因此您可以将NullPointerException缩短为：
public class Shop {
public static enum ShopMode { OPEN, CLOSED, FILLING }
//...
public Shop(int no){
shopID = no;
setup();
}
private void setup(){
inv = Bukkit.createInventory(null, 36, "Shop "+name);
updateItems();
}
private void updateItems(){
//...
}
}
最后，这就是您的代码应该是这样的：
{{1}}