java 如何正确声明子类的实例？
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How do I properly declare an instance of a subclass?
提问by James
I am currently making a text based adventure in Java for the purposes of using it a test platform, to try out new things I learn from this Java book I'm reading.
我目前正在 Java 中进行基于文本的冒险，目的是将其用作测试平台，以尝试从我正在阅读的这本 Java 书中学到的新事物。
I am now trying to declare an instance of a subclass (as the player is scripted to find it).
The parent class is Itemand it has two subclasses: Weaponand Armour.
我现在试图声明一个子类的实例（因为播放器被编写为找到它）。父类是Item，它有两个子类：Weapon和Armour。
However, no matter which way I try and declare it in, the IDE I'm using (Eclipse) flags the line with the following error:
但是，无论我尝试以哪种方式声明它，我正在使用的 IDE (Eclipse) 都会用以下错误标记该行：
No enclosing instance of type Item is accessible. Must qualify the allocation with an enclosing instance of type Item (e.g. x.new A() where x is an instance of Item).
无法访问 Item 类型的封闭实例。必须使用 Item 类型的封闭实例限定分配（例如 xnew A()，其中 x 是 Item 的实例）。
When I attempt to declare it like any of the following:
当我尝试将其声明为以下任何一项时：
Item machinePistol = new Weapon();
Weapon machinePistol = new Weapon();
Item machinePistol = new Item.Weapon();
Weapon machinePistol = new Item.Weapon();
For reference the item class looks like this:
作为参考，项目类如下所示：
package JavaAIO;
public class Item
{
public String itemName;
public double itemWeight;
public class Weapon extends Item
{
public double damage;
public double speed;
}
public class Armour extends Item
{
public double dmgResist;
public double attSpdMod;
}
}
So if anyone could tell me how I could properly instantiate a Weapon (so I can set the values of its fields and give it to the player), I would greatly appreciate it.
因此，如果有人能告诉我如何正确实例化武器（以便我可以设置其字段的值并将其提供给玩家），我将不胜感激。
回答by BalusC
"No enclosing instance of type Item is accessible. Must qualify the allocation with an enclosing instance of type Item (e.g. x.new A() where x is an instance of Item)."
“没有 Item 类型的封闭实例是可访问的。必须用 Item 类型的封闭实例限定分配（例如xnew A()，其中 x 是 Item 的一个实例）。”
It's pretty self-explaining:
这是不言自明的：
Item machinePistol = new Item().new Weapon();
Or:
或者：
Item item = new Item();
Item machinePistol = item.new Weapon();
However, I strongly recommend to put them in their own classes so that you can end up with:
但是，我强烈建议将它们放在自己的类中，以便您最终可以得到：
Item machinePistol = new Weapon();
回答by Joel
To do this:
去做这个：
Item machinePistol = new Item();
Weapon machinePistol = new Item.Weapon();
you would want to declare your inner classes as static:
你想将你的内部类声明为static：
public static class Weapon extends Item
{
public double damage;
public double speed;
}
public static class Armour extends Item
{
public double dmgResist;
public double attSpdMod;
}
Or another (and better) option is to not make them inner classes:
或者另一个（更好）的选择是不让它们成为内部类：
public class Item {}
public class Weapon extends Item {}
public class Armour extends Item {}
Then you can make them like this;
然后你可以把它们做成这样；
Item machinePistol = new Item();
Item machinePistol = new Weapon();
Item someArmour = new Armour();
回答by Hosam Aly
You shouldn't declare the classes Weaponand ArmourinsideItem. Just declare them outside:
你不应该声明 classesWeapon和ArmourinsideItem。只需在外面声明它们：
public class Item {
public String itemName;
public double itemWeight;
}
public class Weapon extends Item {
public double damage;
public double speed;
}
public class Armour extends Item {
public double dmgResist;
public double attSpdMod;
}
So you can instantiate them like this:
所以你可以像这样实例化它们：
Weapon machinePistol = new Weapon();
