"Cannot subclass the final class" 错误，但 class 不是最终的

Question

这是我的代码：

package basic;

public abstract class Entity {}

package characters;

import basic.Entity;

public abstract class Character extends Entity {}

package player;

public class Player extends Character {}

我得到了

The type Player cannot subclass the final class Character.

但我检查了一百万次，但我还没有在我的项目中使用 final 除了 ONCE。给出了什么？

Answer 1

您正在扩展 java.lang.Character（不需要导入，因为它来自 java.lang）。

将 import characters.Character 插入您的 Player 代码。

参考：using package members：

For convenience, the Java compiler automatically imports two entire packages for each source file: (1) the java.lang package and (2) the current package (the package for the current file).

Answer 2

字符是 java.lang 的 class（"char" 的包装 class）。您必须在播放器 class

中导入 characters.Character

package player;
import characters.Character

public class Player extends Character {

}

Answer 3

字符是最终的 class，如 Java 文档中所定义：

public final class Character
extends Object
implements Serializable, Comparable<Character>

所以它不能被子class编辑。

您正在从这个 class 字符隐式导入中收到错误。 小心！.

Answer 4

在这种情况下，我强烈建议使用the fully qualified name of the Character class in the extends clause。

public class Player extends characters.Character {}

有经验的 Java 开发人员知道 java.lang.Character is final 并且不能因此扩展。通过写class Player extends Character，你可能会让他们不知所措。

Every compilation unit implicitly imports every public type name declared in the predefined package java.lang, as if the declaration import java.lang.*; appeared at the beginning of each compilation unit immediately after any package declaration. As a result, the names of all those types are available as simple names in every compilation unit.

^{Java 11 Specification > 7. Packages and Modules > 7.3. Compilation Units}

当然，从标准 java.lang 包中选择一个不与类冲突的名称（如 Person，或 GameCharacter).

"Cannot subclass the final class" 错误，但 class 不是最终的

"Cannot subclass the final class" error, but the class is not final

java

inheritance

package

final-class