java - 为什么 java.lang.Cloneable 没有覆盖 java.lang.Object 中的 clone() 方法？

Question

java.lang.Cloneable 接口(interface)的 Java 规范将自身定义为表示任何扩展它的对象也已经实现了 clone() 方法，该方法在其中处于 hibernate 状态java.lang.Object。具体来说，它说:

A class implements the Cloneable interface to indicate to the java.lang.Object#clone() method that it is legal for that method to make a field-for-field copy of instances of that class.

对我来说，这意味着应该假设每个扩展 Cloneable 的类因此也有一个 public Object clone() 方法。这使得很容易假设以下是一个有效的方法:

public static makeACloneFrom(Cloneable c)
{
  return c.clone();
}

然而，事实并非如此，因为整个 Cloneable 源代码(没有 javadoc)很简单

package java.lang;

public interface Cloneable {
}

这意味着 Cloneable#clone() 不存在(并且尝试编译上面的示例方法会引发编译时错误，提示类似“找不到符号:方法克隆( )")。 Cloneable 的源代码不应该包含一些类似于 public Cloneable clone(); 的东西吗？

为什么我们不允许假设实现 Cloneable 的类具有 public Cloneable clone() 方法？

Answer 1

因为它是一个设计糟糕的界面。

来自 Effective Java (抱歉，Google 图书没有第 2 版的预览):

Item 11: Override clone judiciously

The Cloneable interface was intended as a mixin interface (Item 18) for objects to advertise that they permit cloning. Unfortunately, it fails to serve this purpose. Its primary flaw is that it lacks a clone method, and Object's clone method is protected. You cannot, with resorting to reflection (Item 53), invoke the clone method on an object merely because it implements Cloneable. Even a reflective invocation may fail, as there is no guarantee that the object has an accessible clone method.