为什么有一些什么方法都没有的接口会存在？比如java.lang.Cloneable

/**
 * A class implements the <code>Cloneable</code> interface to
 * indicate to the {@link java.lang.Object#clone()} method that it
 * is legal for that method to make a
 * field-for-field copy of instances of that class.
 * <p>
 * Invoking Object's clone method on an instance that does not implement the
 * <code>Cloneable</code> interface results in the exception
 * <code>CloneNotSupportedException</code> being thrown.
 * <p>
 * By convention, classes that implement this interface should override
 * <tt>Object.clone</tt> (which is protected) with a public method.
 * See {@link java.lang.Object#clone()} for details on overriding this
 * method.
 * <p>
 * Note that this interface does <i>not</i> contain the <tt>clone</tt> method.
 * Therefore, it is not possible to clone an object merely by virtue of the
 * fact that it implements this interface.  Even if the clone method is invoked
 * reflectively, there is no guarantee that it will succeed.
 *
 * @author  unascribed
 * @see     java.lang.CloneNotSupportedException
 * @see     java.lang.Object#clone()
 * @since   JDK1.0
 */
public interface Cloneable {
}

我看到有很多类实现了这个接口，但是我刚开始不明白它的意义，我认为没有意义。

这个接口没有任何抽象出来的方法，所以实现它又能怎么样呢？

看源码的注释：如果一个没有实现Cloneable接口的类对象调用了clone(),结果是抛出异常。

所以这个类有意义：是为了辨别是否具有克隆的能力而存在的。

在Java中为什么实现了Cloneable接口，就能调用Object的clone方法？ - 知乎 (zhihu.com)

具体参考这个博客。