Copying an object using the Cloneable interface and the Object.clone method is error-prone. This is because the Cloneable interface and the clone method are unusual:

• The Cloneable interface has no methods. Its only use is to trigger different behavior of Object.clone.
• Object.clone is protected.
• Object.clone creates a shallow copy without calling a constructor.

The first two points mean that a programmer must do two things to get a useful implementation of clone: first, make the class implement Cloneable to change the behavior of Object.clone so that it makes a copy instead of throwing a CloneNotSupportedException; second, override clone to make it public, to allow it to be called. Another consequence of Cloneable not having any methods is that it does not say anything about an object that implements it, which means that you cannot perform a polymorphic clone operation.

The third point, Object.clone creating a shallow copy, is the most serious one. A shallow copy shares internal state with the original object. This includes private fields that the programmer might not be aware of. A change to the internal state of the original object could affect the copy, and conversely the opposite is true, which could easily lead to unexpected behavior.