What can be the bad example of inheritance in Java?

Question

I know the advantages of inheritance in Java, but it is somewhat difficult for me to accept that it has disadvantages too. Can anybody give me a bad example of inheritance in Java?

Answer 1

• Stack extends Vector. A stack is not a vector.
• Properties extends Hashtable. A table of properties is not a hash table.

See this answer for a quote from Effective Java.

It was easy to write the Stack implementation by using what is already implemented in Vector (likewise for Properties), but it created problems - see here

Answer 2

One example is the old pattern of implementing constant interfaces (an interface containing only immutable fields), then classes wishing to use these constants would implement this interface for convenience. The problem is that your class now inherits the API of this interface and extensions to it's design may harm your API in the future.

The use of constant interfaces in this way is generally considered an anti-pattern these days. Since Java 5, you can use enums instead of interfaces full of constants and static imports instead of defining constant interfaces.

From Effective Java by Josh Bloch:

The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API.