Item 17 - Design and document for inheritance or else prohibit it

From Effective Java 2/e by Joshua Bloch

  1. Class must document its self-use of overridable methods

    public boolean remove(Object o)
      Removes a single instance of the specified element from this collection, if it
      is present (optional operation). More formally, removes an element e such
      that (o==null ? e==null : o.equals(e)), if the collection contains one or
      more such elements. Returns true if the collection contained the specified
      element (or equivalently, if the collection changed as a result of the call).
      This implementation iterates over the collection looking for the specified el-
      ement. If it finds the element, it removes the element from the collection us-
      ing the iterator’s remove method. Note that this implementation throws an
      UnsupportedOperationException if the iterator returned by this collec-
      tion’s iterator method does not implement the remove method.
  2. Class may have to provide hooks into its internal workings in the form of judi- ciously chosen protected methods
  3. The only way to test a class designed for inheritance is to write subclasses
  4. Constructors must not invoke overridable methods
  5. Neither clone nor readObject may invoke an overridable method, directly or indirectly

  6. prohibit subclassing in classes that are not designed and documented to be safely subclassed

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