It is dangerous to subclass a “foreign” class that was not designed and documented for inheritance.
In other words, a subclass depends on the implementation details of its superclass for its proper function. The superclass’s implementation may change from release to release, and if it does, the subclass may break, even though its code has not been touched.
That begs the question: What does it mean for a class to be designed and documented for inheritance?
First, document
A class designed and documented for inheritance must precisely document the effects of overriding any method. In other words, the class must document its self-use of overridable methods.
For each public or protected method, the documentation must indicate which overridable methods the method invokes, in what sequence, and how the results of each invocation affect subsequent processing. (The term overridable means a method is nonfinal and either public or protected.)
More generally, a class must document any circumstances under which it might invoke an overridable method. For example, invocations might come from background threads or static initializers.
A method that invokes overridable methods contains a description of these invocations at the end of its documentation comment. The description is in a special section of the specification, labeled “Implementation Requirements,” which is generated by the @implSpec Javadoc tag. This section describes the inner workings of the method. The following is an example copied from the java.util.AbstractCollection specification:
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 Objects.equals(o, e), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call)…
Implementation Requirements: This implementation iterates over the collection looking for the specified element. If it finds the element, it removes the element from the collection using the iterator’s remove method. Note that this implementation throws an UnsupportedOperationException if the iterator returned by this collection’s iterator method does not implement the remove method and this collection contains the specified object.
The specification documentation leaves no doubt that overriding the iterator method will affect the behavior of the remove method. The documentation also describes exactly how the behavior of the Iterator returned by the iterator method will affect the behavior of the remove method. Contrast this to the situation described in the article “You should favor composition over inheritance in Java. Here’s why.” In that article, the programmer subclassing HashSet simply could not say whether overriding the add method would affect the behavior of the addAll method.
Wait: Doesn’t this violate the dictum that good API documentation should describe what a given method does and not how it does it? Yes; it does! This is an unfortunate consequence of the fact that inheritance violates encapsulation. To document a class so that it can be safely subclassed, you must describe implementation details that should otherwise be left unspecified.
By the way, the @implSpec tag was added in Java 8 and is used heavily in Java 9. This tag should be enabled by default, but as of Java 9, Javadoc ignores the tag unless you pass the command line switch -tag "implSpec:a:Implementation Requirements:".
Second, design
Designing for inheritance involves more than just documenting patterns of self-use. To allow programmers to write efficient subclasses without undue pain, a class may have to provide hooks into its internal workings in the form of judiciously chosen protected methods or, in rare instances, protected fields.
For example, consider the following documentation for the removeRange method from the java.util.AbstractList specification:
protected void removeRange(int fromIndex, int toIndex)
Removes from this list all of the elements whose index is between fromIndex, inclusive, and toIndex, exclusive. Shifts any succeeding elements to the left (reduces their index). This call shortens the list by (toIndex – fromIndex) elements. (If toIndex == fromIndex, this operation has no effect.)
This method is called by the clear operation on this list and its sublists. Overriding this method to take advantage of the internals of the list implementation can substantially improve the performance of the clear operation on this list and its sublists.
Implementation Requirements: This implementation gets a list iterator positioned before fromIndex and repeatedly calls ListIterator.next followed by ListIterator.remove, until the entire range has been removed. Note: If ListIterator.remove requires linear time, this implementation requires quadratic time.
Parameters:
fromIndex - index of first element to be removed.
toIndex - index after last element to be removed.
The removeRange method is of no interest to end users of a List implementation; it is provided solely to make it easy for subclasses to provide a fast clear method on sublists. In the absence of the removeRange method, subclasses would have to make do with quadratic performance when the clear method was invoked on sublists or rewrite the entire subList mechanism from scratch—not an easy task!
How do you decide which protected members to expose when you design a class for inheritance? Unfortunately, there is no magic bullet. The best you can do is to think hard, take your best guess, and then test it by writing subclasses. You should expose as few protected members as possible because each one represents a commitment to an implementation detail. On the other hand, you must not expose too few because a missing protected member can render a class practically unusable for inheritance.
Write subclasses
The only way to test a class designed for inheritance is to write subclasses. If you omit a crucial protected member, trying to write a subclass will make the omission painfully obvious. Conversely, if several subclasses are written and none uses a protected member, you should probably make the class private. Experience shows that three subclasses are usually sufficient to test an extendable class. One or more of these subclasses should be written by someone other than the superclass author.
When you design for inheritance a class that is likely to achieve wide use, realize that you are committing forever to the self-use patterns that you document and to the implementation decisions implicit in its protected methods and fields. These commitments can make it difficult or impossible to improve the performance or functionality of the class in a subsequent release. Therefore, you must test your class by writing subclasses before you release it.
Also, note that the special documentation required for inheritance clutters up normal documentation, which is designed for programmers who create instances of your class and invoke methods on them.
Class restrictions
There are a few more restrictions that a class must obey to allow inheritance.
Constructors must not invoke overridable methods, directly or indirectly. If you violate this rule, program failure will result. The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will get invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected. To make this concrete, the following is a class that violates this rule:
public class Super {
// Broken - constructor invokes an overridable method
public Super() {
overrideMe();
}
public void overrideMe() {
}
}
Here’s a subclass that overrides the overrideMe method, which is erroneously invoked by the Super class’s sole constructor.
public final class Sub extends Super {
// Blank final, set by constructor
private final Instant instant;
Sub() {
instant = Instant.now();
}
// Overriding method invoked by superclass constructor
@Override public void overrideMe() {
System.out.println(instant);
}
public static void main(String[] args) {
Sub sub = new Sub();
sub.overrideMe();
}
}
You might expect this program to print out the value of Instant twice—but instead it prints null the first time because overrideMe is invoked by the Super constructor before the Sub constructor has a chance to initialize the instant field.
Note that this program observes a final field in two different states!
Note also that if overrideMe had invoked any method on instant, it would have thrown a NullPointerException when the Super constructor invoked overrideMe. The only reason this program doesn’t throw a NullPointerException as it stands is that the println method tolerates null parameters.
By the way, it is safe to invoke private methods, final methods, and static methods, none of which are overridable, from a constructor.
The Cloneable and Serializable interfaces present special difficulties when you are designing for inheritance. It is generally not a good idea for a class designed for inheritance to implement either of these interfaces because they place a substantial burden on programmers who extend the class. (There are, however, special actions that you can take to allow subclasses to implement these interfaces without mandating that they do so, but that is beyond the scope of this article.)
If you do decide to implement either Cloneable or Serializable in a class that is designed for inheritance, you should be aware that because the clone and readObject methods behave a lot like constructors, another restriction applies: Neither clone nor readObject may invoke an overridable method, directly or indirectly.
In the case of readObject, the overriding method will run before the subclass’s state has been deserialized. In the case of clone, the overriding method will run before the subclass’s clone method has a chance to fix the clone’s state. In either case, a program failure is likely to follow. In the case of clone, the failure can damage the original object as well as the clone! This can happen, for example, if the overriding method assumes it is modifying the clone’s copy of the object’s deep structure, but the copy hasn’t actually been made yet.
Important note: If you decide to implement Serializable in a class designed for inheritance and the class has a readResolve or writeReplace method, you must make those methods protected rather than private. If these methods are private, they will be silently ignored by subclasses. This is one more case where an implementation detail becomes part of a class’s API to permit inheritance.
Prohibit subclassing where it’s unsafe
By now it should be apparent that designing a class for inheritance requires great effort and places substantial limitations on the class. Designing for inheritance is not a decision to be undertaken lightly.
However, there are some situations where it is clearly the right thing to do, such as for abstract classes, including skeletal implementations of interfaces. There are other situations where it is clearly the wrong thing to do, such as for immutable classes.
But what about ordinary concrete classes? Traditionally, they are neither final nor designed and documented for subclassing, but this state of affairs is dangerous. Each time a change is made in such a class, there is a chance that subclasses extending the class will break. This is not just a theoretical problem. It is not uncommon to receive subclassing-related bug reports after modifying the internals of a nonfinal concrete class that was not designed and documented for inheritance.
The best solution to this problem is to prohibit subclassing in classes that are not designed and documented to be safely subclassed. There are two ways to prohibit subclassing. The easier of the two is to declare the class final. The alternative is to make all the constructors private or package-private and to add public static factories in place of the constructors. Either approach is acceptable.
This advice may be somewhat controversial because many programmers have grown accustomed to subclassing ordinary concrete classes to add facilities—such as instrumentation, notification, and synchronization—or to limit functionality. If a class implements some interface that captures its essence, such as Set, List, or Map, you should feel no compunction about prohibiting subclassing. Using a wrapper class provides a superior alternative to inheritance for augmenting the functionality.
If a concrete class does not implement a standard interface, you may inconvenience some programmers by prohibiting inheritance. If you feel that you must allow inheritance from such a class, one reasonable approach is to ensure that the class never invokes any of its overridable methods and to document this fact. In other words, eliminate the class’s self-use of overridable methods entirely. In doing so, you’ll create a class that is reasonably safe to subclass. Overriding a method will never affect the behavior of any other method.
You can eliminate a class’s self-use of overridable methods mechanically, without changing its behavior. To do so, move the body of each overridable method to a private “helper method” and have each overridable method invoke its private helper method. Then replace each self-use of an overridable method with a direct invocation of the overridable method’s private helper method.
Source: oracle.com
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