Parent issue: #19807

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Documenting Default Constructors

Section 8.8.7 of the Java Language Specification, Second Edition describes a default constructor: If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided. It invokes the superclass constructor with no arguments. The constructor has the same access as its class.

The Javadoc tool generates documentation for default constructors. When it documents such a constructor, Javadoc leaves its description blank, because a default constructor can have no doc comment. The question then arises: How do you add a doc comment for a default constructor? The simple answer is that it is not possible -- and, conveniently, our programming convention is to avoid default constructors. (We considered but rejected the idea that the Javadoc tool should generate a default comment for default constructors.)

Good programming practice dictates that code should never make use of default constructors in public APIs: All constructors should be explicit. That is, all default constructors in public and protected classes should be turned into explicit constructor declarations with the appropriate access modifier. This explicit declaration also gives you a place to write documentation comments.

The reason this is good programming practice is that an explicit declaration helps prevents a class from inadvertently being made instantiable, as the design engineer has to actually make a decision about the constructor's access. We have had several cases where we did not want a public class to be instantiable, but the programmer overlooked the fact that its default constructor was public. If a class is inadvertently allowed to be instantiable in a released version of a product, upward compatibility dictates that the unintentional constructor be retained in future versions. Under these unfortunate circumstances, the constructor should be made explicit and deprecated (using @deprecated).

Note that when creating an explicit constructor, it must match precisely the declaration of the automatically generated constructor; even if the constructor should logically be protected, it must be made public to match the declaration of the automatically generated constructor, for compatibility. An appropriate doc comment should then be provided. Often, the comment should be something as simple as:

/**
* Sole constructor. (For invocation by subclass 
* constructors, typically implicit.)
*/
protected AbstractMap() { }

Guidelines

our programming convention is to avoid default constructors

This guideline is covered by MissingCtor. Proposal: