Contents:
This handout describes how to document the specifications of classes and methods. This document focuses on practical issues.
This document uses the Line class as an example. Notice that we do not provide fields or method bodies in our example. This document covers specifying the behavior of classes and methods (what they should do), not their implementation (what they actually do and how they do it). Abstraction Functions and Representation Invariants covers how to document a class's implementation.
/**
* This class represents the mathematical concept of a line segment.
*
* Specification fields:
* @specfield start-point : point // The starting point of the line.
* @specfield end-point : point // The ending point of the line.
*
* Derived specification fields:
* @derivedfield length : real // length = sqrt((start-point.x - end-point.x)^2 + (start-point.y - end-point.y)^2)
* // The length of the line.
*
* Abstract Invariant:
* A line's start-point must be different from its end-point.
*/
public class Line {
... // Fields not shown.
/**
* @requires p != null && ! p.equals(start-point)
* @modifies this
* @effects Sets end-point to p
*/
public void setEndPoint(Point p) {
...
}
...
}
Because the concepts discussed here are interrelated, it is best to start with a short list of definitions of before diving into the details.
The above concepts are included in a class's external specification (in Javadoc). They aid a client with using the class.
The rest of this document is organized as follows. First, it explains how to document what a class abstractly represents using abstract state, specification fields and derived fields. Then, it explains how to specify method behavior, in terms of abstract state.
To a computer, an instance of an object is a series of zeros and ones. To humans, on the other hand, these zeros and ones represent higher-level abstract ideas: integers, strings, lines, and lists, for example. The abstract value of an object is what the object represents to humans on this abstract level.
The abstract state of an object is what information a humans associate with the abstract value of an object. An abstract value is determined by the abstract state. For example, for the String "cat", humans associate the following sequence of letters: 'c', 'a', and 't'. This sequence of letters is the abstract state of the String.
It is important to note that how the String "cat" is represented concretely (within the Java code) does not affect its abstract state. Whether String internally uses an array, a List or some clever compressed numerical encoding for the the characters 'c', 'a', and 't', the abstract state of "cat" is still a just a sequence of those letters.
More frequently, the abstract values are not simple mathematical objects. In these cases, it's more useful to think about the abstract value as if it were an object with fields. For example, a line has a start and an end; a mailing address has a number, street, city, and zipcode; and a URL has a protocol, host name, and resource name.
Mathematically, this is the same as a tuple; it's just a tuple whose parts are named, rather than simply numbered. So we could specify the abstraction function as a single function that maps representation objects into tuples. But it's convenient, and more readable, to break the abstraction state into several separate parts, where each part is a specification field. (Specification fields are more commonly called abstract fields, because they're fields of the abstract value, as opposed to rep fields which are fields of the representation value. Unfortunately, abstract has another meaning in Java, so we will avoid that potentially confusing terminology.)
Specification fields often (but not always) correspond to observers on the abstract data type. Because the structure of abstract values is a matter of interest to the clients of your class, the specification fields should be listed in the class overview. In CSE 331, we have a Javadoc convention for describing them: @specfield name : type // description. Here's an example:
/**
* Represents an appointment for a meeting.
* @specfield date : Date // The time
* @specfield room : integer // The room number of the meeting's location
* @specfield with : Set<Person> // Who the appointment is with
*/
class Meeting {
By convention, in specification fields, lowercase types like sequence or set refer to mathematical entities. Capitalized types refer to other ADTs (i.e., Java classes). Where you have a choice, prefer a mathematical entity as the type of a spec field; it is better to use sequence than List, for example. It's more elegant, and reduces the coupling between your specification and concrete Java classes.
The presence of a specification field does not imply anything about the interface or implementation of the class. Although spec fields often correspond to observer methods, that's not always true. The interface might not provide any observers that query the spec field's state, so clients of the class might not have access to the information stored in the spec field. Likewise, the implementation might not actually have a concrete field of the spec field's type: that information may be computed from multiple concrete fields, or it might not be available at all. The point is that specification fields are a convenience for use in method specifications and abstraction functions.
Derived fields are information that can be derived from the specification fields and is useful to give a name to. For example, take the class below:
/**
* Represents a square.
* @specfield length : int // The length of the square's sides.
* @derivedfield area : int // area = length^2. The area of the square.
*
* Abstract Invariant:
* length > 0.
*/
class Square {...}
The derived field area can be derived by squaring the length specification field. A derived field's documentation should state how it is derived from the specification fields.
Derived fields' purpose are to help with writing method specifications, abstraction functions, and representation invariants: It is easier to write and understand area than length^2. Since derived fields can be derived from the specification fields, the abstraction function does not need to relate a object's concrete representation to derived fields. (Derived fields should be explained in terms of only specification fields, not representation fields.) Additionally, method specifications do not need to state a method's effects on a derived fields. For example:
/**
* Represents a square.
* @specfield length : int // The length of the square's sides.
* @derivedfield area : int // area = length^2. The area of the square.
*
* Abstract Invariant:
* length > 0.
*/
class Square {
/** The length of the square's sides. */
int size;
// Abstraction Function:
// AF(r) = a square, s, with s.length = r.size.
//
// Representation Invariant:
// size > 0
/** Creates a new Square with length = len.
* @requires len > 0.
* @return a new Square, s, with s.length = len.
*/
public Square(int len) {
if (len <= 0) throw new IllegalArgumentException(len + " < 0");
this.size = len;
}
/** Returns the difference in area between this and s.
* @return this.area - s.area.
*/
public int differenceInArea(Square s) {
return (this.size*this.size) - (s.size*s.size);
}
/** Sets this.length to len.
* @requires len > 0.
* @effects sets this.length to len.
*/
public void setLength(int len) {
if (len <= 0) throw new IllegalArgumentException(len + " < 0");
size = len;
}
}
Notice how the method specification of differenceInArea uses the derived field area to make it easier to explain what it returns.
It is never necessary for a method specification to indicate its effect on derive field specification field because the class documentation has defined the derived specification field in terms of specification fields. Since area is a derived field, the constructor does not need to say what the newly constructed Square's area is. Similarly, the method specification for setLength does not need to document its effect on area . See Abstraction Functions and Method Specifications for further details.
Note that one could have made area a specification field instead of a derived specification field. This would relieve the programmer from the responsibility of documenting how area can be derived from the specification fields. However, in this case, the constructor and setLength would be required to specify their effects on area.
Method specifications describe the behavior of a method in terms of its preconditions and postconditions. Note that method specifications may only refer to specification fields, method arguments, and global variables (aka public static fields), never to the concrete fields of the implementation.
Preconditions are properties that must be true when the method is called. It is the responsibility of the caller to guarantee that these properties hold. If the preconditions do not hold, the method is allowed to behave in absolutely any fashion, including crashing the program, continuing with incorrect results, informing the user of the problem, or gracefully recovering from the problem. Callers should always assume that preconditions are not checked by a method. However, it is good practice — though not required — for a method to check its preconditions (if the check can be performed efficiently) and throw an error if they are not satisfied.
Preconditions are indicated by the "requires" clause in a method specification. If a "requires" clause is omitted from the method specification, it is assumed that the method does not have any preconditions.
Postconditions are properties that a method guarantees will hold when the method exits. However, if the precondition did not hold when the method was called, then all bets are off, and the method may behave in any fashion whatsoever. In particular, if the precondition does not hold upon method entry, then the postcondition need not hold on method exit.
A postcondition can be written as a single complex logical formula, but it is convenient to separate it into logically distinct parts. CSE 331 uses "return", "effects", "throws", and "modifies". (In the descriptions below, "default" indicates what is assumed if that clause is omitted from the method specification.)
Specification fields are useful for writing specifications of the ADT's operations. Here's a specification for a method on the Line class:
/** * @requires l.start-point is equal to this.end-point && l != null. * @return a line segment that is equal to this + l; that is, l appended to this. */ public Line add(Line l);
Specifications may refer to specification fields (such as start-point and end-point), but never to representation fields (such as start and end). Rep fields depend on a particular implementation, so we don't want to expose them in a specification, which can be implemented in many different ways.
When you're writing specifications for operations, you may find it useful to define derived spec fields. A derived spec field is just a spec field that can be written in terms of other spec fields. In other words, it's a shorthand.
You can freely use derived spec fields in a method specification (and easing such specifications is the point of derived spec fields):
/** * @return true is this.length > l.length. */ public boolean longer(Line l);
A subclass often has a different (stronger) specification than its superclass, and often has a larger abstract state than its superclass. When the specification and abstract state are identical to those of the parent (for instance, an implementation of an abstract class), then there is no need to repeat them in the subclass. However, it is helpful to include a brief note indicating that the superclass documentation should be used instead. That note helps readers to distinguish whether the specification is the same, or the author simply didn't document the class.
When the specifications differ, then you have two options. The first option is to repeat, in the subclass, the full superclass documentation. The advantage is that everything is in one place, which may improve understanding. The second option is to augment the existing specification -- for example, to add a few new specification fields and constraints on them. Whichever you do, make sure that you clearly indicate your approach, and that you are consistent throughout the codebase.
Similar rules hold for a method that overrides another method. It's acceptable to leave the Javadoc blank if the specification is identical. (The generated HTML will use the overridden method's Javadoc documentation, but a normal Java comment is a good hint to someone who is reading the source code.) Otherwise, it is usually better to give the complete specification. If you merely augment the overridden method's specification, be sure to refer to it in the documentation.