Google Git
Sign in
eclipse / www.eclipse.org / eclipse / 283e5aa56519b7305d9800532d37435bbb478daf / . / development / java-api-evolution.html
blob: f0bf47cf73f47bf15ffbfb929f4f2a3f70ad71d2 [file] [log] [blame]
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="Author" content="Jim des Rivières">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<link rel="stylesheet" href="../../default_style.css" type="text/css">
<title>Evolving Java-based APIs</title>
</head>
<body>
<h1>
Evolving Java-based APIs</h1>
Jim des Rivi&egrave;res, OTI
<p>Revision history:<br>
June 8, 2001 - revision 1.02 - Added note about breakage due to adding API method
to classes that may be subclassed. <br>
January 15, 2001 - revision 1.01 - Added suggestion about making obsolete hook
methods final. <br>
October 6, 2000 - revision 1.0</p>
This document is about how to evolve Java-based
APIs while maintaining compatibility with existing client code. The main subjects
covered are:
<ul>
<li>
<a href="#API elements">API Java Elements</a></li>
<li>
<a href="#API Prime Directive">API Prime Directive</a></li>
<li>
<a href="#Achieving API Contract Compatibility">Achieving API Contract
Compatibility</a></li>
<li>
<a href="#Achieving API Binary Compatibility">Achieving API Binary Compatibility</a></li>
</ul>
Without loss of generality, we'll assume that there is a generic <b>Component</b>
with a <b>Component API</b>, with one party providing the Component and
controlling its API. The other party, or parties, write <b>Client</b> code
that use the Component's services through its API. This is a very typical
arrangement.
<h2>
<a NAME="API elements"></a>API Java Elements</h2>
All parties need to understand which Java elements (packages, interfaces,
classes, methods, constructors, and fields) are part of the Component API,
which Clients may use, and those which are part of the internal Component
implementation and are off limits to Clients. Having a clearly-defined
and marked boundary between API and non-API is therefore very important.
The following convention uses the visibility-limiting features of the Java
language to distinguish those Java elements which are considered API from
those which are not:
<blockquote>
<li>
<b>API package</b> - a package that contains at least one API class or
API interface. The names of API packages are advertised in the Component
documentation. These names will appear in Client code; the names of non-API
packages should never appear in Client code. Note that Clients must be
prohibited from declaring their code in Component packages (API or otherwise).</li>
<li>
<b>API class</b> - a <code>public</code> class in an API package, or a <code>public</code>
or <code>protected</code> class member declared in, or inherited by, some other
API class or interface. The names of API classes appear in Client code.</li>
<li>
<b>API interface</b> - a <code>public</code> interface in an API package, or
a <code>public</code> or <code>protected</code> interface member declared in, or
inherited by, some other API class or interface. The names of API interfaces
appear in Client code.</li>
<li>
<b>API method</b> - a <code>public</code> or <code>protected</code> method either
declared in, or inherited by, an API class or interface. The names of API
methods appear in Client code.</li>
<li>
<b>API constructor</b> - a <code>public</code> or <code>protected</code> constructor
of an API class. The names of API constructors appear in Client code.</li>
<li>
<b>API field</b> - a <code>public</code> or <code>protected</code> field either
declared in, or inherited by, an API class or interface. The names of API
fields appear in Client code.</li>
</blockquote>
The following elements are not considered API:
<ul>
<li>
Any package that is not advertised in the Component documentation as an
API package.</li>
<li>
All classes and interfaces declared in non-API packages. However, when
API classes and interface extend or implement non-API classes, the non-API
classes and interface may contribute API elements nevertheless.</li>
<li>
Non-<code>public</code> classes and interfaces in API packages.</li>
<li>
Default access and <code>private</code> methods, constructors, fields, and
type members declared in, or inherited by, API classes and interfaces.</li>
</ul>
<h2>
<a NAME="API Prime Directive"></a>API Prime Directive</h2>
As the Component evolves from release to release, there is an absolute
requirement to not break existing Clients that were written in conformance
to Component APIs in an earlier release.
<blockquote>
<p><b>API Prime Directive:</b> <i>When evolving the Component
API from release to release, do not break existing Clients.</i></p>
</blockquote>
Changing an API in a way that is incompatible with existing Clients would
mean that all Clients would need to be revised (and even in instances where
no actual changes are required, the Client code would still need to be
reviewed to ensure that it still works with the revised API). Customers
upgrading to a new release of the Component would need to upgrade all their
Clients at the same time. Since the overall cost of invalidating existing
Client code is usually very high, the more realistic approach is to only
change the API in ways that do not invalidate existing Clients.
<p>As the Component API evolves, all pre-existing Clients are expected
to continue to work, both in principle and in practice.
<p>Suppose a Client was written to a given release of the Component and
abided by the contracts spelled out in the Component API specification.
<br>The first requirement is that when the Component API evolves to follow-on
releases, all pre-existing Client must still be legal according to the
contracts spelled out in the revised Component API specification, without
having to change the Clients source code. This is what is meant by continuing
to work in principle.
<blockquote>
<p><b>API Contract Compatibility:</b> <i>API changes must not
invalidate formerly legal Client code.</i></p>
</blockquote>
Since the set of Clients is open-ended, and we have no way of knowing exactly
which aspects of the API are being counted on, the only safe assumption
to make when evolving APIs is that every aspect of the API matters to some
hypothetical Client, and that any incompatible change to the API contract
will cause that hypothetical Client to fail.
<blockquote>
<p><b>API Usage Assumption:</b> <i>Every aspect of the API matters
to some Client.</i></p>
</blockquote>
Under this assumption, deleting something in the API, or backtracking on
some promise made in the API, will certainly break some Client. For this
reason, obsolete API elements are notoriously difficult to get rid of.
Obsolete API elements should be marked as deprecated and point new customers
at the new API that replaces it, but need to continue working as advertised
for a couple more releases until the expense of breakage is low enough
that it can be deleted.
<p>Clients are generally written in Java and are compiled to standard Java
binary class files. A Client's class files are typically stored in a JAR
file on the Client's library path. It would be unsatisfactory if a Client's
class files, which were compiled against one release of the Component,
do not successfully link and execute correctly with all later releases
of the Component. This is what is meant by continuing to work in practice.
<blockquote>
<p><b>API Binary Compatibility:</b> <i>Pre-existing Client binaries
must link and run with new releases of the Component without recompiling.</i></p>
</blockquote>
Achieving API binary compatibility requires being sensitive to the Java
language's notion of binary compatibility [<a href="http://java.sun.com/docs/books/jls/second_edition/html/binaryComp.doc.html#44872">JLS2, chapter
13</a>].
<p>While the idea that the Java source code for existing Clients should
continue to compile without errors against the revised Component API, this
is not strictly necessary (and not always achievable). For instance, adding
a new public interface to an existing API package may introduce an ambiguous
package reference into source code containing multiple on-demand type (".*")
imports. Similarly, removing a method <code>throws</code> declaration for a
checked exception may cause the compiler to detect dead code in a <code>try</code>-<code>catch</code>
block. Happily, the kinds of problems that could be introduced into Client
source
code can always be easily corrected. The notion of <b>API source compatibility</b>
is not a requirement. (Note: Problems detected by a Java compiler are therefore not
necessarily indicators of any kind of API compatibility that we care about.)
<p>The following sections discuss how API contract and binary compatibility
can be achieved.
<h2>
<a NAME="Achieving API Contract Compatibility"></a>Achieving API Contract
Compatibility</h2>
<blockquote>
<blockquote>
<blockquote>
<blockquote>
<blockquote><i>"How could I have broken anything? All I did was change
a comment."</i></blockquote>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
Since API contracts are captured by the API specification, any change to
the API specification risks making code written against the old specification
incompatible with the revised specification.
<p>The most confining situation is an API that is specified by one party,
implemented by a separate second party, and used by yet a different third
party. For example, a standards body promulgates a pure specification (such
as the HTTP protocol) but leaves it up to others to write browsers and
servers. In such cases, making any changes to the existing specification
will almost certainly break client code, implementations, or both.
<p>Fortunately, the case is typically lop-sided. Most commonly, the party
responsible for specifying the API also provides the sole implementation.
Indeed, this is our earlier assumption about the Component. In this situation,
the API owner can unilaterally decide to change the API specification and
fix up the implementation to match. However, since they can't do anything
about the client code already using the API, the changed API must be contract
compatible with the old API: all existing contractual obligations must
be honored. Contracts can be tightened to allow users to assume more (and
require the implementation to do more); this does not invalidate existing
code which would have been written assuming less. Conversely, contracts
cannot be loosened to require users to assume less, as this could break
existing uses.
<p>Note that in some cases, the contractual roles are reversed. The party
responsible for specifying the API provides the uses, whereas other parties
provide the implementations. Callback interfaces are a prime example of
this situation. Contracts can be loosened to require implementors to provide
less (and allow the client to assume less); this does not invalidate existing
implementations which would have been written under more stringent rules.
Conversely, contracts cannot be tightened to require implementors to provide
more, as this could break existing implementations.
<p>When contemplating changing an existing API contract, the key questions
to ask are:
<ul>
<li>
What roles does the API contract involve?<br>
For a method contract, there is the caller and the implementor. In
the case of frameworks, there is also an additional contract between superclass
and subclass regarding default behavior, extending, and overriding.</li>
<li>Which role or roles will each party play?<br>
For many Component API methods, the Component plays the role of exclusive
implementor and the Client plays the role of caller. In the case of Component
callbacks, the Component plays the caller role and the Client plays the
implementor role. In some cases, the Client might play more than one role.</li>
<li>
Is a role played exclusively by the Component?<br>
Component API changes coincide with Component releases, making it feasible
to change Component code to accommodate the changed APIs.</li>
<li>
For roles played by Clients, would the contemplated API change render invalid
a hypothetical Client making legal usage of the existing API?</li>
</ul>
The following examples illustrate how this analysis is done.
<h3>
Example 1 - Changing a method postcondition</h3>
Standard method contracts have two roles: caller and implementor. Method
postconditions are those things that an implementor must arrange to be
true before returning from the method, and that a caller may presume to
be true after the return. This first example involves a change to a postcondition.
<p>Consider the following API method specification:
<p><code>/** Returns the list of children of this widget.</code>
<br><code>&nbsp;* @return a non-empty list of widgets</code>
<br><code>&nbsp;*/</code>
<br><code>Widget[] getChildren();</code>
<p>The contemplated API change is to allow the empty list of widgets to
be returned as well, as captured by this revised specification:
<p><code>/** Returns the list of children of this widget.</code>
<br><code>&nbsp;* @return a list of widgets</code>
<br><code>&nbsp;*/</code>
<br><code>Widget[] getChildren();</code>
<p>Would this change break compatibility with existing Clients? It depends on the role played by the Client.
<p>Looking at the caller role, this change would break a hypothetical pre-existing
caller that legitimately counts on the result being non-empty. The relevant
snippet from this hypothetical caller might read:
<p><code>Widget[] children = widget.getChildren();</code>
<br><code>Widget firstChild = children[0];</code>
<p>Under the revised contract, this code would be seen to be in error because
it assumes that the result of invoking <code>getChildren</code> is non-empty;
under the previous contract, this assumption was just fine. This API change
weakens a postcondition for the caller, and is not contract compatible
for the caller role. The contemplated change would break Clients playing
the caller role.
<p>Looking at the implementor role, this change would not break a hypothetical
pre-existing implementor which never return empty results anyway. Weakening
a method postcondition is contract compatible for the implementor role.
The contemplated change would not break Clients playing the implementor
role.
<p>So the answer as to whether this change breaks compatibility with existing
Clients hinges on which role(s) the Client plays.
<p>Another form of postcondition change is changing the set of checked
exceptions that a method throws.
<h3>
Example 2 - Changing a method precondition</h3>
Method preconditions are those things that a caller must arrange to be
true before calling the method, and that an implementor may presume to
be true on entry. This second example involves a change to a precondition.
<p>Consider the following API method specification:
<p><code>/** Removes the given widgets from this widget's list of children.</code>
<br><code>&nbsp;* @param widgets a non-empty list of widgets</code>
<br><code>&nbsp;*/</code>
<br><code>void remove(Widget[] widgets);</code>
<p>The contemplated API change is to allow empty lists of widgets to be
passed in as well:
<p><code>/** Removes the given widgets from this widget's list of children.</code>
<br><code>&nbsp;* @param widgets a list of widgets</code>
<br><code>&nbsp;*/</code>
<br><code>void remove(Widget[] widgets);</code>
<p>Would this change break compatibility with existing Clients? Again,
it hinges on the role played by the Client.
<p>Looking at the caller role, this change would not break hypothetical
pre-existing callers since they pass in non-empty lists. However, this
change would break a hypothetical pre-existing implementations that legitimately
assumed that the argument is not empty.
<p>The relevant snippet from this hypothetical implementor might read:
<p><code>Widget firstChild = widgets[0];</code>
<p>Under the revised contract, this code would be seen to be in error because
it assumes that the argument is non-empty; under the previous contract,
this assumption was just fine. This API change weakens a method precondition,
and is not contract compatible for the implementor role. The contemplated
change would break Clients that implement this method.
<h3>
Example 3 - Changing a field invariant</h3>
Fields can be analyzed as having two roles: a getter and a setter. The
Java language does not separate these roles particularly, but it does have
the notion of final fields which eliminates setters from the equation.
(Perhaps a better way to divvy this up is to say that there is a getter
role and a getter/setter role.)&nbsp; The API specification for a field
is usually in the form of an invariant that holds for the lifetime of the
field.
<p>Consider the following API field specification:
<p><code>/** This widget's list of children, or &lt;code>null&lt;/code>.</code>
<br><code>&nbsp;*/</code>
<br><code>Widget[] children;</code>
<p>The contemplated API change is to get rid of the possibility of the
<code>null</code>
value:
<p><code>/** This widget's list of children.</code>
<br><code>&nbsp;*/</code>
<br><code>Widget[] children;</code>
<p>Would this change break compatibility with existing Clients?
<p>This change would break a hypothetical pre-existing setter that legitimately
sets the field to <code>null</code>. On the other hand, it would not break
a hypothetical pre-existing getter that legitimately had to assume that
the field could be <code>null</code>. This API change weakens a field invariant,
and is not contract compatible for the setter role.
<h3>
<a NAME="add method"></a>Example 4 - Adding an API method</h3>
Can adding an API method to a class or interface break compatibility with
existing Clients?
<p>If the method is added to an interface which Clients may implement,
then it is definitely a breaking change.
<p>If the method is added to a class (interface) which Clients are not
allowed to subclass (to implement), then it is not a breaking change.
<p>However, if the method is added to a class which Clients may subclass,
then the change should ordinarily be viewed as a breaking change. The reason
for this harsh conclusion is because of the possibility that a Client's
subclass already has its own implementation of a method by that name. Adding
the API method to the superclass undercuts the Client's code since it would
be sheer coincidence if the Client's existing method met the API contract
of the newly added method. In practice, if the likelihood of this kind
of name coincidence is sufficiently low, this kind of change is often treated
as if it were non-breaking.
<h3>
General Rules for Contract Compatibility</h3>
Whether a particular Component API change breaks or maintains contract
compatibility with hypothetical pre-existing Clients hinges on which role,
or roles, the Client plays in the API contract(s) being changed. The following
table summarizes the pattern seen in the above examples:
<br>&nbsp;
<table BORDER COLS=4 WIDTH="100%" >
<tr>
<td ROWSPAN="2">Method preconditions</td>
<td WIDTH="10%" height="23">Strengthen</td>
<td width="30%"><b><font color="#FF0000">Breaks compatibility for callers</font></b></td>
<td width="30%">Contract compatible for implementors</td>
</tr>
<tr>
<td WIDTH="10%">Weaken</td>
<td>Contract compatible for callers</td>
<td><b><font color="#FF0000">Breaks compatibility for implementors</font></b></td>
</tr>
<tr>
<td ROWSPAN="2">Method postconditions</td>
<td>Strengthen</td>
<td WIDTH="10%">Contract compatible for callers</td>
<td><b><font color="#FF0000">Breaks compatibility for implementors</font></b></td>
</tr>
<tr>
<td WIDTH="10%">Weaken</td>
<td><b><font color="#FF0000">Breaks compatibility for callers</font></b></td>
<td>Contract compatible for implementors</td>
</tr>
<tr>
<td ROWSPAN="2">Field invariants</td>
<td WIDTH="10%">Strengthen</td>
<td>Contract compatible for getters</td>
<td><b><font color="#FF0000">Breaks compatibility for setters</font></b></td>
</tr>
<tr>
<td WIDTH="10%">Weaken</td>
<td><b><font color="#FF0000">Breaks compatibility for getters</font></b></td>
<td>Contract compatible for setters</td>
</tr>
</table>
<h2>
<a NAME="Achieving API Binary Compatibility"></a>Achieving API Binary Compatibility</h2>
<blockquote>
<blockquote><i>"[A]n object-oriented model must be carefully designed so
that class-library transformations that should not break already compiled
applications, indeed, do not break such applications."<br>
</i>---Ira Forman,
Michael Conner, Scott Danforth, and Larry Raper, "Release-to-Release Binary
Compatibility in SOM", in <i>Proceedings of OOPSLA '95.</i></blockquote>
</blockquote>
Achieving API binary compatibility depends in part on the Java language's
notion of binary compatibility:
<blockquote>"A change to a type is <i>binary compatible with </i>(equivalently,
does not <i>break binary compatibility </i>with) preexisting binaries if
preexisting binaries that previously linked without error will continue
to link without error." (<a href="http://java.sun.com/docs/books/jls/second_edition/html/binaryComp.doc.html#44952">JLS2,
13.2</a>)</blockquote>
Reference: <a href="http://java.sun.com/docs/books/jls/second_edition/html/binaryComp.doc.html#44872">Gosling,
Joy, Steele, and Bracha, <i>The Java Language Specification</i>, Second
Edition, Addison-Wesley, 2000; chapter 13 Binary Compatibility</a>.
<p>The tables in the following sections summarize which kinds of changes
break API binary compatibility.
<p>Bear in mind that many changes will have effects in several places.
For example, defining a new public interface with one public method and
adding that interface as the superinterface of an existing interface has
the following ramifications:
<ul>
<li>
a new public API interface is added to an API package</li>
<li>
the superinterface set of the existing API interface has expanded</li>
<li>
a new public API method is added to the existing API interface</li>
</ul>
Each of these individual net effects could break binary compatibility.
Use the tables to determine whether the <i>net effects </i>preserve or
break compatibility.
<h3>
Evolving API packages</h3>
It is always possible to evolve the Component API to include a new API
package. However, once introduced in a release, an API package cannot easily
be withdrawn from service. When an API package becomes obsolete, its API
classes and API interfaces should continue to work but be marked as deprecated.
After a couple of releases, it may be possible to phase out an obsolete
API package.
<p>The names of non-public (non-API) classes and interfaces in API packages
do not appear in Client source code or binaries. Non-API classes and interfaces
can be added or deleted without jeopardizing binary compatibility. However,
once made public in a release, these classes and interfaces are part of
the API and cannot easily be withdrawn from service without breaking existing
Clients. When an API class or interface becomes obsolete, it should continue
to work but be marked as deprecated.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Add API package</td>
<td width="20%">-</td>
<td width="20%">Binary compatible</td>
</tr>
<tr>
<td>Delete API package</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add API interface to API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete API interface from API package</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add API class to API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete API class from API package</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add non-<code>public </code>(non-API) class or interface to API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete non-<code>public</code> (non-API) class or interface from API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>public</code> (non-API) class or interface in API package
to make public (API)</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change <code>public</code> class or interface in API package to make non-<code>public</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Replace API class by API interface of same name</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Replace API interface by API class of same name</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
</table>
<p>(1) API class-interface gender changes break binary compatibility, even
in cases where the class/interface is used by, but not implemented by,
Clients. This is because the Java VM bytecodes for invoking a method declared
in an interface are different from the ones used for invoking a method
declared in a class.
<h3>
Evolving API Interfaces</h3>
Evolving API interfaces is somewhat more straightforward than API classes
since all methods are <code>public</code> and <code>abstract</code>, all fields
are <code>public</code> <code>static</code> and <code>final</code>, all type members
are <code>public</code> and <code>static</code>, and there are no constructors.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="97%" >
<tr>
<td ROWSPAN="2">Add API method</td>
<td width="35%">If method need not be implemented by Client</td>
<td width="25%">Binary compatible (0)</td>
</tr>
<tr>
<td>If method must be implemented by Client</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Delete API method&nbsp;</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td ROWSPAN="2">Add API field</td>
<td>If interface not implementable by Clients</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If interface implementable by Clients</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (2)</td>
</tr>
<tr>
<td>Delete API field</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Expand superinterface set (direct or inherited)</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Contract superinterface set (direct or inherited)</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (3)</td>
</tr>
<tr>
<td>Add, delete, or change static initializers</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td ROWSPAN="2">Add API type member</td>
<td>If interface not implementable by Clients</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If interface implementable by Clients</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (2)</td>
</tr>
<tr>
<td>Delete API type member</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Re-order field, method, and type member declarations</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
</table>
<p>(0) Although adding a new method to an API interface which need not
be reimplemented by Clients does not break binary compatibility, a pre-existing
Client subclass of an existing implementation might still provide a pre-existing
implementation of a method by this name. See <a href="#add method">Example
4</a> in the preceding section for why this breaks API contract compatibility.
<p>(1) Adding a new method to an API interface that is implemented by Clients
(e.g., a callback, listener, or visitor interface) breaks compatibility
because hypothetical pre-existing implementations do not implement the
new method.
<p>(2) Adding an API field to an API interface that is implemented by Clients
(e.g., a callback, listener, or visitor interface) breaks binary compatibility
in a different way. A field added to a superinterface of C may hide an
instance field inherited from a superclass of C, causing linking errors
to be detected. Because of this fact, it is important to distinguish between
API interfaces that Clients should implement from those that Clients should
merely use. API interfaces that Clients should implement should not include
fields.
<p>(3) Shrinking the set of API interfaces that a given API interfaces
extends (either directly or inherited) breaks compatibility because some
casts between API interfaces in hypothetical pre-existing Client code between
will no longer work. However, non-API superinterfaces can be removed without
breaking binary compatibility.
<h4>
Evolving API interfaces - API methods</h4>
All methods in an API interface are implicitly <code>public</code> and <code>abstract</code>,
and are therefore all considered API methods.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Change formal parameter name</td>
<td width="20%" height="0">-</td>
<td width="20%">Binary compatible</td>
</tr>
<tr>
<td>Change method name</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add or delete formal parameter</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change type of a formal parameter</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change result type (including <code>void</code>)</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add checked exceptions thrown</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Add unchecked exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete checked exceptions thrown</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Delete unchecked exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Re-order list of exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
</table>
<p>(1) Adding and deleting checked exceptions declared as thrown by an
API method does not break binary compatibility; however, it breaks contract
compatibility (and source code compatibility).
<h4>
Evolving API interfaces - API fields</h4>
All fields in an API interface are implicitly <code>public</code>, <code>static</code>,
and <code>final</code>; they are therefore all considered API fields.
<p>Because of binary compatibility problems with fields, the Java Language
Specification recommends against using API fields. However, this is not
always possible; in particular, enumeration constants to be used in <code>switch</code>
statements must be defined as API fields.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Change type of API field</td>
<td width="35%">-</td>
<td width="20%"><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td ROWSPAN="2">Change value of API field</td>
<td>If field is compile-time constant value</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (2)</td>
</tr>
<tr>
<td>If field is not compile-time constant value</td>
<td>Binary compatible</td>
</tr>
</table>
<p>(1) All field type changes break binary compatibility, even seemingly
innocuous primitive type widenings like turning a <code>short</code> into an
<code>int</code>.
<p>(2) Java compilers always inline the value of constant fields (ones
with compile-time computable values, whether primitive or <code>String</code>
type). As a consequence, changing the value of an API constant field does
not affect pre-existing Clients. Invariably, this fails to meet the objective
for changing the API field's value in the first place.
<h4>
Evolving API interfaces - API type members</h4>
All type members in an API interface are implicitly <code>public</code> and
<code>static</code>;
they are therefore considered API type members. The rules for evolving
an API type member are basically the same as for API classes and interfaces
declared at the package level.
<h3>
Evolving API Classes</h3>
Evolving API classes is somewhat more complex than API interfaces due to
the wider variety of modifiers, including <code>protected</code> API members.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="97%" >
<tr>
<td ROWSPAN="2">Add API method</td>
<td width="40%">If method need not be reimplemented by Client</td>
<td width="25%">Binary compatible (0)</td>
</tr>
<tr>
<td>If method must be reimplemented by Client</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Delete API method&nbsp;</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td ROWSPAN="2">Add API constructor</td>
<td>If there are other constructors</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If this is only constructor</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (2)</td>
</tr>
<tr>
<td>Delete API constructor</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td ROWSPAN="2">Add API field</td>
<td>If class is not subclassable by Client</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If class is subclassable by Client</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (3)</td>
</tr>
<tr>
<td>Delete API field</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Expand superinterface set (direct or inherited)</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Contract superinterface set (direct or inherited)</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (4)</td>
</tr>
<tr>
<td>Expand superclass set (direct or inherited)</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Contract superclass set (direct or inherited)</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (4)</td>
</tr>
<tr>
<td>Add, delete, or change static or instance initializers</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td ROWSPAN="2">Add API type member</td>
<td>If class is not subclassable by Client</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If class is subclassable by Client</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (3)</td>
</tr>
<tr>
<td>Delete API type member</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Re-order field, method, constructor, and type member declarations</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Add or delete non-API members; that is, <code>private</code> or default
access fields, methods, constructors, and type members</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change <code>abstract</code> to non-<code>abstract</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>abstract</code> to <code>abstract</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (5)</td>
</tr>
<tr>
<td>Change <code>final</code> to non-<code>final</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>final</code> to <code>final</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (6)</td>
</tr>
</table>
<p>(0) Although adding a new method to an API class which need not be reimplemented
by Clients does not break binary compatibility, a pre-existing subclass
might still provide a pre-existing implementation of a method by this name.
See <a href="#add method">Example 4</a> in the preceding section for why
this breaks API contract compatibility.
<p>(1) Adding a new method to an API class that must be reimplemented by
Clients breaks compatibility because pre-existing subclasses would not
provide any such implementation.
<p>(2) Adding the first constructor to an API class causes the compiler
to no longer generate a default (public, 0 argument) constructor, thereby
breaking compatibility with pre-existing code that invoked this API constructor.
To avoid this pitfall, API classes should always explicitly declare at
least one constructor.
<p>(3) Adding a new field to an API class that is subclassed by Clients
breaks binary compatibility. A field in a superinterface of C may hide
an added field inherited from a superclass of C, causing linking errors
to be detected when a static field hides an instance field. Apart from
the binary compatibility issues, it is generally good software engineering
practice that API classes should not expose any fields.
<p>(4) Shrinking an API class's set of API superclasses and superinterfaces
(either directly or inherited) breaks compatibility because some casts
in pre-existing Client code will now longer work. However, non-API superclasses
and superinterfaces can be removed without breaking binary compatibility.
<p>(5) Pre-existing binaries that attempt to create new instances of the
API class will fail with a link-time or runtime error.
<p>(6) Pre-existing binaries that subclass the API class will fail with
a link-time error.
<h4>
Evolving API classes - API methods and constructors</h4>
<table BORDER COLS=3 WIDTH="96%" >
<tr>
<td>Change body of method or constructor</td>
<td width="20%">-</td>
<td width="25%">Binary compatible</td>
</tr>
<tr>
<td>Change formal parameter name</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change method name</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add or delete formal parameter</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change type of a formal parameter</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change result type (including <code>void</code>)</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Add checked exceptions thrown</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Add unchecked exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete checked exceptions thrown</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td>Delete unchecked exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Re-order list of exceptions thrown</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Decrease access; that is, from <code>protected</code> access to default
or <code>private</code> access; or from <code>public</code> access to <code>protected</code>,
default, or <code>private</code> access</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Increase access; that is, from <code>protected</code> access to <code>public</code>
access</td>
<td>-</td>
<td>Binary compatible (2)</td>
</tr>
<tr>
<td>Change <code>abstract</code> to non-<code>abstract</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>abstract</code> to <code>abstract</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (3)</td>
</tr>
<tr>
<td>Change <code>final</code> to non-<code>final</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td ROWSPAN="2">Change non-<code>final</code> to <code>final</code></td>
<td>If method not reimplementable by Clients</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If method reimplementable by Clients</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (4)</td>
</tr>
<tr>
<td>Change <code>static</code> to non-<code>static</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change non-<code>static</code> to <code>static</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Change <code>native</code> to non-<code>native</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>native</code> to <code>native</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change <code>synchronized</code> to non-<code>synchronized</code></td>
<td>-</td>
<td>Binary compatible (5)</td>
</tr>
<tr>
<td>Change non-<code>synchronized</code> to <code>synchronized</code></td>
<td>-</td>
<td>Binary compatible (5)</td>
</tr>
</table>
<p>(1) Adding and deleting checked exceptions declared as thrown by an
API method does not break binary compatibility; however, it breaks contract
compatibility (and source code compatibility).
<p>(2) Perhaps surprisingly, the binary format is defined so that changing
a member or constructor to be more accessible does not cause a linkage
error when a subclass (already) defines a method to have less access.
<p>(3) Pre-existing binaries that invoke the method will fail with a runtime
error.
<p>(4) Pre-existing binaries that reimplement the method will fail with
a link-time error.
<p>(5) Adding or removing the <code>synchronized</code> modifier also has a
bearing on the method's behavior in a multi-threaded world, and may therefore
raise a question of contract compatibility.
<h4>
Evolving API classes - API fields</h4>
Because of binary compatibility problems with fields, the Java Language
Specification recommends against using API fields. However, this is not
always possible; in particular, enumeration constants to be used in <code>switch</code>
statements must be defined as API constant fields.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Change type of API field</td>
<td width="30%">-</td>
<td width="25%"><b><font color="#FF0000">Breaks compatibility</font></b> (1)</td>
</tr>
<tr>
<td ROWSPAN="2">Change value of API field</td>
<td>If field is compile-time constant</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (2)</td>
</tr>
<tr>
<td>If field is not compile-time constant</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Decrease access; that is, from <code>protected</code> access to default
or <code>private</code> access; or from <code>public</code> access to <code>protected</code>,
default, or <code>private</code> access</td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Increase access; that is, from <code>protected</code> access to <code>public</code>
access</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td ROWSPAN="3">Change <code>final</code> to non-<code>final</code></td>
<td>If field is non-static</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>If field is static with compile-time constant value</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (3)</td>
</tr>
<tr>
<td>If field is static with non-compile-time constant value</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>final</code> to <code>final</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (4)</td>
</tr>
<tr>
<td>Change <code>static</code> to non-<code>static</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (5)</td>
</tr>
<tr>
<td>Change non-<code>static</code> to <code>static</code></td>
<td>-</td>
<td><b><font color="#FF0000">Breaks compatibility</font></b> (5)</td>
</tr>
<tr>
<td>Change <code>transient</code> to non-<code>transient</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change non-<code>transient</code> to <code>transient</code></td>
<td>-</td>
<td>Binary compatible</td>
</tr>
</table>
<p>(1) All field type changes break binary compatibility, even seemingly
innocuous primitive type widenings link turning a <code>short</code> into an
<code>int</code>.
<p>(2) Java compilers always inline the value of constant fields (ones
with compile-time computable values, whether primitive or <code>String</code>
type). As a consequence, changing the value of an API constant field does
not affect pre-existing Clients. Invariably, this does not meet the objective
for changing the API field's value.
<p>(3) Java compilers always inline the value of constant fields (ones
with a compile-time computable values, whether primitive or <code>String</code>type).
As a consequence, changing an API constant field into a non-<code>final</code>
one does not propagate to pre-existing Clients. Invariably, this does not
meet the objective for making the API field non-<code>final</code>.
<p>(4) Making an API field final breaks compatibility with pre-existing
binaries that attempt to assign new values to the field.
<p>(5) Changing whether an API field is declared static or not results
in link-time errors where the field is used by a pre-existing binary which
expected a field of the other kind.
<h4>
Evolving API classes - API type members</h4>
The rules for evolving an API type member are basically the same as for
API classes and interfaces declared at the package level, with these additional
rules for changing access modifiers:
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Decrease access; that is, from <code>protected</code> access to default
or <code>private</code> access; or from <code>public</code> access to <code>protected</code>,
default, or <code>private</code> access</td>
<td width="20%">-</td>
<td width="20%"><b><font color="#FF0000">Breaks compatibility</font></b></td>
</tr>
<tr>
<td>Increase access; that is, from <code>protected</code> access to <code>public</code>
access</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
</table>
<h3>
Evolving non-API packages</h3>
The names of non-API packages, classes, and interfaces do not appear in
Client source code or binaries. Consequently, non-API packages, classes,
and interfaces can be added or deleted without jeopardizing binary compatibility.
However, when non-API classes and interfaces containing <code>public</code>
or <code>protected</code> members are among the superclass or superinterface
sets of API classes and interfaces, non-API changes may have ramifications
to API methods, fields, and constructors.
<br>&nbsp;
<table BORDER COLS=3 WIDTH="100%" >
<tr>
<td>Add non-API package</td>
<td width="20%">-</td>
<td width="20%">Binary compatible</td>
</tr>
<tr>
<td>Delete non-API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Add class or interface to non-API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Delete class or interface in a non-API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
<tr>
<td>Change existing class or interface in non-API package</td>
<td>-</td>
<td>Binary compatible</td>
</tr>
</table>
<h2>
<a NAME="Data Compatibility"></a>Data Compatibility</h2>
The Component implementation may need to store and retrieve its internal
data from a file. For example, Microsoft Word stores a document in a file. When one
of these files may live from release to release, clients would break if
the format or interpretation of that data changed in an incompatible way.
<br><b>Data compatibility</b> is an additional issue for components with
persistent data.
<p>The standard technique is to tag all stored data with its format version
number. The format version number is increased when the format is changed
from one release to the next. The Component implementation contains readers
for the current format version and for all past versions, but usually only
the writer for the current format version (unless for some reason there
is an ongoing need to write older versions).
<h2>
<a NAME="Standard Workarounds"></a>Standard Workarounds</h2>
When evolving APIs, the prime directive places serious constraints on how
this can be done.
<p>Here are some standard techniques that come in handy when you're caught
between a rock and a hard place. They're not necessarily pretty, but they
get the job done.
<h3>
Deprecate and Forward</h3>
When some part of the Component API is made obsolete by some new and improved
Component API, the old API should be marked as deprecated using the <code>@deprecated</code>
Javadoc tag (the comment directing the reader attention to the replacement
API). When feasible, the implementation of the old API should forward the
message to the corresponding method in the replacement API; doing so will
mean that any performance improvements or bug fixes made to the implementation
of the new API will automatically be of benefit to clients of the old API.
<h3>
Start over in a New Package</h3>
Even simpler than Deprecate and Forward, the Component API and implementation
can be redone in new packages. The old API and implementation are left
in the old location untouched, except to mark them as deprecated. Old and
new API and implementations co-exist independent of one another.
<h3>
Adding an argument</h3>
Here is a simple technique for adding an argument to a method that is intended
to be overridden by subclasses. For example the <code>Viewer.inputChanged(Object
input)</code> method should get an additional argument <code>Object oldInput</code>.
Adding the argument results in pre-existing clients overridding the wrong
method. The workaround is to call the old method as the default implementation
of the new method:
<p><code>&nbsp;public void inputChanged(Object input, Object oldInput) {</code>
<br><code>&nbsp;&nbsp;&nbsp;&nbsp; inputChanged(input);</code>
<br><code>&nbsp;}</code>
<p>Pre-existing clients which override the old method continue to work;
and all calls to the old method continue to work New or upgraded clients
will override the new method; and all calls to the new method will work,
even if they happen to invoke an old implementation.
<h3>
"2" Convention</h3>
The first release of an API callback-style interface didn't work as well
as hoped. For example, the first release contained:
<p><code>public interface IProgressMonitor {</code>
<br><code>&nbsp; void start();</code>
<br><code>&nbsp; void stop();</code>
<br><code>}</code>
<p>You now wish you had something like:
<p><code>public interface IProgressMonitor {</code>
<br><code>&nbsp; void start(int total);</code>
<br><code>&nbsp; void worked(int units);</code>
<br><code>&nbsp; void stop();</code>
<br><code>}</code>
<p>But it's too late to change <code>IProgressMonitor</code> to be that API.
So you mark <code>IProgressMonitor</code> as deprecated and introduce the new
and improved one under the name <code>IProgressMonitor2</code> (a name everyone
recognizes as the second attempt):
<p><code>public interface IProgressMonitor2 extends IProgressMonitor {</code>
<br><code>&nbsp; void start(int total);</code>
<br><code>&nbsp; void worked(int units);</code>
<br><code>&nbsp; void stop();</code>
<br><code>}</code>
<p>By declaring the new interface to extend the old one, any object of
type <code>IProgressMonitor2</code> can be passed to a method expecting an
old <code>IProgressMonitor</code>.
<h3>
COM Style</h3>
The "COM style" is to not implement interfaces directly but to ask for an interface
by using <code>getAdapter(someInterfaceID)</code>. This allows adding new interfaces
in the implementation without breaking existing classes.
<h3>
Making Obsolete Hook Methods Final</h3>
As a framework evolves, it may sometimes be necessary to break compatibility.
When compatibility is being broken knowingly, there are some tricks that
make it easier for broken clients to find and fix the breakage.
<p>A common situation occurs when the signature of a framework hook method
is changed. Overridding a hook method that is no longer called by the base
class can be tough to track down, especially if the base class contains
a default implementation of the hook method. In order to make this jump
out, the obsolete method should be marked as <code>final</code> in addition
to being deprecated. This ensures that existing subclasses which override
the obsolete method will no longer compile or link.
<h2>
<a NAME="Defective API Specifications"></a>Defective API Specifications</h2>
As hard as one might try, achieving perfect APIs is difficult. The harsh
reality is that some parts of large Component API will be specified better
than others.
<p>One problem is specification bugs---when the API spec actually says
the wrong thing. Every effort should be made to catch these prior to release.
<p>Another problem is underspecification---when the API spec does not specify
enough. In some cases, the implementor will notice this before the API
is ever released. In other cases, the specification will be adequate for
the implementor's needs but inadequate for clients. When an API is released
in advance of serious usage from real clients, it may be discovered too
late that the specification should have been tighter or, even worse, that
the API should have been designed differently.
<p>When you find out that you're saddled with a defective API specification,
these points are worth bearing in mind:
<ul>
<li>
APIs are not sacrosanct; it's just that breaking compatibility is usually
very costly. For a truly unusable feature, the cost is likely much lower.</li>
<li>
Tightening up a seriously weak specification can often be achieved without
breaking compatibility by changing the specification in a way consistent
with the existing implementation. That is, codify more of how it actually
works to ensure that clients that currently work continue to work in subsequent
releases.</li>
<li>
Breaking compatibility in a limited way may be cheaper in the long run
that leaving a bad patch of API as it is.</li>
<li>
If you break compatibility between releases, do it in a controlled way
that only breaks those Clients that actually utilize of the bad parts of
the API. This localizes the pain to affected Clients (and their downstream
customers), rather than foisting a "Big Bang" release on everyone.</li>
<li>
Document all breaking API changes in the release notes. Clients appreciate this
much more than discovering for themselves that you knowingly broke them.</li>
</ul>
<h2>
<a NAME="A Word about Source Code Incompatibilities"></a>A Word about Source
Code Incompatibilities</h2>
While the idea that the Java source code for existing Clients should continue
to compile without errors against the revised Component API, this is not
strictly necessary (and not always achievable). API contract and binary
compatibility are the only hard requirements. Source code incompatibilities
are not worth losing sleep over because the Client's owner can easily correct
these problems if they do arise with only localized editing of the source
code.
<p>The following is a list of known kinds of Java source code incompatibilities
that can arise as APIs evolve:
<ul>
<li>
Ambiguities involving type-import-on-demand declarations.</li>
<ul>
<li>
Triggered by: adding an API class or interface.</li>
<li>
Remedy: add single type import declaration to disambiguate.</li>
<li>
Avoidance strategy: use at most one type-import-on-demand declaration per
compilation unit.</li>
</ul>
<li>
Ambiguities involving overloaded methods.</li>
<ul>
<li>
Triggered by: adding an overloaded API method or constructor.</li>
<li>
Remedy: add casts to disambiguate ambiguously typed arguments.</li>
<li>
Avoidance strategy: put casts on null arguments.</li>
</ul>
<li>
Ambiguities involving field and type member hiding.</li>
<ul>
<li>
Triggered by: adding an API field.</li>
<li>
Remedy: add qualification to disambiguate ambiguous field references.</li>
<li>
Avoidance strategy: none.</li>
</ul>
<li>
Ambiguities involving fields and local variables.</li>
<ul>
<li>
Triggered by: adding an API field.</li>
<li>
Remedy: rename conflicting local variables to avoid new field name.</li>
<li>
Avoidance strategy: don't declared API fields in classes and interfaces
that Clients implement.</li>
</ul>
<li>
Problems involving checked exceptions thrown by methods.</li>
<ul>
<li>
Triggered by: removing checked exceptions from a method's <code>throws</code>
clause.</li>
<li>
Remedy: add or remove exception handlers as required.</li>
<li>
Avoidance strategy: none.</li>
</ul>
</ul>
<p><FONT face="Times New Roman, Times, serif"
size=2>Copyright © 2000, 2002 Object Technology International, Inc.</FONT></p>
</body>
</html>