Introduction
Analysis and Requirements
Development
Coding Standards
Java coding standards should follow the guidelines set forth by Sun Microsystems. These guidelines are titled Code Conventions for the Java Programming Language and found here. Other clarifications should be noted in addition to those found above.
Naming Conventions
packages
lowercase.
Consider using the recommended domain-based conventions described in the Java Language Specification, page 107 as prefixes. (For example, EDU.oswego.cs.dl.)
files
The java compiler enforces the convention that file names have the same base name as the public class they define.
classes:
CapitalizedWithInternalWordsAlsoCapitalized
Exception class:
ClassNameEndsWithException.
Interface. When necessary to distinguish from similarly named classes:
InterfaceNameEndsWithIfc.
Class. When necessary to distinguish from similarly named interfaces:
ClassNameEndsWithImpl OR
ClassNameEndsWithObject
constants (finals):
UPPER_CASE_WITH_UNDERSCORES
private or protected: (pick one!)
firstWordLowerCaseButInternalWordsCapitalized OR
trailingUnderscore_, OR
thisVar (i.e. prefix with this), OR
myVar (i.e. prefix with my), OR
fVar (i.e. prefix with f)
static private or protected:
firstWordLowerCaseButInternalWordsCapitalized OR
twoTrailingUnderscores__
local variables:
firstWordLowerCaseButInternalWordsCapitalized OR
lower_case_with_underscores
methods:
firstWordLowerCaseButInternalWordsCapitalized()
factory method for objects of type X:
newX
converter method that returns objects of type X:
toX
method that reports an attribute x of type X:
X x() or X getX().
method that changes an attribute x of type X:
void x(X value) or void setX(X value).
Recommendations
Minimize * forms of import. Be precise about what you are importing. Check that all declared imports are actually used.
Rationale: Otherwise readers of your code will have a hard time understanding its context and dependencies. Some people even prefer not using import at all (thus requiring that every class reference be fully dot-qualified), which avoids all possible ambiguity at the expense of requiring more source code changes if package names change.
When sensible, consider writing a main for the principal class in each program file. The main should provide a simple unit test or demo.
Rationale: Forms a basis for testing. Also provides usage examples.
For self-standing application programs, the class with main should be separate from those containing normal classes.
Rationale: Hard-wiring an application program in one of its component class files hinders reuse.
Consider writing template files for the most common kinds of class files you create: Applets, library classes, application classes.
Rationale: Simplifies conformance to coding standards.
If you can conceive of someone else implementing a class's functionality differently, define an interface, not an abstract class. Generally, use abstract classes only when they are ``partially abstract''; i.e., they implement some functionality that must be shared across all subclasses.
Rationale: Interfaces are more flexible than abstract classes. They support multiple inheritance and can be used as `mixins' in otherwise unrelated classes.
Consider whether any class should implement Cloneable and/or Serializable.
Rationale: These are ``magic'' interfaces in Java, that automatically add possibly-needed functionality only if so requested.
Declare a class as final only if it is a subclass or implementation of a class or interface declaring all of its non-implementation-specific methods. (And similarly for final methods).
Rationale: Making a class final means that no one ever has a chance to reimplement functionality. Defining it instead to be a subclass of a base that is not final means that someone at least gets a chance to subclass the base with an alternate implementation. Which will essentially always happen in the long run.
Never declare instance variables as public.
Rationale: The standard OO reasons. Making variables public gives up control over internal class structure. Also, methods cannot assume that variables have valid values.
Minimize reliance on implicit initializers for instance variables (such as the fact that reference variables are initialized to null).
Rationale: Minimizes initialization errors.
Minimize statics (except for static final constants).
Rationale: Static variables act like globals in non-OO languages. They make methods more context-dependent, hide possible side-effects, sometimes present synchronized access problems. and are the source of fragile, non-extensible constructions. Also, neither static variables nor methods are overridable in any useful sense in subclasses.
Generally prefer long to int, and double to float. But use int for compatibility with standard Java constructs and classes (for the major example, array indexing, and all of the things this implies, for example about maximum sizes of arrays, etc).
Rationale: Arithmetic overflow and underflow can be 4 billion times less likely with longs than ints; similarly, fewer precision problems occur with doubles than floats. On the other hand, because of limitations in Java atomicity guarantees, use of longs and doubles must be synchronized in cases where use of ints and floats sometimes would not be.
Use final and/or comment conventions to indicate whether instance variables that never have their values changed after construction are intended to be constant (immutable) for the lifetime of the object (versus those that just so happen not to get assigned in a class, but could in a subclass).
Rationale: Access to immutable instance variables generally does not require any synchronization control, but others generally do.
Generally prefer protected to private.
Rationale: Unless you have good reason for sealing-in a particular strategy for using a variable or method, you might as well plan for change via subclassing. On the other hand, this almost always entails more work. Basing other code in a base class around protected variables and methods is harder, since you you have to either loosen or check assumptions about their properties. (Note that in Java, protected methods are also accessible from unrelated classes in the same package. There is hardly ever any reason to exploit this though.)
Avoid unnecessary instance variable access and update methods. Write get/set-style methods only when they are intrinsic aspects of functionality.
Rationale: Most instance variables in most classes must maintain values that are dependent on those of other instance variables. Allowing them to be read or written in isolation makes it harder to ensure that consistent sets of values are always used.
Minimize direct internal access to instance variables inside methods. Use protected access and update methods instead (or sometimes public ones if they exist anyway).
Rationale: While inconvenient and sometimes overkill, this allows you to vary synchronization and notification policies associated with variable access and change in the class and/or its subclasses, which is otherwise a serious impediment to extensiblity in concurrent OO programming. (Note: The naming conventions for instance variables serve as an annoying reminder of such issues.)
Avoid giving a variable the same name as one in a superclass.
Rationale: This is usually an error. If not, explain the intent.
Prefer declaring arrays as Type[] arrayName rather than Type arrayName[].
Rationale: The second form is just for incorrigible C prgrammers.
Ensure that non-private statics have sensible values even if no instances are ever created. (Similarly ensure that static methods can be executed sensibly.) Use static intitializers (static
) if necessary.
Rationale: You cannot assume that non-private statics will be accessed only after instances are constructed.
Write methods that only do ``one thing''. In particular, separate out methods that change object state from those that just rely upon it. For a classic example in a Stack, prefer having two methods Object top() and void removeTop() versus the single method Object pop() that does both.
Rationale: This simplifies (sometimes, makes even possible) concurrency control and subclass-based extensions.
Define return types as void unless they return results that are not (easily) accessible otherwise. (i.e., hardly ever write ``return this'').
Rationale: While convenient, the resulting method cascades (a.meth1().meth2().meth3()) can be the sources of synchronization problems and other failed expectations about the states of target objects.
Avoid overloading methods on argument type. (Overriding on arity is OK, as in having a one-argument version versus a two-argument version). If you need to specialize behavior according to the class of an argument, consider instead choosing a general type for the nominal argument type (often Object) and using conditionals checking instanceof. Alternatives include techniques such as double-dispatching, or often best, reformulating methods (and/or those of their arguments) to remove dependence on exact argument type.
Rationale: Java method resolution is static; based on the listed types, not the actual types of argument. This is compounded in the case of non-Object types with coercion charts. In both cases, most programmers have not committed the matching rules to memory. The results can be counterintuitive; thus the source of subtle errors. For example, try to predict the output of this. Then compile and run.
class Classifier {
String identify(Object x)
String identify(Integer x)
}
class Relay {
String relay(Object obj)
}
public class App {
public static void main(String[] args)
}
Declare all public methods as synchronized; or if not, describe the assumed invocation context and/or rationale for lack of synchronization.
Rationale: In the absence of planning out a set of concurrency control policies, declaring methods as synchronized at least guarantees safety (although not necessarily liveness) in concurrent contexts (every Java program is concurrent to at least some minimal extent). With full synchronization of all methods, the methods may lock up, but the object can never enter in randomly inconsistent states (and thus engage in stupidly or even dangerously wrong behaviors) due to concurrency conflicts. If you are worried about efficiency problems due to synchronization, learn enough about concurrent OO programming to plan out more efficient and/or less deadlock-prone policies.
Prefer synchronized methods to synchronized blocks.
Rationale: Better encsapsulation; less prone to subclassing snags; can be more efficient.
If you override Object.equals, also override Object.hashCode, and vice-versa.
Rationale: Essentially all containers and other utilities that group or compare objects in ways depending on equality rely on hashcodes to indicate possible equality.
Override readObject and WriteObject if a Serializable class relies on any state that could differ across processes, including, in particular, hashCodes and transient fields.
Rationale: Otherwise, objects of the class will not transport properly.
If you think that clone() may be called in a class you write, then explicitly define it (and declare the class to implement Cloneable).
Rationale: The default shallow-copy version of clone might not do what you want.
Always document the fact that a method invokes wait
Rationale: Clients may need to take special actions to avoid nested monitor calls.
Whenever reasonable, define a default (no-argument) constructor so objects can be created via Class.newInstance().
Rationale: This allows classes of types unknown at compile time to be dynamically loaded and instantiated (as is done for example when loading unknown Applets from html pages).
Prefer abstract methods in base classes to those with default no-op implementations. (Also, if there is a common default implementation, consider instead writing it as a protected method so that subclass authors can just write a one-line implementation to call the default.)
Rationale: The Java compiler will force subclass authors to implement abstract methods, avoiding problems occurring when they forget to do so but should have.
Use method equals instead of operator == when comparing objects. In particular, do not use == to compare Strings.
Rationale: If someone defined an equals method to compare objects, then they want you to use it. Otherwise, the default implementation of Object.equals is just to use ==.
Always embed wait statements in while loops that re-wait if the condition being waited for does not hold.
Rationale: When a wait wakes up, it does not know if the condition it is waiting for is true or not.
Use notifyAll instead of notify or resume.
Rationale: Classes that use only notify can normally only support at most one kind of wait condition across all methods in the class and all possible subclasses. And unguarded suspends/resumes are even more fragile.
Declare a local variable only at that point in the code where you know what its initial value should be.
Rationale: Minimizes bad assumptions about values of variables.
Declare and initialize a new local variable rather than reusing (reassigning) an existing one whose value happens to no longer be used at that program point.
Rationale: Minimizes bad assumptions about values of variables.
Assign null to any reference variable that is no longer being used. (This includes, especially, elements of arrays.)
Rationale: Enables garbage collection.
Avoid assignments (``='') inside if and while conditions.
Rationale: There are almost always typos. The java compiler catches cases where ``='' should have been ``=='' except when the variable is a boolean.
Document cases where the return value of a called method is ignored.
Rationale: These are typically errors. If it is by intention, make the intent clear. A simple way to do this is:
int unused = obj.methodReturningInt(args);
Ensure that there is ultimately a catch for all unchecked exceptions that can be dealt with.
Rationale: Java allows you to not bother declaring or catching some common easily-handlable exceptions, for example java.util.NoSuchElementException. Declare and catch them anyway.
Embed casts in conditionals. For example:
C cx = null;
if (x instanceof C) cx = (C)x;
else evasiveAction();
Rationale: This forces you to consider what to do if the object is not an instance of the intended class rather than just generating a ClassCastException.
Document fragile constructions used solely for the sake of optimization.
Rationale: See Jonathan Hardwick's Java Optimization pages.
Do not require 100% conformance to rules of thumb such as the ones listed here!
Rationale: Java allows you program in ways that do not conform to these rules for good reason. Sometimes they provide the only reasonable ways to implement things. And some of these rules make programs less efficient than they might otherwise be, so are meant to be concientiously broken when performance is an issue.
Formatting and Templates
It is recommended that the development team adopt a template if e.g. Eclipse were used for development. IDE is not too important, but the code that we write should be formatted in the same manner. Tabs should not be allowed. And during code reviews they should be removed if found.