10 Subtle Best Practices when Coding Java

This is a list of 10 best practices that are more subtle than your average Josh Bloch Effective Java rule. While Josh Bloch’s list is very easy to learn and concerns everyday situations, this list here contains less common situations involving API / SPI design that may have a big effect nontheless.

I have encountered these things while writing and maintaining jOOQ, an internal DSL modelling SQL in Java. Being an internal DSL, jOOQ challenges Java compilers and generics to the max, combining generics, varargs and overloading in a way that Josh Bloch probably wouldn’t recommend for the “average API”.

jOOQ is the best way to write SQL in Java

Let me share with you 10 Subtle Best Practices When Coding Java:

1. Remember C++ destructors

Remember C++ destructors? No? Then you might be lucky as you never had to debug through any code leaving memory leaks due to allocated memory not having been freed after an object was removed. Thanks Sun/Oracle for implementing garbage collection!

But nonetheless, destructors have an interesting trait to them. It often makes sense to free memory in the inverse order of allocation. Keep this in mind in Java as well, when you’re operating with destructor-like semantics:

  • When using @Before and @After JUnit annotations
  • When allocating, freeing JDBC resources
  • When calling super methods

There are various other use cases. Here’s a concrete example showing how you might implement some event listener SPI:

public void beforeEvent(EventContext e) {
    // Super code before my code

public void afterEvent(EventContext e) {
    // Super code after my code

Another good example showing why this can be important is the infamous Dining Philosophers problem. More info about the dining philosophers can be seen in this awesome post:

The Rule: Whenever you implement logic using before/after, allocate/free, take/return semantics, think about whether the after/free/return operation should perform stuff in the inverse order. tweet this

2. Don’t trust your early SPI evolution judgement

Providing an SPI to your consumers is an easy way to allow them to inject custom behaviour into your library / code. Beware, though, that your SPI evolution judgement may trick you into thinking that you’re (not) going to need that additional parameter. True, no functionality should be added early. But once you’ve published your SPI and once you’ve decided following semantic versioning, you’ll really regret having added a silly, one-argument method to your SPI when you realise that you might need another argument in some cases:

interface EventListener {
    // Bad
    void message(String message);

What if you also need a message ID and a message source? API evolution will prevent you from adding that parameter easily, to the above type. Granted, with Java 8, you could add a defender method, to “defend” you bad early design decision:

interface EventListener {
    // Bad
    default void message(String message) {
        message(message, null, null);
    // Better?
    void message(
        String message,
        Integer id,
        MessageSource source

Note, unfortunately, the defender method cannot be made final.

But much better than polluting your SPI with dozens of methods, use a context object (or argument object) just for this purpose.

interface MessageContext {
    String message();
    Integer id();
    MessageSource source();

interface EventListener {
    // Awesome!
    void message(MessageContext context);

You can evolve the MessageContext API much more easily than the EventListener SPI as fewer users will have implemented it.

The Rule: Whenever you specify an SPI, consider using context / parameter objects instead of writing methods with a fixed amount of parameters. tweet this

Remark: It is often a good idea to also communicate results through a dedicated MessageResult type, which can be constructed through a builder API. This will add even more SPI evolution flexibility to your SPI.

3. Avoid returning anonymous, local, or inner classes

Swing programmers probably have a couple of keyboard shortcuts to generate the code for their hundreds of anonymous classes. In many cases, creating them is nice as you can locally adhere to an interface, without going through the “hassle” of thinking about a full SPI subtype lifecycle.

But you should not use anonymous, local, or inner classes too often for a simple reason: They keep a reference to the outer instance. And they will drag that outer instance to wherevery they go, e.g. to some scope outside of your local class if you’re not careful. This can be a major source for memory leaks, as your whole object graph will suddenly entangle in subtle ways.

The Rule: Whenever you write an anonymous, local or inner class, check if you can make it static or even a regular top-level class. Avoid returning anonymous, local or inner class instances from methods to the outside scope. tweet this

Remark: There has been some clever practice around double-curly braces for simple object instantiation:

new HashMap<String, String>() {{
    put("1", "a");
    put("2", "b");

This leverages Java’s instance initializer as specified by the JLS §8.6. Looks nice (maybe a bit weird), but is really a bad idea. What would otherwise be a completely independent HashMap instance now keeps a reference to the outer instance, whatever that just happens to be. Besides, you’ll create an additional class for the class loader to manage.

4. Start writing SAMs now!

Java 8 is knocking on the door. And with Java 8 come lambdas, whether you like them or not. Your API consumers may like them, though, and you better make sure that they can make use of them as often as possible. Hence, unless your API accepts simple “scalar” types such as int, long, String, Date, let your API accept SAMs as often as possible.

What’s a SAM? A SAM is a Single Abstract Method [Type]. Also known as a functional interface, soon to be annotated with the @FunctionalInterface annotation. This goes well with rule number 2, where EventListener is in fact a SAM. The best SAMs are those with single arguments, as they will further simplify writing of a lambda. Imagine writing

listeners.add(c -> System.out.println(c.message()));

Instead of

listeners.add(new EventListener() {
    public void message(MessageContext c) {

Imagine XML processing through jOOX, which features a couple of SAMs:

    // Find elements with an ID
    .find(c -> $(c).id() != null)
    // Find their  child elements
    .children(c -> $(c).tag().equals("order"))
    // Print all matches
    .each(c -> System.out.println($(c)))

The Rule: Be nice with your API consumers and write SAMs / Functional interfaces already now. tweet this

Remarks: A couple of interesting blog posts about Java 8 Lambdas and improved Collections API can be seen here:

5. Avoid returning null from API methods

I’ve blogged about Java’s NULLs once or twice. I’ve also blogged about Java 8’s introduction of Optional. These are interesting topics both from an academic and from a practical point of view.

While NULLs and NullPointerExceptions will probably stay a major pain in Java for a while, you can still design your API in a way that users will not run into any issues. Try to avoid returning null from API methods whenever possible. Your API consumers should be able to chain methods whenever applicable:


In the above snippet, none of the methods should ever return null. In fact, using null’s semantics (the absence of a value) should be rather exceptional in general. In libraries like jQuery (or jOOX, a Java port thereof), nulls are completely avoided as you’re always operating on iterable objects. Whether you match something or not is irrelevant to the next method call.

Nulls often arise also because of lazy initialisation. In many cases, lazy initialisation can be avoided too, without any significant performance impact. In fact, lazy initialisation should be used carefully, only. If large data structures are involved.

The Rule: Avoid returning nulls from methods whenever possible. Use null only for the “uninitialised” or “absent” semantics. tweet this

6. Never return null arrays or lists from API methods

While there are some cases when returning nulls from methods is OK, there is absolutely no use case of returning null arrays or null collections! Let’s consider the hideous java.io.File.list() method. It returns:

An array of strings naming the files and directories in the directory denoted by this abstract pathname. The array will be empty if the directory is empty. Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

Hence, the correct way to deal with this method is

File directory = // ...

if (directory.isDirectory()) {
    String[] list = directory.list();

    if (list != null) {
        for (String file : list) {
            // ...

Was that null check really necessary? Most I/O operations produce IOExceptions, but this one returns null. Null cannot hold any error message indicating why the I/O error occurred. So this is wrong in three ways:

  • Null does not help in finding the error
  • Null does not allow to distinguish I/O errors from the File instance not being a directory
  • Everyone will keep forgetting about null, here

In collection contexts, the notion of “absence” is best implemented by empty arrays or collections. Having an “absent” array or collection is hardly ever useful, except again, for lazy initialisation.

The Rule: Arrays or Collections should never be null. tweet this

7. Avoid state, be functional

What’s nice about HTTP is the fact that it is stateless. All relevant state is transferred in each request and in each response. This is essential to the naming of REST: Representational State Transfer. This is awesome when done in Java as well. Think of it in terms of rule number 2 when methods receive stateful parameter objects. Things can be so much simpler if state is transferred in such objects, rather than manipulated from the outside. Take JDBC, for instance. The following example fetches a cursor from a stored procedure:

CallableStatement s =
  connection.prepareCall("{ ? = ... }");

// Verbose manipulation of statement state:
s.registerOutParameter(1, cursor);
s.setString(2, "abc");
ResultSet rs = s.getObject(1);

// Verbose manipulation of result set state:

These are the things that make JDBC such an awkward API to deal with. Each object is incredibly stateful and hard to manipulate. Concretely, there are two major issues:

  • It is very hard to correctly deal with stateful APIs in multi-threaded environments
  • It is very hard to make stateful resources globally available, as the state is not documented
State is like a box of chocolates

Theatrical poster for Forrest Gump, Copyright © 1994 by Paramount Pictures. All Rights Reserved. It is believed that the above usage fulfils what is known as Fair Use

The Rule: Implement more of a functional style. Pass state through method arguments. Manipulate less object state. tweet this

8. Short-circuit equals()

This is a low-hanging fruit. In large object graphs, you can gain significantly in terms of performance, if all your objects’ equals() methods dirt-cheaply compare for identity first:

public boolean equals(Object other) {
    if (this == other) return true;

    // Rest of equality logic...

Note, other short-circuit checks may involve null checks, which should be there as well:

public boolean equals(Object other) {
    if (this == other) return true;
    if (other == null) return false;

    // Rest of equality logic...

The Rule: Short-circuit all your equals() methods to gain performance. tweet this

9. Try to make methods final by default

Some will disagree on this, as making things final by default is quite the opposite of what Java developers are used to. But if you’re in full control of all source code, there’s absolutely nothing wrong with making methods final by default, because:

  • If you do need to override a method (do you really?), you can still remove the final keyword
  • You will never accidentally override any method anymore

This specifically applies for static methods, where “overriding” (actually, shadowing) hardly ever makes sense. I’ve come across a very bad example of shadowing static methods in Apache Tika, recently. Consider:

TikaInputStream extends TaggedInputStream and shadows its static get() method with quite a different implementation.

Unlike regular methods, static methods don’t override each other, as the call-site binds a static method invocation at compile-time. If you’re unlucky, you might just get the wrong method accidentally.

The Rule: If you’re in full control of your API, try making as many methods as possible final by default. tweet this

10. Avoid the method(T…) signature

There’s nothing wrong with the occasional “accept-all” varargs method that accepts an Object... argument:

void acceptAll(Object... all);

Writing such a method brings a little JavaScript feeling to the Java ecosystem. Of course, you probably want to restrict the actual type to something more confined in a real-world situation, e.g. String.... And because you don’t want to confine too much, you might think it is a good idea to replace Object by a generic T:

void acceptAll(T... all);

But it’s not. T can always be inferred to Object. In fact, you might as well just not use generics with the above methods. More importantly, you may think that you can overload the above method, but you cannot:

void acceptAll(T... all);
void acceptAll(String message, T... all);

This looks as though you could optionally pass a String message to the method. But what happens to this call here?

acceptAll("Message", 123, "abc");

The compiler will infer <? extends Serializable & Comparable<?>> for T, which makes the call ambiguous!

So, whenever you have an “accept-all” signature (even if it is generic), you will never again be able to typesafely overload it. API consumers may just be lucky enough to “accidentally” have the compiler chose the “right” most specific method. But they may as well be tricked into using the “accept-all” method or they may not be able to call any method at all.

The Rule: Avoid “accept-all” signatures if you can. And if you cannot, never overload such a method. tweet this


Java is a beast. Unlike other, fancier languages, it has evolved slowly to what it is today. And that’s probably a good thing, because already at the speed of development of Java, there are hundreds of caveats, which can only be mastered through years of experience.

jOOQ is the best way to write SQL in Java

Stay tuned for more top 10 lists on the subject!

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21 responses to “10 Subtle Best Practices when Coding Java”

  1. Peter Verhas says :

    As for the SPI thing in point #2. While this is true that creating SPI and for that matter creating API is more an art than engineering I would not state those techniques without critic. When you create any xPI (unless x = chocolate) you can give more or less freedom to the user. Giving less freedom will help the casual user avoid wrong use during compile time. After all that is, for example, why you implement JOOX and JOOQ as fluent API and not just simple method chaining. If done wrong being too restrictive may prevent backward compatible development of the xPI. On the loose end you can pass Map objects as parameters and then you have the total freedom what and how you implement. But this was more than 15 years ago when I was programming Perl and I did not start Java in 2006 to do it the Perl style. So better let’s not get that far, or you may face in an imaginary project that I will reject your git pull request.

    What you propose is somewhere in the middle. Sounds reasonable, but certain circumstances may prove it too restrictive or too loose. Java API was regarded as fairly good at the time. Nowadays we ask many questions, like “why exception is not an interface”, “why there is no immutableX in JDK extended by the interface X (Set, Map etc)” and my questions similar.

    At the bottom line, there is only one thing sure: whatever xPI you design, it will not be a good one in the long run. The only question is: how long will it be good. But then if it is good for a long time, perhaps because there is no use and development of it, which would also be sad. You just never know.

    • lukaseder says :

      After all that is, for example, why you implement JOOX and JOOQ as fluent API and not just simple method chaining.

      Note, jOOX performs method chaining, just like jQuery.

      why exception is not an interface

      Now THAT would be awesome :-)

      At the bottom line, there is only one thing sure: whatever xPI you design, it will not be a good one in the long run. The only question is: how long will it be good.

      Nicely put. Obviously, with semantic versioning, you’re allowed to break your xPI in each major release. So that should define the timespan of how long an xPI should remain “good” before you need to alter it incompatibly.

      • Peter Verhas says :

        This is not about semantic versioning. With semantic versioning you are allowed to break your xPI in each major release. very true. However without it you are also allowed. Semver is only a tool to deliver the message to the users of the xPI what changes there are between releases.

        Breaking backward compatibility is a marketing decision. (Marketing in the wider meaning, including market research.) What are the advantages of the new xPI and what are the disadvantages of backward compatibility flaws. There will be customers jumping on the train at the very first moment (hopefully not falling off under the wheels), and others using the old xPI for a long time before changing. Some may churn stopping to use your product on the long run. You should have a model and estimate your income and costs measured in dollars in case of a commercial product or in other measurable quantity in case of OS project and try to maximize the difference (profit). You have to make the new version easy to accommodate to change for from the old xPI in the mutual interest for the change among your customers. If your change is radical, hard to accomplish be prepared and have costs allocated in due time to support the old version for a long time.

        There are many factors, and they are more marketing that software engineering.

        EXAMPLE: There is log4j still being the most widely used logging framework. And there is slf4j and logback, developed by the same person, and much more advanced than the last version of log4j. Users still use the old version. Why? I see two reasons:

        1. Because log4j is the standard logging framework and management does not like us to use something that is not widely used. (Why they did not name it log4j version whatever the next version is? Probably a marketing mistake.)

        2. Log4j is good enough, there is no demanding need for the change.

        3. API is not fully compatible, needs some work and (more important!) understanding some new structure. Should I waste my brain for such technology as logging? Logging is just the least important tool that I use and it should just work.

        • lukaseder says :

          True words. As always, it’s about the added value to customers. This includes incompatible changes that will result in excess work and potential malfunction at the use site. “Cleaning up dirty internals” is not a good reason to publish a new version with a new API. Improving performance by 80% or adding 70 new features is.

          Some companies are incredibly successful in making their customers believe that previous versions are no longer supported and they indeed need the latest minor version because of an improved button rendering and a better exposé animation and they even pay for it (Apple).

          Other companies may have made an early mistake in their sales process and need to support their legacy for ages providing patches, security updates, service packs and what not (Microsoft, Sun Microsystems with the JVM).

          And yes, then, there are the popular yet “irrelevant” log4j-style libraries which have come to be taken for granted. I’ve had an interesting talk with Thomas Müller, the H2 developer, about attaining market share. It’s just extremely hard, especially if you’re in a business where everything has been said.

  2. Caesar Ralf says :

    Awesome post. Although I already follow these principles for sometime, it’s always good to see some programmers that we respect following the same thing.

    Keep the good work o/

  3. tomekkaczanowski says :

    Ad. 9 – what about mocking? many decent mocking frameworks do not play nicely with final methods

    cheers! (and thanks for this very interesting article!)

    • lukaseder says :

      As I said – some will disagree on this ;-)

      With respect to mocking, from my jOOQ experience, I can say this:

      • I have never felt the urge for mocking. I personally prefer testing a blackbox, which probably qualifies as integration testing rather than unit testing
      • jOOQ’s API is implemented through interfaces. They can still be mocked, even if the implementation is mostly final
      • The example mostly referred to static final methods, where mocking is hard anyway (which is why I like “blackbox-testing”)
  4. Dan says :

    It might help if you actually tell us what SPI means. One of these things?


  5. Ric says :

    I don’t understand your point 9 “Try to make methods final by default” for static methods. Even if your static method is final, it doesn’t prevent someone from defiining a method with the same name and signature in a subclass.

    • Ric says :

      Sorry, I was wrong in my comment dated March 2, I have just written. Delete it.

    • lukaseder says :

      I actually ran into an issue with the example from Apache Tika once. The subtype shadowed the supertype’s static method(s), “overriding” it with a different semantics. This can lead to very dangerous bugs.

      It’s a corner-case, though. If your API is not widely reused, you probably don’t need to worry about making all static methods final as well.

  6. JP says :

    Nice list. Returning Empty object instead of Null has yield more benefit for us then optional or any other trick, Agree that its not always possible, but whenever possible, I prefer it over Optional. It result in much cleaner and readable code. By the I have also shared few practices to avoid NPE in Java, may be your reader find it useful as well.

    • lukaseder says :

      Yes, the “null object pattern” or “empty object instead of null” can help, but it is only possible, if you’re constructing your own types. You cannot apply that technique with, for instance, java.lang.Integer. So, I wonder if from a consistency point of view, this is really a viable solution in an application?

  7. Charles Roth says :

    Great article, thank you. Slight disagreement on “final”, though. The “if you are in full control of all source code” is the tipping point for me. If someone else is ever going to need to use (extend, fix, test, etc.) that software, ‘final’ frequently puts a big orange-painted concrete traffic block in their way. So I guess I’m agreeing with your point, but disagreeing with the premise: we’re never in full control throughout all time of any software — so don’t make it harder for the next guy/gal.

    • lukaseder says :

      Thanks for the feedback. Yes, that part about final may be a bit controversial. Although, as an API designer, I don’t want API consumers tweaking things that were not meant to be tweaked. I personally prefer the “classic” approach of feature request, careful design and planning and then implementation of a clean SPI. After all, in the case of Open Source, there is always a way around such limitations. Be it overriding a method, or patching its sources.

      • Charles Roth says :

        Fair enough. Sometimes I have to finagle my way around someone else’s (final) API when I need to mock something in order to unit-test my code. That can be a nuisance. OTOH, if the API Is carefully designed (as I think you’re suggesting, if I understand you correctly) to an interface, then it’s not an issue.

        • lukaseder says :

          OTOH, if the API Is carefully designed (as I think you’re suggesting, if I understand you correctly) to an interface, then it’s not an issue.

          Yes, because all the potential mocking “contact points” are well-defined in SPIs. In a database API, this could go as far as providing a single MockConnection to mock the entirety of JDBC.

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