摘要：本文详细描述了如何使用 Java8 新特性来避免 null 检查，全文分析的十分深入，包括了 Optional 的若干操作，值得参考。
Tired of Null Pointer Exceptions? Consider Using Java SE 8’s Optional!
by Raoul-Gabriel Urma
Make your code more readable and protect it against null pointer exceptions.
Published March 2014
A wise man once said you are not a real Java programmer until you’ve dealt with a null pointer exception. Joking aside, the null reference is the source of many problems because it is often used to denote the absence of a value. Java SE 8 introduces a new class called java.util.Optional that can alleviate some of these problems.
Let’s start with an example to see the dangers of null. Let’s consider a nested object structure for a Computer, as illustrated in Figure 1.
Figure 1: A nested structure for representing a Computer
What’s possibly problematic with the following code?
String version = computer.getSoundcard().getUSB().getVersion();
This code looks pretty reasonable. However, many computers (for example, the Raspberry Pi) don’t actually ship with a sound card. So what is the result of getSoundcard()?
A common (bad) practice is to return the null reference to indicate the absence of a sound card. Unfortunately, this means the call to getUSB() will try to return the USB port of a null reference, which will result in a NullPointerException at runtime and stop your program from running further. Imagine if your program was running on a customer’s machine; what would your customer say if the program suddenly failed?
To give some historical context, Tony Hoare—one of the giants of computer science—wrote, “I call it my billion-dollar mistake. It was the invention of the null reference in 1965. I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement.”
所有讨论 Null Exception 的引用。
What can you do to prevent unintended null pointer exceptions? You can be defensive and add checks to prevent null dereferences, as shown in Listing 1:
String version = "UNKNOWN";
if(computer != null){
Soundcard soundcard = computer.getSoundcard();
if(soundcard != null){
USB usb = soundcard.getUSB();
if(usb != null){
version = usb.getVersion();
}
}
}
Listing 1
However, you can see that the code in Listing 1 quickly becomes very ugly due to the nested checks. Unfortunately, we need a lot of boilerplate code to make sure we don’t get a NullPointerException. In addition, it’s just annoying that these checks get in the way of the business logic. In fact, they are decreasing the overall readability of our program.
Furthermore, it is an error-prone（易于出错的） process; what if you forget to check that one property could be null? I will argue in this article that using null to represent the absence of a value is a wrong approach. What we need is a better way to model the absence and presence of a value.
To give some context, let’s briefly look at what other programming languages have to offer.
What Alternatives to Null Are There?
Languages such as Groovy have a safe navigation operator represented by “?.“ to safely navigate through potential null references. (Note that it is soon to be included in C#, too, and it was proposed for Java SE 7 but didn’t make it into that release.) It works as follows:
String version = computer?.getSoundcard()?.getUSB()?.getVersion();
In this case, the variable version will be assigned to null if computer is null, or getSoundcard() returns null, or getUSB() returns null. You don’t need to write complex nested conditions to check for null.
In addition, Groovy also includes the Elvis operator “?:“ (if you look at it sideways, you’ll recognize Elvis’ famous hair), which can be used for simple cases when a default value is needed. In the following, if the expression that uses the safe navigation operator returns null, the default value "UNKNOWN" is returned; otherwise, the available version tag is returned.
String version =
computer?.getSoundcard()?.getUSB()?.getVersion() ?: "UNKNOWN";
Other functional languages, such as Haskell and Scala, take a different view. Haskell includes a Maybe type, which essentially encapsulates an optional value. A value of type Maybe can contain either a value of a given type or nothing. There is no concept of a null reference. Scala has a similar construct called Option[T] to encapsulate the presence or absence of a value of type T. You then have to explicitly check whether a value is present or not using operations available on the Option type, which enforces the idea of “null checking.” You can no longer “forget to do it” because it is enforced by the type system.
OK, we diverged a bit and all this sounds fairly abstract. You might now wonder, “so, what about Java SE 8?”
Optional in a Nutshell
Java SE 8 introduces a new class called java.util.Optional<t> that is inspired from the ideas of Haskell and Scala. It is a class that encapsulates an optional value, as illustrated in Listing 2 below and in Figure 1. You can view Optional as a single-value container that either contains a value or doesn’t (it is then said to be “empty”), as illustrated in Figure 2.
Figure 2: An optional sound card
We can update our model to make use of Optional, as shown in Listing 2:
1 | public class Computer { |
Listing 2
The code in Listing 2 immediately shows that a computer might or might not have a sound card (the sound card is optional). In addition, a sound card can optionally have a USB port. This is an improvement, because this new model can now reflect clearly whether a given value is allowed to be missing. Note that similar ideas have been available in libraries such as Guava.
But what can you actually do with an Optional<soundcard> object? After all, you want to get to the USB port’s version number. In a nutshell, the Optional class includes methods to explicitly deal with the cases where a value is present or absent. However, the advantage compared to null references is that the Optional class forces you to think about the case when the value is not present. As a consequence, you can prevent unintended null pointer exceptions.
It is important to note that the intention of the Optional class is not to replace every single null reference. Instead, its purpose is to help design more-comprehensible APIs so that by just reading the signature of a method, you can tell whether you can expect an optional value. This forces you to actively unwrap an Optional to deal with the absence of a value.
Patterns for Adopting Optional
Enough talking; let’s see some code! We will first explore how typical null-check patterns can be rewritten using Optional. By the end of this article, you will understand how to use Optional, as shown below, to rewrite the code in Listing 1 that was doing several nested null checks:
1 | String name = computer.flatMap(Computer::getSoundcard) |
Note: Make sure you brush up on the Java SE 8 lambdas and method references syntax (see “Java 8: Lambdas“) as well as its stream pipelining concepts (see “Processing Data with Java SE 8 Streams“).
Creating Optional objects
First, how do you create Optional objects? There are several ways:
Here is an empty Optional:
Optional<soundcard> sc = Optional.empty();
And here is an Optional with a non-null value:
SoundCard soundcard = new Soundcard();
Optional<soundcard> sc = Optional.of(soundcard);
If soundcard were null, a NullPointerException would be immediately thrown (rather than getting a latent error once you try to access properties of the soundcard).
Also, by using ofNullable, you can create an Optional object that may hold a null value:
Optional<soundcard> sc = Optional.ofNullable(soundcard);
If soundcard were null, the resulting Optional object would be empty.
Do Something If a Value Is Present
Now that you have an Optional object, you can access the methods available to explicitly deal with the presence or absence of values. Instead of having to remember to do a null check, as follows:
SoundCard soundcard = ...;
if(soundcard != null){
System.out.println(soundcard);
}
You can use the ifPresent() method, as follows:
Optional<soundcard> soundcard = ...;
soundcard.ifPresent(System.out::println);
You no longer need to do an explicit null check; it is enforced by the type system. If the Optional object were empty, nothing would be printed. ifPresent就是表示如果对象不为空则执行某些操作。
You can also use the isPresent() method to find out whether a value is present in an Optional object. In addition, there’s a get() method that returns the value contained in the Optional object, if it is present. Otherwise, it throws a NoSuchElementException. The two methods can be combined, as follows, to prevent exceptions:
if(soundcard.isPresent()){
System.out.println(soundcard.get());
}
However, this is not the recommended use of Optional (it’s not much of an improvement over nested null checks), and there are more idiomatic alternatives, which we explore below. 这用用法实际上还是一个 if 语句，所以作者不推荐。
Default Values and Actions
A typical pattern is to return a default value if you determine that the result of an operation is null. In general, you can use the ternary operator, as follows, to achieve this:
Soundcard soundcard =
maybeSoundcard != null ? maybeSoundcard
: new Soundcard("basic_sound_card");
Using an Optional object, you can rewrite this code by using the orElse() method, which provides a default value if Optional is empty:
Soundcard soundcard = maybeSoundcard.orElse(new Soundcard("defaut"));
Similarly, you can use the orElseThrow() method, which instead of providing a default value if Optional is empty, throws an exception:
Soundcard soundcard =
maybeSoundCard.orElseThrow(IllegalStateException::new);
Rejecting Certain Values Using the filter Method
Often you need to call a method on an object and check some property. For example, you might need to check whether the USB port is a particular version. To do this in a safe way, you first need to check whether the reference pointing to a USB object is null and then call the getVersion() method, as follows:
USB usb = ...;
if(usb != null && "3.0".equals(usb.getVersion())){
System.out.println("ok");
}
This pattern can be rewritten using the filter method on an Optional object, as follows:
Optional<usb> maybeUSB = ...;
maybeUSB.filter(usb -> "3.0".equals(usb.getVersion())
.ifPresent(() -> System.out.println("ok"));
The filter method takes a predicate as an argument. If a value is present in the Optional object and it matches the predicate, the filter method returns that value; otherwise, it returns an empty Optional object. You might have seen a similar pattern already if you have used the filter method with the Stream interface.
Extracting and Transforming Values Using the map Method
Another common pattern is to extract information from an object. For example, from a Soundcard object, you might want to extract the USB object and then further check whether it is of the correct version. You would typically write the following code:
if(soundcard != null){
USB usb = soundcard.getUSB();
if(usb != null && "3.0".equals(usb.getVersion()){
System.out.println("ok");
}
}
We can rewrite this pattern of “checking for null and extracting” (here, the Soundcard object) using the map method.
Optional<usb> usb = maybeSoundcard.map(Soundcard::getUSB);
There’s a direct parallel to the map method used with streams. There, you pass a function to the map method, which applies this function to each element of a stream. However, nothing happens if the stream is empty.
The map method of the Optional class does exactly the same: the value contained inside Optional is “transformed” by the function passed as an argument (here, a method reference to extract the USB port), while nothing happens if Optional is empty.
Finally, we can combine the map method with the filter method to reject a USB port whose version is different than 3.0:
maybeSoundcard.map(Soundcard::getUSB)
.filter(usb -> "3.0".equals(usb.getVersion())
.ifPresent(() -> System.out.println("ok"));
Awesome; our code is starting to look closer to the problem statement and there are no verbose null checks getting in our way!
Cascading Optional Objects Using the flatMap Method
You’ve seen a few patterns that can be refactored to use Optional. So how can we write the following code in a safe way?
String version = computer.getSoundcard().getUSB().getVersion();
Notice that all this code does is extract one object from another one, which is exactly what the map method is for. Earlier in the article, we changed our model so a Computer has an Optional<soundcard> and a Soundcard has an Optional<usb>, so we should be able to write the following:
String version = computer.map(Computer::getSoundcard)
.map(Soundcard::getUSB)
.map(USB::getVersion)
.orElse("UNKNOWN");
Unfortunately, this code doesn’t compile. Why? The variable computer is of type Optional<computer>, so it is perfectly correct to call the map method. However, getSoundcard() returns an object of type Optional<soundcard>. This means the result of the map operation is an object of type Optional<optional<soundcard>>. As a result, the call to getUSB() is invalid because the outermost Optional contains as its value another Optional, which of course doesn’t support the getUSB() method. Figure 3 illustrates the nested Optional structure you would get. 在多层次获取对象时，实际上是获取的 Optional 的对象，这样在第二次实际上获得了 Optional
Figure 3: A two-level Optional
So how can we solve this problem? Again, we can look at a pattern you might have used previously with streams: the flatMap method. With streams, the flatMap method takes a function as an argument, which returns another stream. This function is applied to each element of a stream, which would result in a stream of streams. However, flatMap has the effect of replacing each generated stream by the contents of that stream. In other words, all the separate streams that are generated by the function get amalgamated or “flattened” into one single stream. What we want here is something similar, but we want to “flatten” a two-level Optional into one.
Well, here’s good news: Optional also supports a flatMap method. Its purpose is to apply the transformation function on the value of an Optional (just like the map operation does) and then flatten the resulting two-level Optional into a single one. Figure 4 illustrates the difference between map and flatMap when the transformation function returns an Optional object.
Figure 4: Using map versus flatMap with Optional
So, to make our code correct, we need to rewrite it as follows using flatMap:
String version = computer.flatMap(Computer::getSoundcard)
.flatMap(Soundcard::getUSB)
.map(USB::getVersion)
.orElse("UNKNOWN");
The first flatMap ensures that an Optional<soundcard> is returned instead of an Optional<optional<soundcard>>, and the second flatMap achieves the same purpose to return an Optional<usb>. Note that the third call just needs to be a map() because getVersion() returns a String rather than an Optional object.
Wow! We’ve come a long way from writing painful nested null checks to writing declarative code that is composable, readable, and better protected from null pointer exceptions.
Conclusion
In this article, we have seen how you can adopt the new Java SE 8 java.util.Optional<t>. The purpose of Optional is not to replace every single null reference in your codebase but rather to help design better APIs in which—just by reading the signature of a method—users can tell whether to expect an optional value. In addition, Optional forces you to actively unwrap an Optional to deal with the absence of a value; as a result, you protect your code against unintended null pointer exceptions.
See Also
Acknowledgments
Thanks to Alan Mycroft and Mario Fusco for going through the adventure of writing Java 8 in Action: Lambdas, Streams, and Functional-style Programming with me.
About the Author
Raoul-Gabriel Urma (@raoulUK) is currently completing a PhD in computer science at the University of Cambridge, where he does research in programming languages. He’s a coauthor of the upcoming book Java 8 in Action: Lambdas, Streams, and Functional-style Programming, published by Manning. He is also a regular speaker at major Java conferences (for example, Devoxx and Fosdem) and an instructor. In addition, he has worked at several well-known companies—including Google’s Python team, Oracle’s Java Platform group, eBay, and Goldman Sachs—as well as for several startup projects.
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