Hindari Check for Null Statement di Java

1. Ikhtisar

Umumnya, variabel null , referensi, dan koleksi sulit ditangani dalam kode Java. Tidak hanya sulit diidentifikasi, tetapi juga rumit untuk ditangani.

Faktanya, setiap kesalahan dalam menangani null tidak dapat diidentifikasi pada waktu kompilasi dan menghasilkan NullPointerException pada waktu proses.

Dalam tutorial ini, kita akan melihat kebutuhan untuk memeriksa null di Java dan berbagai alternatif yang membantu kita menghindari pemeriksaan null dalam kode kita.

2. Apa Itu NullPointerException?

Menurut Javadoc untuk NullPointerException , ini dilemparkan saat aplikasi mencoba menggunakan null dalam kasus di mana objek diperlukan, seperti:

  • Memanggil metode instance dari objek null
  • Mengakses atau memodifikasi bidang objek null
  • Mengambil panjang null seolah-olah itu adalah array
  • Mengakses atau memodifikasi slot null seolah-olah itu adalah sebuah array
  • Melempar null seolah-olah itu adalah nilai Throwable

Mari kita lihat dengan cepat beberapa contoh kode Java yang menyebabkan pengecualian ini:

public void doSomething() { String result = doSomethingElse(); if (result.equalsIgnoreCase("Success")) // success } } private String doSomethingElse() { return null; }

Di sini, kami mencoba memanggil metode panggilan untuk referensi null . Ini akan menghasilkan NullPointerException.

Contoh umum lainnya adalah jika kita mencoba mengakses array null :

public static void main(String[] args) { findMax(null); } private static void findMax(int[] arr) { int max = arr[0]; //check other elements in loop }

Ini menyebabkan NullPointerException di baris 6.

Jadi, mengakses kolom , metode, atau indeks apa pun dari objek null menyebabkan NullPointerException , seperti yang dapat dilihat dari contoh di atas.

Cara umum untuk menghindari NullPointerException adalah dengan memeriksa null :

public void doSomething() { String result = doSomethingElse(); if (result != null && result.equalsIgnoreCase("Success")) { // success } else // failure } private String doSomethingElse() { return null; }

Di dunia nyata, programmer merasa sulit untuk mengidentifikasi objek mana yang bisa nol. Strategi yang aman secara agresif adalah dengan memeriksa nol untuk setiap objek. Namun, ini menyebabkan banyak pemeriksaan null yang berlebihan dan membuat kode kita kurang dapat dibaca.

Di beberapa bagian berikutnya, kita akan membahas beberapa alternatif di Java yang menghindari redundansi seperti itu.

3. Penanganan null Melalui Kontrak API

Seperti yang dibahas di bagian terakhir, mengakses metode atau variabel objek null menyebabkan NullPointerException. Kami juga membahas bahwa memberi centang null pada objek sebelum mengaksesnya menghilangkan kemungkinan NullPointerException.

Namun, seringkali ada API yang dapat menangani nilai null . Sebagai contoh:

public void print(Object param) { System.out.println("Printing " + param); } public Object process() throws Exception { Object result = doSomething(); if (result == null) { throw new Exception("Processing fail. Got a null response"); } else { return result; } }

The print () metode panggilan hanya akan mencetak “null” tetapi tidak akan melemparkan sebuah pengecualian. Demikian pula, proses () tidak akan pernah mengembalikan null dalam responsnya. Ini lebih seperti melempar Exception.

Jadi untuk kode klien yang mengakses API di atas, tidak perlu pemeriksaan null .

Namun, API tersebut harus membuatnya eksplisit dalam kontrak mereka. Tempat umum bagi API untuk menerbitkan kontrak semacam itu adalah JavaDoc .

Namun, ini tidak memberikan indikasi yang jelas tentang kontrak API dan karenanya bergantung pada pengembang kode klien untuk memastikan kepatuhannya.

Di bagian selanjutnya, kita akan melihat bagaimana beberapa IDE dan alat pengembangan lainnya membantu pengembang dalam hal ini.

4. Mengotomatiskan Kontrak API

4.1. Menggunakan Analisis Kode Statis

Alat analisis kode statis membantu meningkatkan kualitas kode secara signifikan. Dan beberapa alat seperti itu juga memungkinkan pengembang untuk mempertahankan kontrak nol . Salah satu contohnya adalah FindBugs.

FindBugs membantu mengelola nol kontrak melalui @Nullable dan @NonNull penjelasan. Kita dapat menggunakan anotasi ini di atas metode, bidang, variabel lokal, atau parameter apa pun. Ini membuatnya eksplisit untuk kode klien apakah tipe yang dianotasi bisa nihil atau tidak. Mari kita lihat contohnya:

public void accept(@Nonnull Object param) { System.out.println(param.toString()); }

Here, @NonNull makes it clear that the argument cannot be null. If the client code calls this method without checking the argument for null, FindBugs would generate a warning at compile time.

4.2. Using IDE Support

Developers generally rely on IDEs for writing Java code. And features such as smart code completion and useful warnings, like when a variable may not have been assigned, certainly help to a great extent.

Some IDEs also allow developers to manage API contracts and thereby eliminate the need for a static code analysis tool. IntelliJ IDEA provides the @NonNull and @Nullable annotations. To add the support for these annotations in IntelliJ, we must add the following Maven dependency:

 org.jetbrains annotations 16.0.2 

Now, IntelliJ will generate a warning if the null check is missing, like in our last example.

IntelliJ also provides a Contract annotation for handling complex API contracts.

5. Assertions

Until now, we've only talked about removing the need for null checks from the client code. But, that is rarely applicable in real-world applications.

Now, let's suppose that we're working with an API that cannot accept null parameters or can return a null response that must be handled by the client. This presents the need for us to check the parameters or the response for a null value.

Here, we can use Java Assertions instead of the traditional null check conditional statement:

public void accept(Object param){ assert param != null; doSomething(param); }

In line 2, we check for a null parameter. If the assertions are enabled, this would result in an AssertionError.

Although it is a good way of asserting pre-conditions like non-null parameters, this approach has two major problems:

  1. Assertions are usually disabled in a JVM
  2. A false assertion results in an unchecked error that is irrecoverable

Hence, it is not recommended for programmers to use Assertions for checking conditions. In the following sections, we'll discuss other ways of handling null validations.

6. Avoiding Null Checks Through Coding Practices

6.1. Preconditions

It's usually a good practice to write code that fails early. Therefore, if an API accepts multiple parameters that aren't allowed to be null, it's better to check for every non-null parameter as a precondition of the API.

For example, let's look at two methods – one that fails early, and one that doesn't:

public void goodAccept(String one, String two, String three) { if (one == null || two == null || three == null) { throw new IllegalArgumentException(); } process(one); process(two); process(three); } public void badAccept(String one, String two, String three) { if (one == null) { throw new IllegalArgumentException(); } else { process(one); } if (two == null) { throw new IllegalArgumentException(); } else { process(two); } if (three == null) { throw new IllegalArgumentException(); } else { process(three); } }

Clearly, we should prefer goodAccept() over badAccept().

As an alternative, we can also use Guava's Preconditions for validating API parameters.

6.2. Using Primitives Instead of Wrapper Classes

Since null is not an acceptable value for primitives like int, we should prefer them over their wrapper counterparts like Integer wherever possible.

Consider two implementations of a method that sums two integers:

public static int primitiveSum(int a, int b) { return a + b; } public static Integer wrapperSum(Integer a, Integer b) { return a + b; }

Now, let's call these APIs in our client code:

int sum = primitiveSum(null, 2);

This would result in a compile-time error since null is not a valid value for an int.

And when using the API with wrapper classes, we get a NullPointerException:

assertThrows(NullPointerException.class, () -> wrapperSum(null, 2));

There are also other factors for using primitives over wrappers, as we covered in another tutorial, Java Primitives versus Objects.

6.3. Empty Collections

Occasionally, we need to return a collection as a response from a method. For such methods, we should always try to return an empty collection instead of null:

public List names() { if (userExists()) { return Stream.of(readName()).collect(Collectors.toList()); } else { return Collections.emptyList(); } }

Hence, we've avoided the need for our client to perform a null check when calling this method.

7. Using Objects

Java 7 introduced the new Objects API. This API has several static utility methods that take away a lot of redundant code. Let's look at one such method, requireNonNull():

public void accept(Object param) { Objects.requireNonNull(param); // doSomething() }

Now, let's test the accept() method:

assertThrows(NullPointerException.class, () -> accept(null));

So, if null is passed as an argument, accept() throws a NullPointerException.

This class also has isNull() and nonNull() methods that can be used as predicates to check an object for null.

8. Using Optional

8.1. Using orElseThrow

Java 8 introduced a new Optional API in the language. This offers a better contract for handling optional values compared to null. Let's see how Optional takes away the need for null checks:

public Optional process(boolean processed) { String response = doSomething(processed); if (response == null) { return Optional.empty(); } return Optional.of(response); } private String doSomething(boolean processed) { if (processed) { return "passed"; } else { return null; } }

By returning an Optional, as shown above, the process method makes it clear to the caller that the response can be empty and must be handled at compile time.

This notably takes away the need for any null checks in the client code. An empty response can be handled differently using the declarative style of the Optional API:

assertThrows(Exception.class, () -> process(false).orElseThrow(() -> new Exception()));

Furthermore, it also provides a better contract to API developers to signify to the clients that an API can return an empty response.

Although we eliminated the need for a null check on the caller of this API, we used it to return an empty response. To avoid this, Optional provides an ofNullable method that returns an Optional with the specified value, or empty, if the value is null:

public Optional process(boolean processed) { String response = doSomething(processed); return Optional.ofNullable(response); }

8.2. Using Optional with Collections

While dealing with empty collections, Optional comes in handy:

public String findFirst() { return getList().stream() .findFirst() .orElse(DEFAULT_VALUE); }

This function is supposed to return the first item of a list. The Stream API's findFirst function will return an empty Optional when there is no data. Here, we have used orElse to provide a default value instead.

This allows us to handle either empty lists, or lists, which after we have used the Stream library's filter method, have no items to supply.

Alternatively, we can also allow the client to decide how to handle empty by returning Optional from this method:

public Optional findOptionalFirst() { return getList().stream() .findFirst(); }

Therefore, if the result of getList is empty, this method will return an empty Optional to the client.

Using Optional with collections allows us to design APIs that are sure to return non-null values, thus avoiding explicit null checks on the client.

It's important to note here that this implementation relies on getList not returning null. However, as we discussed in the last section, it's often better to return an empty list rather than a null.

8.3. Combining Optionals

When we start making our functions return Optional we need a way to combine their results into a single value. Let's take our getList example from earlier. What if it were to return an Optional list, or were to be wrapped with a method that wrapped a null with Optional using ofNullable?

Our findFirst method wants to return an Optional first element of an Optional list:

public Optional optionalListFirst() { return getOptionalList() .flatMap(list -> list.stream().findFirst()); }

By using the flatMap function on the Optional returned from getOptional we can unpack the result of an inner expression that returns Optional. Without flatMap, the result would be Optional . The flatMap operation is only performed when the Optional is not empty.

9. Libraries

9.1. Using Lombok

Lombok is a great library that reduces the amount of boilerplate code in our projects. It comes with a set of annotations that take the place of common parts of code we often write ourselves in Java applications, such as getters, setters, and toString(), to name a few.

Another of its annotations is @NonNull. So, if a project already uses Lombok to eliminate boilerplate code, @NonNull can replace the need for null checks.

Before we move on to see some examples, let's add a Maven dependency for Lombok:

 org.projectlombok lombok 1.18.6 

Now, we can use @NonNull wherever a null check is needed:

public void accept(@NonNull Object param){ System.out.println(param); }

So, we simply annotated the object for which the null check would've been required, and Lombok generates the compiled class:

public void accept(@NonNull Object param) { if (param == null) { throw new NullPointerException("param"); } else { System.out.println(param); } }

If param is null, this method throws a NullPointerException. The method must make this explicit in its contract, and the client code must handle the exception.

9.2. Using StringUtils

Generally, String validation includes a check for an empty value in addition to null value. Therefore, a common validation statement would be:

public void accept(String param){ if (null != param && !param.isEmpty()) System.out.println(param); }

This quickly becomes redundant if we have to deal with a lot of String types. This is where StringUtils comes handy. Before we see this in action, let's add a Maven dependency for commons-lang3:

 org.apache.commons commons-lang3 3.8.1 

Let's now refactor the above code with StringUtils:

public void accept(String param) { if (StringUtils.isNotEmpty(param)) System.out.println(param); }

So, we replaced our null or empty check with a static utility method isNotEmpty(). This API offers other powerful utility methods for handling common String functions.

10. Kesimpulan

Dalam artikel ini, kami melihat berbagai alasan untuk NullPointerException dan mengapa sulit diidentifikasi. Kemudian, kami melihat berbagai cara untuk menghindari redundansi dalam kode seputar memeriksa null dengan parameter, tipe kembalian, dan variabel lainnya.

Semua contoh tersedia di GitHub.