Tuesday, May 28, 2013

Java Tips and Best practices to avoid NullPointerException in Java Applications

A NullPointerException in Java application is best way to solve it and that is also key to write robust programs which can work smoothly. As it said “prevention is better than cure”, same is true with nasty NullPointerException. Thankfully by applying some defensive coding techniques and following contract between multiple part of application, you can avoid NullPointerException in Java to a good extent. By the way this is the second post on NullPointerException in Javarevisited, In last post we have discussed about common cause of NullPointerException in Java and in this tutorial,  we will learn some Java coding techniques and best practices, which can be used to avoid NullPointerException in Java. Following these Java tips also minimize number of !=null check, which litter lot of Java code. As an experience Java programmer, you may be aware of some of these techniques and already following it in your project, but for freshers and intermediate developers, this can be good learning. By the way, if you know any other Java tips to avoid NullPointerException and reduce null checks in Java, then please share with us.


Java Tips and Best practices to avoid NullPointerException 

Avoid NullPointerException in Java by following coding best practice and tipsThese are simple techniques, which is very easy to follow, but has significant impact on code quality and robustness. In my experience, just first tip is resulted in significant improvement in code quality. As I said earlier, if you know any other Java tips or best practice, which can help to reduce null check, then you can share with us by commenting on this article.

1) Call equals() and equalsIgnoreCase() method on known String literal rather unknown object
Always call equals() method on known String which is not null. Since equals() method is symmetric, calling a.equals(b) is same as calling b.equals(a), and that’s why many programmer don’t pay attention on object a and b. One side effect of this call can result in NullPointerException, if caller is null.


Object unknownObject = null;

//wrong way - may cause NullPointerException
if(unknownObject.equals("knownObject")){
   System.err.println("This may result in NullPointerException if unknownObject is null");
}

//right way - avoid NullPointerException even if unknownObject is null
if("knownObject".equals(unknownObject)){
    System.err.println("better coding avoided NullPointerException");
}


This is the most easy Java tip or best practice to avoid NullPointerException, but results in tremendous improvement, because of equals()being a common method.

2) Prefer valueOf() over toString() where both return same result
Since calling toString() on null object throws NullPointerException, if we can get same value by calling valueOf() then prefer that, as passing null to  valueOf() returns "null", specially in case of wrapper classes  like Integer, Float, Double or BigDecimal.


BigDecimal bd = getPrice();
System.out.println(String.valueOf(bd)); //doesn’t throw NPE
System.out.println(bd.toString()); //throws "Exception in thread "main" java.lang.NullPointerException"


Follow this Java tips, if you are unsure about object being null or not.

3) Using null safe methods and libraries
There are lot of open source library out there, which does the heavy lifting of checking null for you. One of the most common one is StringUtils from Apache commons. You can use StringUtils.isBlank(), isNumeric(), isWhiteSpace() and other utility methods without worrying of  NullPointerException.


//StringUtils methods are null safe, they don't throw NullPointerException
System.out.println(StringUtils.isEmpty(null));
System.out.println(StringUtils.isBlank(null));
System.out.println(StringUtils.isNumeric(null));
System.out.println(StringUtils.isAllUpperCase(null));

Output:
true
true
false
false


But before reaching to any conclusion don't forget to read the documentation of Null safe methods and classes. This is another Java best practices, which doesn't require much effort, but result in great improvements.


4) Avoid returning null from method, instead return empty collection or empty array.
This Java best practice or tips is also mentioned by Joshua Bloch in his book Effective Java which is another good source of better programming in Java. By returning empty collection or empty array you make sure that basic calls like size(), length() doesn't fail with NullPointerException. Collections class provides convenient empty List, Set and Map as Collections.EMPTY_LIST, Collections.EMPTY_SET and Collections.EMPTY_MAP which can be used accordingly. Here is code example


public List getOrders(Customer customer){
   List result = Collections.EMPTY_LIST;
   return result;
}


Similarly you can use Collections.EMPTY_SET and Collections.EMPTY_MAP instead of returning null.


5)  Use of annotation @NotNull and @Nullable
While writing method you can define contracts about nullability, by declaring whether a method is null safe or not, by using annotations like @NotNull and @Nullable. Modern days compiler, IDE or tool can read this annotation and assist you to put a missing null check, or may inform you about an unnecessary null check, which is cluttering your code. IntelliJ IDE and findbugs already supports such annotation. These annotations are also part of JSR 305, but even in the absence of any tool or IDE support, this  annotation itself work as documentation. By looking @NotNull and @Nullable, programmer can himself decide whether to check for null or not. By the way ,this is relatively new best practice for Java programmers and it will take some time to get adopted.

6)  Avoid unnecessary autoboxing and unboxing in your code
Despite of other disadvantages like creating temporary object, autoboxing are also prone to NullPointerException, if the wrapper class object is null. For example,  following code will fail with NullPointerException if person doesn't have phone number and instead return null.


Person ram = new Person("ram");
int phone = ram.getPhone();


Not just equality but < , > can also throw NullPointerException if used along autoboxing and unboxing. See this article to learn more pitfalls of autoboxing and unboxing in Java.


7) Follow Contract and define reasonable default value
One of the best way to avoid NullPointerException in Java is as simple as defining contracts and following them. Most of the NullPointerException occurs because Object is created with incomplete information or all required dependency is not provided. If you don't allow to create incomplete object and gracefully deny any such request you can prevent lots of NullPointerException down the road. Similarly if  Object is allowed to be created, than you should work with reasonable default value. for example an Employee object can not be created without id and name, but can have an optional phone number. Now if Employee doesn't have phone number than instead of returning null, return default value e.g. zero, but that choice has to be carefully taken sometime checking for null is easy rather than calling an invalid number. One same note, by defining what can be null and what can not be null, caller can make an informed decision. Choice of failing fast or accepting null is also an important design decision you need to take and adhere consistently.


8)  If you are using database for storing your domain object such as Customers, Orders etc than you should define your null-ability constraints on database itself. Since database can acquire data from multiple sources, having null-ability check in DB will ensure data integrity. Maintaining null constraints on database will also help in reducing null check in Java code. While loading objects from database you will be sure, which field can be null and which field is not null, this will minimize unnecessary != null check in code.


9) Use Null Object Pattern
This is another way of avoiding NullPointerExcpetion in Java. If a method returns an object, on which caller, perform some operations e.g. Collection.iterator() method returns Iterator, on which caller performs traversal. Suppose if a caller doesn’t have any Iterator, it can return Null object instead of null. Null object is a special object, which has different meaning in different context, for example, here an empty Iterator, calling hasNext() on which returns false, can be a null object. Similarly in case of method, which returns Container or Collection types, empty object should be used instead of returning null. I am planning to write a separate article on Null Object pattern, where I will share few more examples of NULL objects in Java.

That’s all guys, these are couple of easy to follow Java tips and best practices to avoid NullPointerException. You would appreciate, how useful these tips can be, without too much of effort. If you are using any other tip to avoid this exception, which is not included in this list, than please share with us via comment, and I will include them here.

23 comments :

Javier said...

In point 3 you can add com.google.common.base.Preconditions.checkNotNull, a static method that returns the type provided as argument, so that you can use the function as it were the original object

Anonymous said...

One of the tip, which I have learned hard way is : avoid chaining of methods, like following code

String city = getPerson(id).getAddress().getCity();

Now this method will throw NullPointerException if person is null, or address is null.Instead of chaining multiple methods, you can create a separate method say getCityFromPerson(int id), that will help you during debugging to find out which field is null.

Stevan said...

How can you miss simplest of simple, prefer primitive over Object. Instead of returning Integer, Boolean or Double return primitives like int, boolean and double. Since primitive can not be null, you are sure there would not be any NullPointerException.

Anonymous said...

com.google.common.base.Optional is one more elegant way to deal with nulls

surajtamang said...

Excellent Post Javin. Hoping to read the "Null Object Pattern" soon.

Suraj

Javin @ Connection refused exception in Java said...

Thanks @Anonymous and @suraj, I see use of Optional from Guava another good way to deal with null. Thanks for pointing that.

Anonymous said...

The best practice I think is to consider null references as actual errors. Using null references for program flow should be avoided, and null checks should almost only be necessary for inputs from outside the system.

treaz said...

Jarvin, how is the example at "6) Avoid unnecessary autoboxing and unboxing in your code" related to autoboxing? I think that it's really misleading regarding what autoboxing is...

Javin @ linked list loop detection said...

Hi treaz, It's Javin not Jarvin :), Anyway would you please elaborate, why you think it's misleading? Autoboxing can throw NullPointerException and that's why returning primitive is preferred over wrapper class object e.g. Integer or Boolean, so a code like this

int id = getId();

where getId() returns Integer will fail with NullPointerException, if getId() return a null object.

Srujan kumar Gulla said...

About point 2: String.valueOf(null) - also returns a null pointer excepiton. How is better than null.toString()? There are other advantages to valueOf() mentioned in Effective Java Chapter 1 but not what you mentioned.

Jesus Rafael Lopez Ibarra said...

Hello Srujan Kumar it appears that in that special case it throws and exception , but when it receives a null object it does not:

look:

Object c=null;
String.valueOf(c); /// is OK
String.valueOf(null); // trows Exception


I think that String.valueOf(null); is not a common sentence to use ;D

Ravi said...

@Jesus Rafael Lopez Ibarra, that's something very good detail about valueOf() method, didn't know that it handle null literal that way.

SethW from Firebox said...

Thanks a lot for these tips. I've been trying to get some useful tips on null pointer exceptions for some time now.

Anonymous said...

It's a very useful info towards to valueof() method of String class

Steven Scholnick said...

#4 should use generics and the solution would be slightly different:

public List getOrders(Customer customer){
List result = Collections.emptyList();
return result;
}

The emptyList() method is generics aware, while EMPTY_LIST is just constant (If I remember correctly, it is a zero length list of Objects).

Anonymous said...

Hey Javin,

thank you for the tips. Occasionally, I still use null as return value for single object being able to indicate that there something missing ... and if I have to deal with an NPE then I obviously didn't consider every possible situation :) In addition, I usually start my Eclipse instance in debug mode with an debug point on Exception class (don't forget to check "Subclasses of this exception"). I also add an Filter to the package name of my company. So each time there is an unexpected exception, Eclipse will stop an I have enough time to investigate.

@Anonymous
I like method chaining then it makes my actual intention more obvious (temporary variables would add more details that the user needs to skip). While debugging this chain, you could used the following Eclipse features:

- "Inspect" (Shift+Ctrl+I)
- "Step into Selection" (Ctrl+Alt+left mouse) to jump right into the selected method while debugging

Best regards,

Kon

AbdulBasit Shaikh said...

Hi guys,

The best way according to me to handle null check is to write one utility class which has conditions for all the objects like map, String, int and object itself. So that utility will have one method which accept the object and check for the null, if the passed object is null then it will return a false otherwise true. You have to make sure that before using any object in your code, you should have null check by using the method of utility class. For e.g if(utility.exists(object)){}. Might be helpful.

venkateswarlu boya said...

null!=Object,;

Object!=null ;

which one throws null pointer exception

Anonymous said...

Java 8 has added a class called Optional which will make null handling and avoiding null pointer exception more easy. If you are not using Java 8, then can still uses Google Guava's Optional class, which is even better than Java 8, as it can be used inside foreach loop because it implements Iterable as well. Optional provide clean API and force user to think about what to do when something is null.

Frank Raiser said...

I beg to differ that Guava Optional is better than Java 8 Optional. In fact, Guava's Optional does NOT implement Iterable (just check the API docs). On the contrary, Java 8's Optional is in fact a Monad, which is why it is far superior to Guava's.

Given Java 8's Optional#ofNullable(T) there is yet another way to get rid of NPEs:
1. Eliminate the null symbol from your codebase completely. Nothing justifies writing null anywhere these days.
2. Wrap all results to external APIs (libraries, etc.) into Optional via ofNullable (possibly with deep nesting).

Unfortunately, Java sucks, so 2 would have to be applied to all Java JDK API methods as well. I have no idea how someone could introduce Optional, yet still allow Map#get to return null. The price for downward compatibility has become outrageous already. It basically cripples the language.

PS @Javin: Please improve your command of the English language. Even as a non-native speaker, reading your strange grammatical constructions hurts.

Anonymous said...

Collections.EMPTY_LIST and Collections.emptyList(). Both the methods are immutable. Can you give an example where it can be used?

GOPI said...

Below coding guidelines may help to avoid NPE which in turn leads to silent failures as these cannot be traced out through compiler.
1. use "null == object" instead of "object == null" as if we miss out '=' in latter one then leads to NPE if we reference "object" but in the former one it will be a compiler error.
2. use "null != object" instead of "object != null" as if we miss out '!' in latter one then leads to NPE if we reference "object" but in the former one it will be a compiler error.

Javin Paul said...

@GOPI interesting, Surely I can't deny that such mistakes did happen in real world. Thanks for those tips.

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