In this post, we feature a comprehensive example on java.lang.NullPointerException. In Java, a special
null value can be assigned to an object’s reference and denotes that the object is currently pointing to unknown piece of data. A
NullPointerException is thrown when an application is trying to use or access an object whose reference equals to
null. The following cases throw that exception:
- Invoking a method from a
- Accessing or modifying a
- Taking the length of
null, as if it were an array.
- Accessing or modifying the slots of
nullobject, as if it were an array.
null, as if it were a
- When you try to synchronize over a
1. Why do we need the null value?
As already mentioned,
null is a special value used in Java. It is extremely useful in coding some design patterns, such as Null Object pattern and Singleton pattern. The Null Object pattern provides an object as a surrogate for the lack of an object of a given type. The Singleton pattern ensures that only one instance of a class is created and also, aims for providing a global point of access to the object.
For example, a sample way to create at most one instance of a class is to declare all its constructors as private and then, create a public method that returns the unique instance of the class:
In this example, we declare a static instance of the Singleton class. That instance is initialized at most once inside the
getInstance method. Notice the use of the
null value that enables the unique instance creation.
2. How to avoid the java.lang.NullPointerException
In order to avoid the
NullPointerException, ensure that all your objects are initialized properly, before you use them. Notice that, when you declare a reference variable, you are really creating a pointer to an object. You must verify that the pointer is not null, before you request the method or a field from the object.
Also, if the exception is thrown, use the information residing in the exception’s stack trace. The execution’s stack trace is provided by the JVM, in order to enable the debugging of the application. Locate the method and the line where the exception was caught and then, figure out which reference equals to null in that specific line.
In the rest of this section, we will describe some techniques that deal with the aforementioned exception. However, they do not eliminate the problem and the programmer should always be careful while coding an application.
2.1 String comparison with literals
A very common case in an application’s execution code involves the comparison between a String variable and a literal. The literal may be a String or the element of an Enum. Instead of invoking the method from the null object, consider invoking it from the literal. For example, observe the following case:
The above code snippet will throw a
NullPointerException. However, if we invoke the method from the literal, the execution flow continues normally:
2.2 Check the arguments of a method
Before executing the body of your own method, be sure to check its arguments for null values. Continue with the execution of the method, only when the arguments are properly checked. Otherwise, you can throw an
IllegalArgumentException and notify the calling method that something is wrong with the passed arguments.
2.3 Prefer String.valueOf() method instead of toString()
When your application’s code requires the String representation of an object, avoid using the object’s
toString method. If your object’s reference equals to
NullPointerException will be thrown.
Instead, consider using the static
String.valueOf method, which does not throw any exceptions and prints
"null", in case the function’s argument equals to
2.4 Use the Ternary Operator
First, the boolean expression is evaluated. If the expression is true then, the value1 is returned, otherwise, the value2 is returned. We can use the
ternary operator for handling null pointers as follows:
The message variable will be empty if
str’s reference is null. Otherwise, if
str points to actual data, the message will retrieve the first 10 characters of it.
2.5 Create methods that return empty collections instead of null
A very good technique is to create methods that return an empty collection, instead of a
null value. Your application’s code can iterate over the empty collection and use its methods and fields, without throwing a
NullPointerException. For example:
2.6 Make use of Apache’s StringUtils class
Commons Lang is a library that provides helper utilities for the
java.lang API, such as String manipulation methods. A sample class that provides String manipulation is
StringUtils.java, which handles
null input Strings quietly.
2.7 Use the contains(), containsKey(), containsValue() methods
If your application code makes use of collections, such as
Maps, consider using the contains,
containsValue methods. For example, retrieve the value of a specific key, after you have verified its existence in the map:
In the above snippet, we don’t check if the key actually exists inside the
Map and thus, the returned value can be
null. The safest way is the following:
2.8 Check the return value of external methods
It is very common in practice to make use of external libraries. These libraries contain methods that return a reference. Make sure that the returned reference is not
null. Also, consider reading the Javadoc of the method, in order to better understand its functionality and its return values.
2.9 Use Assertions
Assertions are very useful while testing your code and can be used, in order to avoid executing code snippets that will throw a
NullPointerException. Java Assertions are implemented with the assert keyword and throw an
Notice that you must explicitly enable the assertion flag of the JVM, by executing it with the
–ea argument. Otherwise, the assertions will be completely ignored.
A sample example using Java assertions is the following:
If you execute the above code snippet and pass a null argument to
getLength, the following error message will appear:
2.10 Unit Tests
Unit tests can be extremely useful while testing the functionality and correctness of your code. Devote some time to write a couple tests cases that verify that no
NullPointerException is thrown, while your application’s code undergoes a specific execution flow.
3. Existing NullPointerException safe methods
3.1 Accessing static members or methods of a class
When your code attempts to access a static variable or method of a class, even if the object’s reference equals to
null, the JVM does not throw a
NullPointerException. This is due to the fact that the Java compiler stores the static methods and fields in a special place, during the compilation procedure. Thus, the static fields and methods are not associated with objects, rather with the name of the class.
For example, the following code does not throw a
Notice, that despite the fact that the instance of the
SampleClass equals to
null, the method will be executed properly. However, when it comes to static methods or fields, it is better to access them in a static way, such as
3.2 The instanceof operator
instanceof operator can be used, even if the object’s reference equals to
instanceof operator returns false when the reference equals to null and does not throw a
NullPointerException. For example consider the following code snippet:
The result of the execution is, as expected:
This was a tutorial on how to handle java.lang.NullPointerException – Java’s