Being the most common exception in Java, the infamous NullPointerException is probably the first exception a new Java developer experiences.
To offer helpful information to developers and improve the usability of the language, Java 14 introduces improved NullPointerException messages which point out precisely the variable that was null.
NPEs before Java 14
Let’s say we have the following classes.
class Person {
Name name;
}
class Name {
String firstName;
String lastName;
}
A NullPointerException occurs when we try to dereference a variable that points to nothing.
Person person = null;
System.out.println(person.name);
// Exception in thread "main" java.lang.NullPointerException
// at mypackage.MyClass.main(MyClass.java:7)
Reading the error message, it’s fairly obvious that person is null.
However, when the code is more complex, the error message is not that helpful anymore.
Imagine we get a reference to a Person object and then try to access its fields.
var person = getPersonFromDatabase();
System.out.println(person.name.firstName);
// Exception in thread "main" java.lang.NullPointerException
// at mypackage.MyClass.main(MyClass.java:7)
From the error message we can infer that either person or name was null.
If the issue is reproducible and we have access to the source code we could debug the problem by attaching a debugger or create intermediate local variables.
However, when the error is found from logs from a production server, we don’t have a clear indication which variable was null.
New and improved NPE error messages
Java 14 introduces Helpful NullPointerExceptions that improve NPE error messages by describing precisely which variable was null.
If we were to run the previous example in Java 14, we would get a different error message.
var person = getPersonFromDatabase();
System.out.println(person.name.firstName);
// Exception in thread "main" java.lang.NullPointerException: Cannot read field "firstName" because "person.name" is null
// at mypackage.MyClass.main(MyClass.java:7)
This time, the error message is more usable and there’s no need to hunt down which variable is null.
Summary
By pointing out which variable was null in the error message greatly improves the usability of the language.
Keep in mind that in Java 14, helpful NullPointerException messages are turned off by default.
You need to start the JVM with the -XX:+ShowCodeDetailsInExceptionMessages command line flag.
Helpful error messages will be enabled by default in Java 15.