Java Null Pointer Exception (NPE): Enhanced Error Reporting with Helpful NPE

Discover the Helpful NullPointerException feature introduced in Java 14, which improves error messages by providing more detailed information about the source of a NullPointerException. Learn how this enhancement helps developers quickly identify and fix null reference issues in Java applications, moving beyond just the filename, method, and line number.

Java - Null Pointer Exception (NPE)

Introduction to Helpful NullPointerException

In Java 14, a feature called Helpful NullPointerException was introduced as a preview, which later became a standard part of the JDK. This enhancement was designed to provide more detailed error messages compared to the traditional NullPointerException, which only reported the filename, method, and line number where the issue occurred.

Traditional NullPointerException in Java

Before the Java 14 enhancement, when a null object was accessed, the following error would occur:

Code Snippet


// Create an employee without a department
Employee emp = new Employee(101, "Alice", null);

// Attempt to get the name of the department (which is null)
String dept = emp.getDept().getName();

Output


Exception in thread "main" java.lang.NullPointerException
at com.example.Main.main(Main.java:10)

Informative NullPointerException in Java

When debugging, such messages aren’t very helpful, especially when multiple levels of object nesting are involved. To solve this, Java 14 introduced more informative error messages that help pinpoint the exact issue. Here's an example of the new message:


Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.example.Department.getName()" because the return value of "com.example.Employee.getDept()" is null
at com.example.Main.main(Main.java:10)

To enable this feature in Java 14, you had to run your program with the following flag:

Command


java -XX:+ShowCodeDetailsInExceptionMessages Main

From Java 20 onwards, this flag is enabled by default.

Example: NullPointerException for Null Objects

In this example, we create an employee object without a department, which leads to a NullPointerException when trying to access the department’s name. The enhanced error message helps identify the null reference.

Code Snippet


package com.example;

public class Main {
public static void main(String[] args) {
    // Create an employee without a department
    Employee emp = new Employee(101, "Alice", null);

    // This will throw a NullPointerException
    String deptName = emp.getDept().getName();
    System.out.println(deptName);
}  
}

class Employee {
int id;
String name;
Department dept;

public Employee(int id, String name, Department dept) {
    this.id = id;
    this.name = name;
    this.dept = dept;
}

public Department getDept() {
    return dept;
}
}

class Department {
String name;

public String getName() {
    return name;
}
}

Output


Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.example.Department.getName()" because the return value of "com.example.Employee.getDept()" is null
at com.example.Main.main(Main.java:10)

NullPointerException for Local Variables

In Java 14, the JVM also enhanced error messages for null local variables. If a local variable is null, the JVM prints "" in the error message. For example:

Code Snippet


String str = null;
System.out.println(str.length());

Output


Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "" is null
at com.example.Main.main(Main.java:8)

To get the actual variable name in error messages in Java 14, you had to compile the program with the -g flag:

Command


javac -g Main.java

In Java 20 and beyond, this behavior is enabled by default, as shown in the next example.

Example: NullPointerException for Local Null Variable

This example demonstrates how the JVM provides an enhanced error message when a null local variable is used:

Code Snippet


package com.example;

public class Main {
public static void main(String[] args) {
    // Declare a null string
    String name = null;

    // Accessing a method of a null variable will throw NullPointerException
    System.out.println(name.length());
}  
}

Output


Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "name" is null
at com.example.Main.main(Main.java:8)