Java Helpful NullPointerExceptions Demo

This repository contains an example code that uses the DOM API to discover an XML document.

A typical usage of this API can be seen in line 83:

department.getElementsByTagName("name").item(0).getChildNodes().item(0).getNodeValue();

If any method returns a null in this chain, it will blow up with a NullPointerException. But can you tell which?

In the old days of Java all we got is:

Exception in thread "main" java.lang.NullPointerException
        at Main.processEmployee(Main.java:83)

Finding out the exact source of the problem is left as an exercise for the reader.

However, since Java 14 with Helpful NullPointerExceptions the message contains a more precise reason:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "org.w3c.dom.Node.getChildNodes()" because the return value of "org.w3c.dom.NodeList.item(int)" is null
        at Main.processEmployee(Main.java:83)

Usage

With Java 15 and above

# Compile
javac -source 15 Main.java

# Helpful NullPointerExceptions are enabled by default
java -cp . Main

With Java 14

# Compile
javac --enable-preview -source 14 Main.java

# Run with Helpful NullPointerExceptions enabled
java -cp . --enable-preview -XX:+ShowCodeDetailsInExceptionMessages Main

# By default Helpful NullPointerExceptions are not enabled
java -cp . --enable-preview Main

dodie/java-helpful-npe-demo

Java Helpful NullPointerExceptions Demo

Usage

With Java 15 and above

With Java 14