NullPointerException

Why This Happens

A NullPointerException occurs when you try to use an object reference that is null. This can happen when calling a method on null, accessing a field of null, or indexing a null array. It is the most common runtime exception in Java.

The Problem

String name = null;
int length = name.length(); // NullPointerException

The Fix

String name = null;
if (name != null) {
    int length = name.length();
}

Step-by-Step Fix

1. 1

   Identify the null reference

   Read the stack trace to find the exact line number. The variable being dereferenced on that line is null.

2. 2

   Trace where null originates

   Follow the variable backwards to find where it was assigned null or where an expected initialization was skipped.

3. 3

   Add a null check or fix initialization

   Either add a null check before usage, use Optional, or ensure the variable is properly initialized before the call.