java – What is a NullPointerException, and how do I fix it?

A NullPointerException (NPE) in Java occurs when a program attempts to access or modify an object that is null. Common causes include calling methods, accessing fields, or performing operations on an uninitialized or null object.

Example:

Fixing NPE:

1. Initialize objects before use:
   java

   CopyEdit

   String str = "";

2. Add null checks:
   java

   CopyEdit

   if (str != null) {
System.out.println(str.length());
}

3. Use Optional (Java 8+):
   java

   CopyEdit

   Optional.ofNullable(str).ifPresent(s -> System.out.println(s.length()));

4. Debug stack trace to locate the problematic code.