How to Resolve NullPointerException When Using List.add in Java

What causes NullPointerException when using list.add in Java, and how can it be resolved?

List<String> myList = null; 
myList.add("Hello"); // This will cause NullPointerException

A NullPointerException in Java occurs when your code attempts to use a null reference as if it were an initialized object. Specifically, calling the `add` method on a null list reference will result in this exception. Understanding the root causes and knowing how to handle them is essential for robust programming.

// Corrected code to prevent NullPointerException
List<String> myList = new ArrayList<>();
myList.add("Hello"); // No exception will be thrown.

Causes

• The list reference is not initialized (it is null).
• The list reference is set to null after initialization due to some code logic.
• A method mistakenly returns null instead of an initialized list.

Solutions

• Ensure that the list is properly initialized before calling methods on it. For example: ```java List<String> myList = new ArrayList<>(); myList.add("Hello"); ``` This guarantees that 'myList' is not null when 'add' is called.
• Use null checks before operating on the list. For instance: ```java if (myList != null) { myList.add("Hello"); } else { myList = new ArrayList<>(); myList.add("Hello"); } ```
• Utilize Java's Optional class to handle null values more elegantly.

Mistake: Forgetting to initialize the List before use.

Solution: Always initialize your List with new ArrayList() or equivalent before using it.

Mistake: Assuming a method returning a List will not return null.

Solution: Check the method documentation and ensure proper null checks.