IllegalArgumentException

Why This Happens

Methods throw IllegalArgumentException when a parameter value is invalid. This is a deliberate validation check by the method author. Common cases include negative sizes, null where non-null is required, out-of-range enum values, or invalid format strings.

The Problem

// Setting negative capacity
ArrayList<String> list = new ArrayList<>(-1); // IllegalArgumentException

The Fix

ArrayList<String> list = new ArrayList<>(10); // Valid positive capacity

Step-by-Step Fix

1. 1

   Identify the invalid argument

   Read the exception message to understand which argument is invalid and why.

2. 2

   Check method documentation

   Read the Javadoc for the method to understand the valid range and constraints for each parameter.

3. 3

   Validate before calling

   Add validation for the argument value before passing it to the method, with a meaningful error message if invalid.