What is a key difference between arrays and ArrayLists in Java?

The key difference highlighted in the correct answer is that ArrayLists in Java are designed to store objects, which means they can only hold references to objects rather than primitive data types directly. This is important to understand because it reflects the nature of how ArrayLists are implemented in Java.

ArrayLists can dynamically resize themselves as elements are added or removed, making them flexible and adaptable for various data sizes. However, to store primitive data types like int, char, or boolean, you would need to use their corresponding wrapper classes (e.g., Integer, Character, Boolean). This distinction is crucial because it affects how programmers declare, manipulate, and interact with collections of different types of data.

In contrast, arrays are more basic structures that can indeed store any data type, both primitive and object types, but they do not have the dynamic resizing capability of ArrayLists; their size is fixed once they are created. The characteristics of these two types of collections influence their use in programming, with ArrayLists providing more flexibility at the cost of storing only object references, whereas arrays provide straightforward access to both primitives and objects without any additional overhead.