What happens when you try to add an incompatible data type to an ArrayList?

When an incompatible data type is added to an ArrayList, it throws a runtime exception. This is because ArrayLists in Java utilize generics, allowing you to specify the type of elements they contain. When you try to add an object of a different type that is not compatible with the specified type for that ArrayList, the compiler does not catch this at compile time (assuming you're using a raw type without generics), but the Java Virtual Machine (JVM) does enforce the type rules at runtime.

For example, if you declare an ArrayList to hold integers and attempt to add a string, it will not compile or run successfully, resulting in a ClassCastException or a similar runtime exception being thrown when the type checking occurs. This behavior effectively safeguards type integrity within the collection, ensuring that only compatible data types can be stored, which helps to prevent potential errors during program execution.