What type of value can an array index be?

An array index in Java must evaluate to a nonnegative integer. This is because array indices are used to access specific elements within the array, and these indices are zero-based. This means that the first element of the array is accessed with an index of 0, the second element with an index of 1, and so forth. When you think about the purpose of indexing an array, it becomes clear that negative values or values that are not integers wouldn't make sense in that context, as they would reference positions that don't correspond to any valid element of the array.

For example, if you have an array of size 5, valid indices would be 0 through 4; using any negative integer or a non-integer (like a floating-point number) would lead to errors or exceptions in the program. This highlights the importance of using expressions that resolve to nonnegative integers when working with arrays in Java. Additionally, array indices cannot be based on string values, which further underscores the requirement for specific types of expressions to satisfy the indexing criteria.