An integer that can be used to index any of the given sequences.

For example, an expression with type @IndexFor({"a", "b"}) is non-negative and is less than both a.length and b.length. The sequences a and b might have different lengths.

The String.charAt(int) method is declared as

class String {
  char charAt(@IndexFor("this") index) { ... }
}

Writing @IndexFor("arr") is equivalent to writing @NonNegative @LTLengthOf("arr"), and that is how it is treated internally by the checker. Thus, if you write an @IndexFor("arr") annotation, you might see warnings about @NonNegative or @LTLengthOf.