kotlin-stdlib / kotlin.collections / ArrayList / indexOf indexOf Common 1.0 fun indexOf(element: @UnsafeVariance E): Int(source) JS 1.1 open fun indexOf(element: E): Int(source) Native 1.3 fun indexOf(element: E): Int(source) Returns the index of the first occurrence of the specified element in the list, or -1 if the specified element is not contained in the list.