What is a requirement for an array index in Java?

In Java, an array index must always be less than the size of the array. This requirement ensures that any attempt to access an element of the array is limited to valid positions within its defined boundaries. When Java arrays are declared, they are given a fixed size, and the valid indices range from 0 to one less than this size. For example, if an array has a size of 5, the valid indices available would be 0, 1, 2, 3, and 4. Attempting to access an index that equals or exceeds the size of the array results in an ArrayIndexOutOfBoundsException, indicating that the index is invalid. This rule helps maintain data integrity and prevents runtime errors caused by out-of-bounds access. Hence, ensuring that any index used is strictly less than the array's size is crucial for correct and safe array manipulation in Java.