Arrays are of fixed size, which means they cannot grow or shrink as needed. Vectors are implemented as dynamically resizable arrays, allowing them to grow and shrink as needed. This is useful for constant growth of the data; for example, a program that reads text files line by line will be able to grow in tandem with the size of the file. Vectors are generally more efficient than arrays. This is because vectors are implemented as arrays of references (java.lang.Objects), whereas arrays are implemented as arrays of objects.