In contrast to languages such as C, the Java language has a dynamic memory allocation model, so arrays can be dimensioned at run time.  You can write a program that works out how big an array needs to be and THEN dimension it to allocate the memory.  Alas, once an array is dimensioned even in Java you can't come back later and simply extend it - after all, there's probably something else in the computer's memory directly after the end of your array by this time.

Java supports a number of collection types (via standard classes) that let you define structures to hold a whole series of objects, even if you don't know how many objects you'll want to hold in your structure. Vectors (or also ArrayLists in Java 2) can hold as many objects as you like, and you don't have to tell Java how many.   The information isn't stored sequentially in memory, so their use isn't as efficient as an array, and they're not a part of the base Java language so you have to access them through an object syntax, but they do solve the problems of changing an array size even after it has some data stored in it!

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