Java Array size

You can't change array size once it's created,but you can use System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length) to copy the value of the array to another bigger array,don't worry about its speed,because it's built-in function and implemented with JNI,so it's very fast

C does not have the fundamental concept of an "array" as Java does; in C, you'd

malloc(some_size * sizeof(one_element))

and affect it to a one_element * (of course, that is a gross description).

In Java, arrays are equally dynamically allocated; if you know the size, at runtime, then you can, for an array of SomeType and of size someSize do:

final SomeType[] myArray = new SomeType[someSize];

In essence, it's quite the same; including the fact that in both cases arrays are NOT resizable, but with a huge difference on what happens if you specify an invalid index:

• in Java, this leads to an IndexOutOfBoundsException;
• in C, this is undefined behavior.

All in all, apart from the consequences of using "arrays" incorrectly, what goes in C and what goes in Java only really differs by the syntax to create the array to begin with...

As others have mentioned, you cannot do this. But instead you can use ArrayList (or any other List) and where needed, convert it to simple array, like this:

ArrayList<String> arrayList = new ArrayList<>();
String strings[] = (String[])arrayList.toArray();

In C you can't define the size of the array while the program is running. Is it possible in Java (or just right as concept)?

Yes you can define the size of an array at runtime, just not redefine it. This will create an object on the heap with either enough space to hold the required number of primitives (in case of a primitive array) or object references (think of them as pointers).

If you want to redefine the size of an array you'd need to create a new one and copy the old (System.arraycopy() or Arrays.copyOf(...)).

In my case I have to use an array and not an arraylist because I'm drawing a Polyline on a Panel

Well, you could still use a list and call toArray(...) on it. This also is an example of creating an array at runtime.

Since you want to eventually call Graphics.drawPolyline(...) you'd have to either maintain two List<Integer> or preferably a List<Point> and construct the x and y arrays internally out of that list.