Since car has not been initialized, it has no length, its value is null. However, the compiler won't even allow you to compile that code as is, throwing the following error: variable car might not have been initialized.
You need to initialize it first, and then you can use .length:
String car[] = new String[] { "BMW", "Bentley" };
System.out.println(car.length);
If you need to initialize an empty array, you can use the following:
String car[] = new String[] { }; // or simply String car[] = { };
System.out.println(car.length);
If you need to initialize it with a specific size, in order to fill certain positions, you can use the following:
String car[] = new String[3]; // initialize a String[] with length 3
System.out.println(car.length); // 3
car[0] = "BMW";
System.out.println(car.length); // 3
However, I'd recommend that you use a List instead, if you intend to add elements to it:
List<String> cars = new ArrayList<String>();
System.out.println(cars.size()); // 0
cars.add("BMW");
System.out.println(cars.size()); // 1