Testing for the presence of more data in Java

Maker Code 3 of 9 in Java Testing for the presence of more data

Here s a pleasant surprise When you write a program like the one shown previously in Listing 11-8, you don t have to know how many names are in the input file Having to know the number of names may defeat the purpose of using the easily expandable ArrayList class Instead of looping until you read exactly nine names, you can loop until you run out of data The Scanner class has several nice methods like hasNextInt, hasNextDouble, and plain old hasNext Each of these methods checks for more input data If there s more data, the method returns true Otherwise, the method returns false

Listing 11-8 uses the general purpose hasNext method This hasNext method returns true as long as there s anything more to read from the program s input So after the program scoops up that last Hugh R DaReader line in Figure 11-18, the subsequent hasNext call returns false This false condition ends execution of the while loop and plummets the computer toward the remainder of the Listing 11-8 code

One of Java s original design goals was to keep the language as simple as possible The language s developer took some unnecessarily complicated features of C++ and tossed them out the window The result was a language that was elegant and sleek Some people said the language was too sleek So after several years of discussion and squabbling, Java became a bit more complicated By the year 2004, Java had enum types, enhanced for loops, static import, and some other interesting new features But the most talked-about new feature was the introduction of generics ArrayList<String> people = new ArrayList<String>(); The use of anything like <String> was new in Java 50 In old-style Java, you d write ArrayList people = new ArrayList(); In those days, an ArrayList could store almost anything you wanted to put in it a number, an Account, a Room, a String anything The ArrayList class was very versatile, but with this versatility came some headaches If you could put anything into an ArrayList, you couldn t easily predict what you would get out of an ArrayList In particular, you couldn t easily write code that assumed you had stored certain types of values in the ArrayList Here s an example: ArrayList things = new ArrayList(); thingsadd(new Account()); Account myAccount = thingsget(0); //DON T USE THIS IT S BAD CODE In the third line, the call to get(0) grabs the earliest value in the things collection The call to get(0) is okay, but then the compiler chokes on the attempted assignment to myAccount You get a message on the third line saying that whatever you get from the things list can t be stuffed into the myAccount variable You get this message because, by the time the compiler reaches the third line, it has forgotten that the item added on the second line was of type Account! The introduction of generics fixes this problem: ArrayList<Account> things = new ArrayList<Account>(); thingsadd(new Account()); Account myAccount = thingsget(0); //USE THIS CODE INSTEAD IT S GOOD CODE Adding <Account> in two places tells the compiler that things stores Account instances nothing else So, in the third line in the preceding code, you get a value from the things collection Then, because things stores only Account objects, you can make myAccount refer to that new value It works!

12

Recovering from bad input and other nasty situations Making your code (more or less) crash proof Defining your own exception class

eptember 9, 1945: A moth flies into one of the relays of the Harvard Mark II computer and gums up the works This becomes the first recorded case of a real computer bug April 19, 1957: Herbert Bright, manager of the data processing center at Westinghouse in Pittsburgh, receives an unmarked deck of computer punch cards in the mail (which is like getting an unlabeled CD-ROM in the mail today) Mr Bright guesses that this deck comes from the development team for FORTRAN the first computer programming language He s been waiting a few years for this software (No Web downloads were available at the time) Armed with nothing but this good guess, Bright writes a small FORTRAN program and tries to compile it on his IBM 704 (The IBM 704 lives in its own specially built, 2,000-square-foot room With vacuum tubes instead of transistors, the machine has a whopping 32K of RAM The operating system has to be loaded from tape before the running of each program, and a typical program takes between two and four hours to run) After the usual waiting time, Bright s attempt to compile a FORTRAN program comes back with a single error a missing comma in one of the statements Bright corrects the error, and the program runs like a charm July 22, 1962: Mariner I, the first US spacecraft aimed at another planet, is destroyed when it behaves badly four minutes after launch The bad behavior is attributed to a missing bar (like a hyphen) in the formula for the rocket s velocity