6: Controlling Program Flow with Loops in Java

Creator Code 39 Full ASCII in Java 6: Controlling Program Flow with Loops

In Listing 6-4, the actual file-handling statements deserve some attention These statements involve the use of classes, objects, and methods Many of the meaty details about these things are in other chapters, like s 7 and 9 Even so, I can t do any harm by touching on some highlights right here So, you can find a class in the Java language API named javaioFile The statement File evidence = new File( c:\\cookedBookstxt ); creates a new object in the computer s memory This object, formed from the javaioFile class, describes everything that the program needs to know about the disk file c:\cookedBookstxt (In Java, when you want to indicate a backslash inside a double-quoted String literal, you use a double backslash instead) From this point on in Listing 6-4, the variable evidence refers to the disk file c:\cookedBookstxt After you ve got all this javaioFile stuff in your head, the only thing left to know is that the evidence object, being an instance of the javaioFile class, has a delete method (What can I say It s in the API documentation) When you call evidencedelete, the computer gets rid of the file for you

A bunch of statements surrounded by curly braces form a block If you declare a variable inside a block, you generally can t use that variable outside the block For instance, in Listing 6-4, you get an error message if you make the following change: do { outprint( Delete evidence (y/n) ); char reply = myScannerfindWithinHorizon( , 0)charAt(0); } while (reply != y && reply != n ); if (reply == y ) With the declaration char reply inside the loop s curly braces, no use of the name reply makes sense anywhere outside the braces When you try to compile this code, you get three error messages two for the reply words in while (reply != y && reply != n ), and a third for the if statement s reply

So in Listing 6-4, your hands are tied The program s first real use of the reply variable is inside the loop But, to make that variable available after the loop, you have to declare reply before the loop In this situation, you re best off declaring the reply variable without initializing the variable Very interesting! To read more about variable initializations, see 4 To read more about blocks, see 5

I ll never forget the first time I played Clue We bought the board game during a family vacation in Niagara Falls (Niagara Falls ) Cousin Alan and I sat in the hotel room playing the game for hours on end I scribbled complicated assertions in my little detective s notebook At one point, I bluffed by suggesting three cards that I had in my hand Alan took the bait and made an accusation that was wrong on all three counts Because he d lost the game, and because I was being so smug about it, he beat the living daylights out of me It was heaven If I had the chance, I d do it all over again The material in this section applies to Java 5, Java 6, or whatever higher version number comes along in the next few years But this section s material doesn t work with older versions of Java versions such as 13, 14, and so on For a bit more about Java s version numbers, see 2

The for loop in Listing 6-2 counts from 1 to 10 And its friend (the for loop in Listing 6-3) counts 1, 2, 3 This counting is very nice, but sometimes it s not the most natural way to think about a problem Take, for instance, the listing of all possible accusations in the board game Clue The suspects aren t numbered from 1 to 6, and neither are the rooms or weapons You can number all these things, but why bother with numbering when the programming language provides a simpler solution Java s enhanced for loop lets you cycle through groups of things without creating a counting variable All you have to do is define the group Listing 6-5 shows you what to do