Part II: Writing Your Own Java Programs in Java

Creation Code 39 Extended in Java Part II: Writing Your Own Java Programs

Listing 6-2 is very nice, but the program in that listing doesn t do anything interesting For a more eye-catching example, see Listing 6-3 In Listing 6-3, I make good on a promise I made in 5 The program in Listing 6-3 prints all the lyrics of the hit single, Al s All Wet (You can find the lyrics in 5)

Listing 6-3:

import static javalangSystemout; class AlsAllWet { public static void main(String args[]) { for (int verse = 1; verse <= 3; verse++) { outprint( Al s all wet ); outprintln( Oh, why is Al all wet Oh, ); outprint( Al s all wet cause ); outprintln( he s standing in the rain ); outprintln( Why is Al out in the rain ); switch (verse) { case 1: outprintln ( That s because he has no brain ); break; case 2: outprintln ( That s because he is a pain ); break; case 3: outprintln ( Cause this is the last refrain ); break; } switch (verse) { case 3: outprintln( Last refrain, last refrain, ); case 2: outprintln( He s a pain, he s a pain, ); case 1: outprintln( Has no brain, has no brain, ); } outprintln( In the rain, in the rain );

Listing 6-3 (continued)

outprintln( Ohhhhhhhh ); outprintln(); } outprint( Al s all wet ); outprintln( Oh, why is Al all wet Oh, ); outprint( Al s all wet cause ); outprintln( he s standing in the rain ); } } Listing 6-3 is nice because it combines many of the ideas from s 5 and 6 In Listing 6-3, two switch statements are nested inside a for loop One of the switch statements uses break statements; the other switch statement uses fall-through As the value of the for loop s counter variable (verse) goes from 1 to 2 and then to 3, all the cases in the switch statements are executed When the program is near the end of its run and execution has dropped out of the for loop, the program s last four statements print the song s final verse When I boldly declare that a for statement is for counting, I m stretching the truth just a bit Java s for statement is very versatile You can use a for statement in situations that have nothing to do with counting For instance, a statement with no update part, such as for (i = 0; i < 10; ), just keeps on going The looping ends when some action inside the loop assigns a big number to the variable i You can even create a for statement with nothing inside the parentheses The loop for ( ; ; ) runs forever, which is good if the loop controls a serious piece of machinery Usually, when you write a for statement, you re counting how many times to repeat something But, in truth, you can do just about any kind of repetition with a for statement Listing 6-3 uses break statements to jump out of a switch But a break statement can also play a role inside a loop To see an example, visit this book s Web site

Repeating Until You Get What You Want (Java do Statements)

Fools rush in where angels fear to tread Alexander Pope Today, I want to be young and foolish (or, at the very least, foolish) Look back at Figure 6-2 and notice how Java s while loop works As execution

enters a while loop, the computer checks to make sure that the loop s condition is true If the condition isn t true, the statements inside the loop are never executed not even once In fact, you can easily cook up a while loop whose statements are never executed (although I can t think of a reason why you would ever want to do it) int twoPlusTwo = 2 + 2; while (twoPlusTwo == 5) { outprintln( Are you kidding ); outprintln( 2 + 2 doesn t equal 5 ); outprint( Everyone knows that ); outprintln( 2 + 2 equals 3 ); } In spite of this silly twoPlusTwo example, the while statement turns out to be the most versatile of Java s looping constructs In particular, the while loop is good for situations in which you must look before you leap For example: While money is in my account, write a mortgage check every month When you first encounter this statement, if your account has a zero balance, you don t want to write a mortgage check not even one check But at times (not many), you want to leap before you look Take, for instance, the situation in which you re asking the user for a response Maybe the user s response makes sense, but maybe it doesn t If it doesn t, you want to ask again Maybe the user s finger slipped, or perhaps the user didn t understand the question Figure 6-5 shows some runs of a program to delete a file Before deleting the file, the program asks the user whether making the deletion is okay If the user answers y or n, the program proceeds according to the user s wishes But if the user enters any other character (any digit, uppercase letter, punctuation symbol, or whatever), the program asks the user for another response