Experiments with Variables in Java

Creator Code 39 in Java Experiments with Variables

One summer during my college days, I was sitting on the front porch, loafing around, talking with someone I d just met I think her name was Janine Where are you from I asked Mars, she answered She paused to see whether I d ask a follow-up question

As it turned out, Janine was from Mars, Pennsylvania, a small town about 20 miles north of Pittsburgh Okay, so what s my point The point is that the meaning of a name depends on the context If you re just north of Pittsburgh and ask, How do I get to Mars from here you may get a sensible, nonchalant answer But if you ask the same question standing on a street corner in Manhattan, you ll probably arouse some suspicion (Okay, knowing Manhattan, people would probably just ignore you) Of course, the people who live in Mars, Pennsylvania, are very much aware that their town has an oddball name Fond memories of teenage years at Mars High School don t prevent a person from knowing about the big red planet On a clear evening in August, you can still have the following conversation with one of the local residents: You: How do I get to Mars Local resident: You re in Mars, pal What particular part of Mars are you looking for You: No, I don t mean Mars, Pennsylvania I mean the planet Mars Local resident: Oh, the planet! Well, then, catch the 8:19 train leaving for Cape Canaveral No, wait, that s the local train That d take you through West Virginia So the meaning of a name depends on where you re using the name Although most English-speaking people think of Mars as a place with a carbon dioxide atmosphere, some folks in Pennsylvania think about all the shopping they can do in Mars And those folks in Pennsylvania really have two meanings for the name Mars In Java, those names may look like this: Mars and planetsMars

Your first experiment is shown in Listings 10-6 and 10-7 The listings code highlights the difference between variables that are declared inside and outside methods

Listing 10-6:

import static javalangSystemout; class EnglishSpeakingWorld { String mars = red planet ; void visitPennsylvania() {

outprintln( visitPA is running: ); String mars = Janine s home town ;

outprintln(mars); outprintln(thismars); } }

Listing 10-7:

Calling the Code of Listing 10-6

import static javalangSystemout; class GetGoing { public static void main(String args[]) { outprintln( main is running: ); EnglishSpeakingWorld e = new EnglishSpeakingWorld(); //outprintln(mars); cannot resolve symbol outprintln(emars); evisitPennsylvania(); } } Figure 10-7 shows a run of the code in Listings 10-6 and 10-7 Figure 10-8 shows a diagram of the code s structure In the GetGoing class, the main method creates an instance of the EnglishSpeakingWorld class The variable e refers to the new instance The new instance is an object with a variable named mars inside it That mars variable has value red planet The mars variable is called an instance variable, because the variable belongs to an object an instance of the EnglishSpeakingWorld class Now look at the main method in Listing 10-7 Inside the GetGoing class s main method, you aren t permitted to write outprintln(mars) In other words, a bare-faced reference to any mars variable is a definite no-no The mars variable that I mention in the previous paragraph belongs to the EnglishSpeakingWorld object, not the GetGoing class However, inside the GetGoing class s main method, you can certainly write emas because the e variable refers to your EnglishSpeakingWorld object That s nice

Near the bottom of the code, the visitPennsylvania method is called When you re inside visitPennsylvania, you have another declaration of a mars variable, whose value is Janine s home town This particular mars variable is called a method-local variable because it belongs to just one method the visitPennsylvania method

Figure 10-7: A run of the code in Listings 10-6 and 10-7

EnglishSpeakingWorld mars (instance variable) red planet visitPennsylvania mars (method-local variable) Janine's home town

Figure 10-8: The structure of the code in Listings 10-6 and 10-7