Returning an object from a method in Java

Generation Code39 in Java Returning an object from a method

Believe it or not, the previous sections on parameter passing left one nook and cranny of Java methods unexplored When you call a method, the method can return something right back to the calling code In previous chapters and sections, I return primitive values, such as int values, or nothing (otherwise known as void) In this section, I return a whole object It s an object of type City from Listing 10-12 The code that makes this happen is in Listing 10-14

class TrackPopulation4 { public static void main(String args[]) { City smackoverAR = new City(); smackoverARpopulation = 2232; smackoverAR = doBirth(smackoverAR); Systemoutprintln(smackoverARpopulation); } static City doBirth(City aCity) { City myCity = new City(); myCitypopulation = aCitypopulation + 1; return myCity; } } If you run the code in Listing 10-14, you get the number 2,233 That s good The code works by telling the doBirth method to create another City instance In the new instance, the value of population is 2333 (Figure 10-13)

After the doBirth method is executed, that City instance is returned to the main method Then, back in the main method, that instance (the one that doBirth returns) is assigned to the smackoverAR variable (See Figure 10-14) Now smackoverAR refers to a brand-new City instance an instance whose population is 2,233

In Listing 10-14, notice the type consistency in the calling and returning of the doBirth method: The smackoverAR variable has type City The smackoverAR variable is passed to the aCity parameter, which is also of type City The myCity variable is of type City The myCity variable is sent back in the doBirth method s return statement That s consistent, because the doBirth method s header begins with the promise public static City the promise to return an object of type City The doBirth method returns an object of type City Back in the main method, the object that the call to doBirth returns is assigned to the smackoverAR variable, and (you guessed it) the smackoverAR variable is of type City Aside from being very harmonious, all this type agreement is absolutely necessary If you write a program in which your types don t agree with one another, the compiler spits out an unsympathetic incompatible types message

Dora Kermongoos and her newborn baby daughter are safe, healthy, and resting happily in their Smackover, Arkansas, home

11

Dealing with several values at once Creating values as you get a program running Impressing other programmers with fancy generic types

elcome to the Java Motel! No haughty bellhops, no overpriced room service, none of the usual silly puns Just a clean double room that s a darn good value!

The Java Motel, with its ten comfortable rooms, sits in a quiet place off the main highway Aside from a small, separate office, the motel is just one long row of ground floor rooms Each room is easily accessible from the spacious front parking lot Oddly enough, the motel s rooms are numbered 0 through 9 I could say that the numbering is a fluke something to do with the builder s original design plan But the truth is that starting with 0 makes the examples in this chapter easier to write Anyway, you re trying to keep track of the number of guests in each room Because you have ten rooms, you may think about declaring ten variables: int guestsInRoomNum0, guestsInRoomNum1, guestsInRoomNum2, guestsInRoomNum3, guestsInRoomNum4, guestsInRoomNum5, guestsInRoomNum6, guestsInRoomNum7, guestsInRoomNum8, guestsInRoomNum9;

Doing it this way may seem a bit inefficient But inefficiency isn t the only thing wrong with this code Even more problematic is the fact that you can t loop through these variables To read a value for each variable, you have to copy the nextInt method ten times guestsInRoomNum0 = diskScannernextInt(); guestsInRoomNum1 = diskScannernextInt(); guestsInRoomNum2 = diskScannernextInt(); and so on Surely a better way exists That better way involves an array An array is a row of values, like the row of rooms in a one-floor motel To picture the array, just picture the Java Motel: First, picture the rooms, lined up next to one another Next, picture the same rooms with their front walls missing Inside each room you can see a certain number of guests If you can, forget that the two guests in Room 9 are putting piles of bills into a big briefcase Ignore the fact that the guests in Room 6 haven t moved away from the TV set in a day and a half Instead of all these details, just see numbers In each room, see a number representing the count of guests in that room (If freeform visualization isn t your strong point, look at Figure 11-1)