Pulling a Switcheroo

In Book II, Chapter 4, you find out about the workhorses of Java decision-making: boolean expressions and the mighty if statement. In this chapter, you discover another Java tool for decision making: the switch statement. The switch statement is a pretty limited beast, but it excels at one particular type of decision making: choosing one of several actions based on a value stored in an integer variable. As it turns out, the need to do just that comes up a lot. So you want to keep the switch statement handy when such a need arises.

else-if Monstrosities

Many applications call for a simple logical selection of things to be done depending on some value that controls everything. As I describe in Book II, Chapter 4, such things are usually handled with big chains of else-if statements all strung together.

Unfortunately, these things can quickly get out of hand. else-if chains can end up looking like DNA double-helix structures, or those things that dribble down from the tops of the computer screens in The Matrix.

For example, Listing 6-1 shows a bit of a program that might be used to decode error codes in a Florida or Ohio voting machine.

Listing 6-1: The else-if Version of the Voting Machine Error Decoder

import java.util.Scanner;

public class VoterApp
{

   static Scanner sc = new Scanner(System.in);

   public static void main(String[] args)
   {

      System.out.println
     ("Welcome to the voting machine "
     + "error code decoder.\n\n"
     + "If your voting machine generates "
     + "an error code,\n"
     + "you can use this program to determine "
     + "the exact\ncause of the error.\n");
      System.out.print("Enter the error code: ");
      int err = sc.nextInt();

      String msg;
      if (err==1)
     msg = "Voter marked more than one candidate.\n"
      + "Ballot rejected.";
      else if (err==2)
     msg = "Box checked and write-in candidate "
      + "entered.\nBallot rejected.";
      else if (err==3)
     msg = "Entire ballot was blank.\n"
      + "Ballot filled in according to "
      + "secret plan.";
      else if (err==4)
     msg = "Nothing unusual about the ballot.\n"
      + "Voter randomly selected for tax audit.";
      else if (err==5)
     msg = "Voter filled in every box.\n"
      + "Ballot counted twice.";
      else if (err==6)
     msg = "Voter drooled in voting machine.\n"
      + "Beginning spin cycle.";
      else if (err==7)
     msg = "Voter lied to pollster after voting.\n"
      + "Voter's ballot changed "
      + "to match polling data.";
      else
     msg = "Voter filled out ballot correctly.\n"
      + "Ballot discarded anyway.";
      System.out.println(msg);
   }
}

Wow! And this program has to decipher just seven different error codes. What if the machine had 500 different codes?