numberOfBunnies becomes 28 in Java

Making Code 3 of 9 in Java numberOfBunnies becomes 28

Figure 4-11: A run of the code in Figure 4-10

The word before has two different meanings: You put ++ before the variable The computer adds 1 to the variable s value before the variable is used in any other part of the statement To understand this, look at the bold line in Figure 4-10 The computer adds 1 to numberOfBunnies (raising the value of numberOfBunnies to 29), and then the computer prints the number 29 on-screen With outprintln(++numberOfBunnies), the computer adds 1 to numberOfBunnies before printing the new value of numberOfBunnies on-screen An alternative to preincrement is postincrement (The post stands for after) The word after has two different meanings: You put ++ after the variable The computer adds 1 to the variable s value after the variable is used in any other part of the statement To see more clearly how postincrement works, look at the bold line in Figure 4-12 The computer prints the old value of numberOfBunnies (which is 28) on the screen, and then the computer adds 1 to numberOfBunnies, which raises the value of numberOfBunnies to 29

import static javalangSystemout; class preIncrementDemo { public static void main(String args[]) { int numberOfBunnies = 27; numberOfBunnies++; outprintln(numberOfBunnies); outprintln(numberOfBunnies++); outprintln(numberOfBunnies); } }

With outprintln(numberOfBunnies++), the computer adds 1 to numberOfBunnies after printing the old value that numberOfBunnies already had

A run of the code in Figure 4-12 is shown in Figure 4-13 Compare Figure 4-13 with the run in Figure 4-11: With preincrement in Figure 4-11, the second number is 29 With postincrement in Figure 4-13, the second number is 28 In Figure 4-13, the number 29 doesn t show up on-screen until the end of the run, when the computer executes one last outprintln (numberOfBunnies)

Figure 4-13: A run of the code in Figure 4-12

Are you trying to decide between using preincrement or postincrement Try no longer Most programmers use postincrement In a typical Java program, you often see things like numberOfBunnies++ You seldom see things like ++numberOfBunnies In addition to preincrement and postincrement, Java has two operators that use -- These operators are called predecrement and postdecrement With predecrement (--numberOfBunnies), the computer subtracts 1 from the variable s value before the variable is used in the rest of the statement With postdecrement (numberOfBunnies--), the computer subtracts 1 from the variable s value after the variable is used in the rest of the statement Instead of writing ++numberOfBunnies, you could achieve the same effect by writing numberOfBunnies = numberOfBunnies + 1 So some people conclude that Java s ++ and -- operators are for saving keystrokes to keep those poor fingers from overworking themselves This is entirely incorrect The best reason for using ++ is to avoid the inefficient and error-prone practice of writing the same variable name, such as numberOfBunnies, twice in the same statement If you write numberOfBunnies only once (as you do when you use ++ or --), the computer has to figure out what numberOfBunnies means only once On top of that, when you write numberOfBunnies only once, you have only one chance (instead of two

chances) to type the variable name incorrectly With simple expressions like numberOfBunnies++, these advantages hardly make a difference But with more complicated expressions, like inventoryItems[(quantityReceived-*itemsPerBox+17)]++, the efficiency and accuracy that you gain by using ++ and -- are significant

You can describe the pre- and postincrement and pre- and postdecrement operators in two ways: the way everyone understands them and the right way The way that I explain the concept in most of this section (in terms of time, with before and after) is the way that everyone understands it Unfortunately, the way everyone understands the concept isn t really the right way When you see ++ or --, you can think in terms of time sequence But occasionally some programmer uses ++ or -- in a convoluted way, and the notions of before and after break down So, if you re ever in a tight spot, think about these operators in terms of statements and expressions

First, remember that a statement tells the computer to do something, and an expression has a value (I discuss statements in 3, and I describe expressions elsewhere in this chapter) Which category does numberOfBunnies++ belong to The surprising answer is both The Java code numberOfBunnies++ is both a statement and an expression Assume that, before the computer executes the code outprintln(numberOfBunnies++), the value of numberOfBunnies is 28 As a statement, numberOfBunnies++ tells the computer to add 1 to numberOfBunnies As an expression, the value of numberOfBunnies++ is 28, not 29 So, even though the computer adds 1 to numberOfBunnies, the code outprintln (numberOfBunnies++) really means outprintln(28) Now, almost everything you just read about numberOfBunnies++ is true about ++numberOfBunnies The only difference is that as an expression, ++numberOfBunnies behaves in a more intuitive way As a statement, ++numberOfBunnies tells the computer to add 1 to numberOfBunnies As an expression, the value of ++numberOfBunnies is 29 So, with outprintln(++numberOfBunnies), the computer adds 1 to the variable numberOfBunnies, and the code outprintln(++numberOfBunnies) really means outprintln(29)