Control Flow

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Java, like any programming language, supports both conditional statements and loops to determine control flow. We start with the conditional statements and then move on to loops. We end with the somewhat cumbersome switch statement that you can use when you have to test for many values of a single expression.

C++ NOTE

The Java control flow constructs are identical to those in C and C++, with a few exceptions. There is no goto, but there is a "labeled" version of break that you can use to break out of a nested loop (where you perhaps would have used a goto in C). Finally. JDK 5.0 adds a variant of the for loop that has no analog in C or C++. It is similar to the foreach loop in C#.

Block Scope

Before we get into the actual control structures, you need to know more about blocks.

A block or compound statement is any number of simple Java statements that are surrounded by a pair of braces. Blocks define the scope of your variables. Blocks can be nested inside another block. Here is a block that is nested inside the block of the main method.

 public static void main(String[] args) {    int n;    . . .    {       int k;       . . .    } // k is only defined up to here } 

However, you may not declare identically named variables in two nested blocks. For example, the following is an error and will not compile:

 public static void main(String[] args) {    int n;    . . .    {       int k;       int n; // error--can't redefine n in inner block       . . .    } } 

C++ NOTE

In C++, it is possible to redefine a variable inside a nested block. The inner definition then shadows the outer one. This can be a source of programming errors; hence, Java does not allow it.

Conditional Statements

The conditional statement in Java has the form

if (condition) statement

The condition must be surrounded by parentheses.

In Java, as in most programming languages, you will often want to execute multiple statements when a single condition is true. In this case, you use a block statement that takes the form

{
   statement₁
   statement₂
   . . .
}

For example:

 if (yourSales >= target) {    performance = "Satisfactory";    bonus = 100; } 

In this code all the statements surrounded by the braces will be executed when yourSales is greater than or equal to target. (See Figure 3-8.)

Figure 3-8. Flowchart for the if statement

NOTE

A block (sometimes called a compound statement) allows you to have more than one (simple) statement in any Java programming structure that might otherwise have a single (simple) statement.

The more general conditional in Java looks like this (see Figure 3-9):

if (condition) statement₁ else statement₂

Figure 3-9. Flowchart for the if/else statement

For example:

 if (yourSales >= target) {    performance = "Satisfactory";    bonus = 100 + 0.01 * (yourSales - target); } else {    performance = "Unsatisfactory";    bonus = 0; } 

The else part is always optional. An else groups with the closest if. Thus, in the statement

 if (x <= 0) if (x == 0) sign = 0; else sign = -1; 

the else belongs to the second if.

Repeated if . . . else if . . . alternatives are common (see Figure 3-10). For example:

 if (yourSales >= 2 * target) {    performance = "Excellent";    bonus = 1000; } else if (yourSales >= 1.5 * target) {    performance = "Fine";    bonus = 500; } else if (yourSales >= target) {    performance = "Satisfactory";    bonus = 100; } else {    System.out.println("You're fired"); } 

Figure 3-10. Flowchart for the if/else if (multiple branches)

Loops

The while loop executes a statement (which may be a block statement) while a condition is TRue. The general form is

while (condition) statement

The while loop will never execute if the condition is false at the outset (see Figure 3-11).

Figure 3-11. Flowchart for the while statement

The program in Example 3-3 determines how long it will take to save a specific amount of money for your well-earned retirement, assuming that you deposit the same amount of money per year and that the money earns a specified interest rate.

In the example, we are incrementing a counter and updating the amount currently accumulated in the body of the loop until the total exceeds the targeted amount.

 while (balance < goal) {    balance += payment;    double interest = balance * interestRate / 100;    balance += interest;    years++; } System.out.println(years + " years."); 

(Don't rely on this program to plan for your retirement. We left out a few niceties such as inflation and your life expectancy.)

A while loop tests at the top. Therefore, the code in the block may never be executed. If you want to make sure a block is executed at least once, you will need to move the test to the bottom. You do that with the do/while loop. Its syntax looks like this:

do statement while (condition);

This loop executes the statement (which is typically a block) and only then tests the condition. It then repeats the statement and retests the condition, and so on. The code in Example 3-4 computes the new balance in your retirement account and then asks if you are ready to retire:

 do {    balance += payment;    double interest = balance * interestRate / 100;    balance += interest;    year++;    // print current balance    . . .    // ask if ready to retire and get input    . . . } while (input.equals("N")); 

As long as the user answers "N", the loop is repeated (see Figure 3-12). This program is a good example of a loop that needs to be entered at least once, because the user needs to see the balance before deciding whether it is sufficient for retirement.

Example 3-3. Retirement.java

  1. import java.util.*;  2.  3. public class Retirement  4. {  5.    public static void main(String[] args)  6.    {  7.       // read inputs  8.       Scanner in = new Scanner(System.in);  9. 10.       System.out.print("How much money do you need to retire? "); 11.       double goal = in.nextDouble(); 12. 13.       System.out.print("How much money will you contribute every year? "); 14.       double payment = in.nextDouble(); 15. 16.       System.out.print("Interest rate in %: "); 17.       double interestRate = in.nextDouble(); 18. 19.       double balance = 0; 20.       int years = 0; 21. 22.       // update account balance while goal isn't reached 23.       while (balance < goal) 24.       { 25.          // add this year's payment and interest 26.          balance += payment; 27.          double interest = balance * interestRate / 100; 28.          balance += interest; 29.          years++; 30.       } 31. 32.       System.out.println("You can retire in " + years + " years."); 33.    } 34. } 

Example 3-4. Retirement2.java

  1. import java.util.*;  2.  3. public class Retirement2  4. {  5.    public static void main(String[] args)  6.    {  7.       Scanner in = new Scanner(System.in);  8.  9.       System.out.print("How much money will you contribute every year? "); 10.       double payment = in.nextDouble(); 11. 12.       System.out.print("Interest rate in %: "); 13.       double interestRate = in.nextDouble(); 14. 15.       double balance = 0; 16.       int year = 0; 17. 18.       String input; 19. 20.       // update account balance while user isn't ready to retire 21.       do 22.       { 23.          // add this year's payment and interest 24.          balance += payment; 25.          double interest = balance * interestRate / 100; 26.          balance += interest; 27. 28.          year++; 29. 30.          // print current balance 31.          System.out.printf("After year %d, your balance is %,.2f%n", year, balance); 32. 33.          // ask if ready to retire and get input 34.          System.out.print("Ready to retire? (Y/N) "); 35.          input = in.next(); 36.       } 37.       while (input.equals("N")); 38.    } 39.} 

Figure 3-12. Flowchart for the do/while statement

Determinate Loops

The for loop is a general construct to support iteration that is controlled by a counter or similar variable that is updated after every iteration. As Figure 3-13 shows, the following loop prints the numbers from 1 to 10 on the screen.

 for (int i = 1; i <= 10; i++)     System.out.println(i); 

Figure 3-13. Flowchart for the for statement

The first slot of the for statement usually holds the counter initialization. The second slot gives the condition that will be tested before each new pass through the loop, and the third slot explains how to update the counter.

Although Java, like C++, allows almost any expression in the various slots of a for loop, it is an unwritten rule of good taste that the three slots of a for statement should only initialize, test, and update the same counter variable. One can write very obscure loops by disregarding this rule.

Even within the bounds of good taste, much is possible. For example, you can have loops that count down:

 for (int i = 10; i > 0; i--)    System.out.println("Counting down . . . " + i); System.out.println("Blastoff!"); 

CAUTION

Be careful about testing for equality of floating-point numbers in loops. A for loop that looks like this

 for (double x = 0; x != 10; x += 0.1) . . . 

may never end. Because of roundoff errors, the final value may not be reached exactly. For example, in the loop above, x jumps from 9.99999999999998 to 10.09999999999998 because there is no exact binary representation for 0.1.

When you declare a variable in the first slot of the for statement, the scope of that variable extends until the end of the body of the for loop.

 for (int i = 1; i <= 10; i++) {    . . . } // i no longer defined here 

In particular, if you define a variable inside a for statement, you cannot use the value of that variable outside the loop. Therefore, if you wish to use the final value of a loop counter outside the for loop, be sure to declare it outside the loop header!

 int i; for (i = 1; i <= 10; i++) {    . . . } // i still defined here 

On the other hand, you can define variables with the same name in separate for loops:

 for (int i = 1; i <= 10; i++) {    . . . } . . . for (int i = 11; i <= 20; i++) // ok to define another variable named i {    . . . } 

A for loop is merely a convenient shortcut for a while loop. For example,

 for (int i = 10; i > 0; i--)    System.out.println("Counting down . . . " + i); 

can be rewritten as

 int i = 10; while (i > 0) {    System.out.println("Counting down . . . " + i);    i--; } 

Example 3-5 shows a typical example of a for loop.

The program computes the odds on winning a lottery. For example, if you must pick 6 numbers from the numbers 1 to 50 to win, then there are (50 x 49 x 48 x 47 x 46 x 45)/(1 x 2 x 3 x 4 x 5 x 6) possible outcomes, so your chance is 1 in 15,890,700. Good luck!

In general, if you pick k numbers out of n, there are

possible outcomes. The following for loop computes this value:

 int lotteryOdds = 1; for (int i = 1; i <= k; i++)    lotteryOdds = lotteryOdds * (n - i + 1) / i; 

NOTE

See page 82 for a description of the "generalized for loop" (also called "for each" loop) that was added to the Java language in JDK 5.0.

Example 3-5. LotteryOdds.java

  1. import java.util.*;  2.  3. public class LotteryOdds  4. {  5.    public static void main(String[] args)  6.    {  7.       Scanner in = new Scanner(System.in);  8.  9.       System.out.print("How many numbers do you need to draw? "); 10.       int k = in.nextInt(); 11. 12.       System.out.print("What is the highest number you can draw? "); 13.       int n = in.nextInt(); 14. 15.       /* 16.          compute binomial coefficient 17.          n * (n - 1) * (n - 2) * . . . * (n - k + 1) 18.          ------------------------------------------- 19.          1 * 2 * 3 * . . . * k 20.       */ 21. 22.       int lotteryOdds = 1; 23.       for (int i = 1; i <= k; i++) 24.          lotteryOdds = lotteryOdds * (n - i + 1) / i; 25. 26.       System.out.println("Your odds are 1 in " + lotteryOdds + ". Good luck!"); 27.    } 28. } 

Multiple Selections The switch Statement

The if/else construct can be cumbersome when you have to deal with multiple selections with many alternatives. Java has a switch statement that is exactly like the switch statement in C and C++, warts and all.

For example, if you set up a menuing system with four alternatives like that in Figure 3-14, you could use code that looks like this:

 Scanner in = new Scanner(System.in); System.out.print("Select an option (1, 2, 3, 4) "); int choice = in.nextInt(); switch (choice) {    case 1:       . . .       break;    case 2:       . . .       break;    case 3:       . . .       break;    case 4:       . . .       break;    default:       // bad input       . . .       break; } 

Figure 3-14. Flowchart for the switch statement

Execution starts at the case label that matches the value on which the selection is performed and continues until the next break or the end of the switch. If none of the case labels match, then the default clause is executed, if it is present.

Note that the case labels must be integers or enumerated constants. You cannot test strings. For example, the following is an error:

 String input = . . .; switch (input) // ERROR {    case "A": // ERROR       . . .       break;    . . . } 

PITFALL

It is possible for multiple alternatives to be triggered. If you forget to add a break at the end of an alternative, then execution falls through to the next alternative! This behavior is plainly dangerous and a common cause for errors. For that reason, we never use the switch statement in our programs.

Statements That Break Control Flow

Although the designers of Java kept the goto as a reserved word, they decided not to include it in the language. In general, goto statements are considered poor style. Some programmers feel the anti-goto forces have gone too far (see, for example, the famous article of Donald Knuth called "Structured Programming with goto statements"). They argue that unrestricted use of goto is error prone but that an occasional jump out of a loop is beneficial. The Java designers agreed and even added a new statement, the labeled break, to support this programming style.

Let us first look at the unlabeled break statement. The same break statement that you use to exit a switch can also be used to break out of a loop. For example,

 while (years <= 100) {    balance += payment;    double interest = balance * interestRate / 100;    balance += interest;    if (balance >= goal) break;    years++; } 

Now the loop is exited if either years > 100 occurs at the top of the loop or balance >= goal occurs in the middle of the loop. Of course, you could have computed the same value for years without a break, like this:

 while (years <= 100 && balance < goal) {    balance += payment;    double interest = balance * interestRate / 100;    balance += interest;    if (balance < goal)       years++; } 

But note that the test balance < goal is repeated twice in this version. To avoid this repeated test, some programmers prefer the break statement.

Unlike C++, Java also offers a labeled break statement that lets you break out of multiple nested loops. Occasionally something weird happens inside a deeply nested loop. In that case, you may want to break completely out of all the nested loops. It is inconvenient to program that simply by adding extra conditions to the various loop tests.

Here's an example that shows the break statement at work. Notice that the label must precede the outermost loop out of which you want to break. It also must be followed by a colon.

 Scanner in = new Scanner(System.in); int n; read_data: while (. . .) // this loop statement is tagged with the label {    . . .    for (. . .) // this inner loop is not labeled    {       System.out.print("Enter a number >= 0: ");       n = in.nextInt();       if (n < 0) // should never happen can't go on          break read_data;          // break out of read_data loop       . . .    } } // this statement is executed immediately after the labeled break if (n < 0) // check for bad situation {    // deal with bad situation } else {    // carry out normal processing } 

If there was a bad input, the labeled break moves past the end of the labeled block. As with any use of the break statement, you then need to test whether the loop exited normally or as a result of a break.

NOTE

Curiously, you can apply a label to any statement, even an if statement or a block statement, like this:

label:
{
   . . .
   if (condition) break label; // exits block
   . . .
}
// jumps here when the break statement executes

Thus, if you are lusting after a goto and if you can place a block that ends just before the place to which you want to jump, you can use a break statement! Naturally, we don't recommend this approach. Note, however, that you can only jump out of a block, never into a block.

Finally, there is a continue statement that, like the break statement, breaks the regular flow of control. The continue statement transfers control to the header of the innermost enclosing loop. Here is an example:

 Scanner in = new Scanner(System.in); while (sum < goal) {    System.out.print("Enter a number: ");    n = in.nextInt();    if (n < 0) continue;    sum += n; // not executed if n < 0 } 

If n < 0, then the continue statement jumps immediately to the loop header, skipping the remainder of the current iteration.

If the continue statement is used in a for loop, it jumps to the "update" part of the for loop. For example, consider this loop.

 for (count = 1; count <= 100; count++) {    System.out.print("Enter a number, -1 to quit: ");    n = in.nextInt();    if (n < 0) continue;    sum += n; // not executed if n < 0 } 

If n < 0, then the continue statement jumps to the count++ statement.

There is also a labeled form of the continue statement that jumps to the header of the loop with the matching label.

TIP

Many programmers find the break and continue statements confusing. These statements are entirely optional you can always express the same logic without them. In this book, we never use break or continue.

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