Java’s iteration statements are for, while, and do-while. These statements create what we commonly call loops. As you probably know, a loop repeatedly executes the same set of instructions until a termination condition is met. As you will see, Java has a loop to fit any programming need.
while
The while loop is Java’s most fundamental looping statement. It repeats a statement or block while its controlling expression is true. Here is its general form:
while(condition) {
// body of loop
}
The condition can be any Boolean expression. The body of the loop will be executed as long as the conditional expression is true. When condition becomes false, control passes to the next line of code immediately following the loop. The curly braces are unnecessary if only a single statement is being repeated.
Here is a while loop that counts down from 10, printing exactly ten lines of “tick”:
// Demonstrate the while loop.
class While {
public static void main(String args[]) {
int n = 10;
while(n > 0) {
System.out.println("tick " + n);
n--;
}
}
}
When you run this program, it will “tick” ten times:
tick 10
tick 9
tick 8
tick 7
tick 6
tick 5
tick 4
tick 3
tick 2
tick 1
Since the while loop evaluates its conditional expression at the top of the loop, the body of the loop will not execute even once if the condition is false to begin with. For example, in the following fragment, the call to println( ) is never executed:
int a = 10, b = 20;
while(a > b)
System.out.println("This will not be displayed");
The body of the while (or any other of Java’s loops) can be empty. This is because a null statement (one that consists only of a semicolon) is syntactically valid in Java. For example, consider the following program:
// The target of a loop can be empty.
class NoBody {
public static void main(String args[]) {
int i, j;
i = 100;
j = 200;
// find midpoint between i and j
while(++i < --j) ; // no body in this loop
System.out.println("Midpoint is " + i);
}
}
This program finds the midpoint between i and j. It generates the following output:
Midpoint is 150
Here is how the while loop works. The value of i is incremented, and the value of j is decremented. These values are then compared with one another. If the new value of i is still less than the new value of j, then the loop repeats. If i is equal to or greater than j, the loop stops. Upon exit from the loop, i will hold a value that is midway between the original values of i and j. (Of course, this procedure only works when i is less than j to begin with.) As you can see, there is no need for a loop body; all of the action occurs within the conditional expression, itself. In professionally written Java code, short loops are frequently coded without bodies when the controlling expression can handle all of the details itself.
do-while
As you just saw, if the conditional expression controlling a while loop is initially false, then the body of the loop will not be executed at all. However, sometimes it is desirable to execute the body of a while loop at least once, even if the conditional expression is false to begin with. In other words, there are times when you would like to test the termination expression at the end of the loop rather than at the beginning. Fortunately, Java supplies a loop that does just that: the do-while. The do-while loop always executes its body at least once, because its conditional expression is at the bottom of the loop. Its general form is
do {
// body of loop
} while (condition);
Each iteration of the do-while loop first executes the body of the loop and then evaluates the conditional expression. If this expression is true, the loop will repeat. Otherwise, the loop terminates. As with all of Java’s loops, condition must be a Boolean expression.
Here is a reworked version of the “tick” program that demonstrates the do-while loop. It generates the same output as before.
// Demonstrate the do-while loop.
class DoWhile {
public static void main(String args[]) {
int n = 10;
do {
System.out.println("tick " + n);
n--;
} while(n > 0);
}
}
The loop in the preceding program, while technically correct, can be written more efficiently as follows:
do {
System.out.println("tick " + n);
} while(--n > 0);
In this example, the expression (– –n > 0) combines the decrement of n and the test for zero into one expression. Here is how it works. First, the – –n statement executes, decrementing n and returning the new value of n. This value is then compared with zero. If it is greater than zero, the loop continues; otherwise it terminates.
The do-while loop is especially useful when you process a menu selection, because you will usually want the body of a menu loop to execute at least once. Consider the following program which implements a very simple help system for Java’s selection and iteration statements:
// Using a do-while to process a menu selection
class Menu {
public static void main(String args[])
throws java.io.IOException {
char choice;
do {
System.out.println("Help on:");
System.out.println(" 1. if");
System.out.println(" 2. switch");
System.out.println(" 3. while");
System.out.println(" 4. do-while");
System.out.println(" 5. for\n");
System.out.println("Choose one:");
choice = (char) System.in.read();
} while( choice < '1' || choice > '5');
System.out.println("\n");
switch(choice) {
case '1':
System.out.println("The if:\n");
System.out.println("if(condition) statement;");
System.out.println("else statement;");
break;
case '2':
System.out.println("The switch:\n");
System.out.println("switch(expression) {");
System.out.println(" case constant:");
System.out.println(" statement sequence");
System.out.println(" break;");
System.out.println(" // ...");
System.out.println("}");
break;
case '3':
System.out.println("The while:\n");
System.out.println("while(condition) statement;");
break;
case '4':
System.out.println("The do-while:\n");
System.out.println("do {");
System.out.println(" statement;");
System.out.println("} while (condition);");
break;
case '5':
System.out.println("The for:\n");
System.out.print("for(init; condition; iteration)");
System.out.println(" statement;");
break;
}
}
}
Here is a sample run produced by this program:
Help on:
1. if
2. switch
3. while
4. do-while
5. for
Choose one:
4
The do-while:
do {
statement;
} while (condition);
In the program, the do-while loop is used to verify that the user has entered a valid choice. If not, then the user is reprompted. Since the menu must be displayed at least once, the do-while is the perfect loop to accomplish this.
A few other points about this example: Notice that characters are read from the keyboard by calling System.in.read( ). This is one of Java’s console input functions. Although Java’s console I/O methods won’t be discussed in detail until Chapter 12, System.in.read( ) is used here to obtain the user’s choice. It reads characters from standard input (returned as integers, which is why the return value was cast to char). By default, standard input is line buffered, so you must press ENTER before any characters that you type will be sent to your program.
Java’s console input is quite limited and awkward to work with. Further, most real-world Java programs and applets will be graphical and window-based. For these reasons, not much use of console input has been made in this book. However, it is useful in this context. One other point: Because System.in.read( ) is being used, the program must specify the throws java.io.IOException clause. This line is necessary to handle input errors. It is part of Java’s exception handling features, which are discussed in Chapter 10.
for
You were introduced to a simple form of the for loop in Chapter 2. As you will see, it is a powerful and versatile construct. Here is the general form of the for statement:
for(initialization; condition; iteration) {
// body
}
If only one statement is being repeated, there is no need for the curly braces.
The for loop operates as follows. When the loop first starts, the initialization portion of the loop is executed. Generally, this is an expression that sets the value of the loop control variable, which acts as a counter that controls the loop. It is important to understand that the initialization expression is only executed once. Next, condition is evaluated. This must be a Boolean expression. It usually tests the loop control variable against a target value. If this expression is true, then the body of the loop is executed. If it is false, the loop terminates. Next, the iteration portion of the loop is executed. This is usually an expression that increments or decrements the loop control variable. The loop then iterates, first evaluating the conditional expression, then executing the body of the loop, and then executing the iteration expression with each pass. This process repeats until the controlling expression is false.
Here is a version of the “tick” program that uses a for loop:
// Demonstrate the for loop.
class ForTick {
public static void main(String args[]) {
int n;
for(n=10; n>0; n--)
System.out.println("tick " + n);
}
}
Declaring Loop Control Variables Inside the for Loop
Often the variable that controls a for loop is only needed for the purposes of the loop and is not used elsewhere. When this is the case, it is possible to declare the variable inside the initialization portion of the for. For example, here is the preceding program recoded so that the loop control variable n is declared as an int inside the for:
// Declare a loop control variable inside the for.
class ForTick {
public static void main(String args[]) {
// here, n is declared inside of the for loop
for(int n=10; n>0; n--)
System.out.println("tick " + n);
}
}
When you declare a variable inside a for loop, there is one important point to remember: the scope of that variable ends when the for statement does. (That is, the scope of the variable is limited to the for loop.) Outside the for loop, the variable will cease to exist. If you need to use the loop control variable elsewhere in your program, you will not be able to declare it inside the for loop.
When the loop control variable will not be needed elsewhere, most Java programmers declare it inside the for. For example, here is a simple program that tests for prime numbers. Notice that the loop control variable, i, is declared inside the for since it is not needed elsewhere.
// Test for primes.
class FindPrime {
public static void main(String args[]) {
int num;
boolean isPrime = true;
num = 14;
for(int i=2; i <= num/2; i++) {
if((num % i) == 0) {
isPrime = false;
break;
}
}
if(isPrime) System.out.println("Prime");
else System.out.println("Not Prime");
}
}
Using the Comma
There will be times when you will want to include more than one statement in the initialization and iteration portions of the for loop. For example, consider the loop in the following program:
class Sample {
public static void main(String args[]) {
int a, b;
b = 4;
for(a=1; a<b; a++) {
System.out.println("a = " + a);
System.out.println("b = " + b);
b--;
}
}
}
As you can see, the loop is controlled by the interaction of two variables. Since the loop is governed by two variables, it would be useful if both could be included in the for statement, itself, instead of b being handled manually. Fortunately, Java provides a way to accomplish this. To allow two or more variables to control a for loop, Java permits you to include multiple statements in both the initialization and iteration portions of the for. Each statement is separated from the next by a comma.
Using the comma, the preceding for loop can be more efficiently coded as shown here:
// Using the comma.
class Comma {
public static void main(String args[]) {
int a, b;
for(a=1, b=4; a<b; a++, b--) {
System.out.println("a = " + a);
System.out.println("b = " + b);
}
}
}
In this example, the initialization portion sets the values of both a and b. The two comma-separated statements in the iteration portion are executed each time the loop repeats. The program generates the following output:
a = 1
b = 4
a = 2
b = 3
If you are familiar with C/C++, then you know that in those languages the comma is an operator that can be used in any valid expression. However, this is not the case with Java. In Java, the comma is a separator that applies only to the for loop.
Some for Loop Variations
The for loop supports a number of variations that increase its power and applicability. The reason it is so flexible is that its three parts, the initialization, the conditional test, and the iteration, do not need to be used for only those purposes. In fact, the three sections of the for can be used for any purpose you desire. Let’s look at some examples.
One of the most common variations involves the conditional expression. Specifically, this expression does not need to test the loop control variable against some target value. In fact, the condition controlling the for can be any Boolean expression. For example, consider the following fragment:
boolean done = false;
for(int i=1; !done; i++) {
// ...
if(interrupted()) done = true;
}
In this example, the for loop continues to run until the boolean variable done is set to true. It does not test the value of i.
Here is another interesting for loop variation. Either the initialization or the iteration expression or both may be absent, as in this next program:
// Parts of the for loop can be empty.
class ForVar {
public static void main(String args[]) {
int i;
boolean done = false;
i = 0;
for( ; !done; ) {
System.out.println("i is " + i);
if(i == 10) done = true;
i++;
}
}
}
Here, the initialization and iteration expressions have been moved out of the for. Thus, parts of the for are empty. While this is of no value in this simple example—indeed, it would be considered quite poor style—there can be times when this type of approach makes sense. For example, if the initial condition is set through a complex expression elsewhere in the program or if the loop control variable changes in a nonsequential manner determined by actions that occur within the body of the loop, it may be appropriate to leave these parts of the for empty.
Here is one more for loop variation. You can intentionally create an infinite loop (a loop that never terminates) if you leave all three parts of the for empty. For example:
for( ; ; ) {
// ...
}
This loop will run forever, because there is no condition under which it will terminate. Although there are some programs, such as operating system command processors, that require an infinite loop, most “infinite loops” are really just loops with special termination requirements. As you will soon see, there is a way to terminate a loop—even an infinite loop like the one shown—that does not make use of the normal loop conditional expression.
Nested Loops
Like all other programming languages, Java allows loops to be nested. That is, one loop may be inside another. For example, here is a program that nests for loops:
// Loops may be nested.
class Nested {
public static void main(String args[]) {
int i, j;
for(i=0; i<10; i++) {
for(j=i; j<10; j++)
System.out.print(".");
System.out.println();
}
}
}
The output produced by this program is shown here:
..........
.........
........
.......
......
.....
....
...
..
.
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