Java while Loop
In this chapter you will learn:
- How to use the basic Java While Loop
- How to use do-while statement
- How to create a simple help menu with while loop and switch statement
Java While Loop Syntax
The while loop repeats a statement or block while its controlling
condition is true.
Here is its general form:
while(condition) {
// body of loop
}
Please note:
- The
conditioncan be anyBooleanexpression. - The body of the loop will be executed as long as the conditional
conditionistrue. - 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":
public class Main {
public static void main(String args[]) {
int n = 10;// j a va 2 s . c o m
while (n > 0) {
System.out.println("n:" + n);
n--;
}
}
}
When you run this program, you will get the following result:
The body of the while loop will not execute if the condition is false.
For example, in the following fragment, the call to println() is never executed:
public class Main {
public static void main(String[] argv) {
int a = 10, b = 20;
while (a > b) {
System.out.println("This will not be displayed");
}// j av a 2 s .com
System.out.println("You are here");
}
}
The output:
The body of the while can be empty.
For example, consider the following program:
public class Main {
public static void main(String args[]) {
int i, j;/* j a va 2s .c o m*/
i = 10;
j = 20;
// find midpoint between i and j
while (++i < --j)
;
System.out.println("Midpoint is " + i);
}
}
The while loop in the code above has no loop body and i and
j are calculated in the while loop condition statement.
It generates the following output:
do-while statement
To execute the body of a while loop at least once, you can use the do-while loop. Its general form is:
do {
// body of loop
} while (condition);
Here is an example to show how to use a do-while loop.
public class Main {
public static void main(String args[]) {
int n = 10;//from ja v a2 s. co m
do {
System.out.println("n:" + n);
n--;
} while (n > 0);
}
}
The output:
The loop in the preceding program can be written as follows:
public class Main {
public static void main(String args[]) {
int n = 10;/* ja v a 2s .co m*/
do {
System.out.println("n:" + n);
} while (--n > 0);
}
}
The output is identical the result above:
Help menu with While and Switch
The following program implements a very simple help system with do-while
loop and switch statement.
public class Main {
public static void main(String args[]) throws java.io.IOException {
char choice;//from j av a 2 s .co m
do {
System.out.println("Help on:");
System.out.println(" 1. A");
System.out.println(" 2. B");
System.out.println(" 3. C");
System.out.println(" 4. D");
System.out.println(" 5. E");
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("A");
break;
case '2':
System.out.println("B");
break;
case '3':
System.out.println("C");
break;
case '4':
System.out.println("D");
break;
case '5':
System.out.println("E");
break;
}
}
}
Here is a sample run produced by this program:
Next chapter...
What you will learn in the next chapter:
- How to write for loop
- How to use variables inside the for loop
- How to use comma in the for loop statement
- How to write different forms of for loop
- How to write nested for loops