Java while Loops
Jakob Jenkov |
The Java while loop is similar to the for loop. The while loop
enables your Java program to repeat a set of operations while a certain conditions is true.
The Java while loop exist in two variations. The commonly used while loop and the
less often do while version. I will cover both while loop versions in this text.
The Java while Loop
Let us first look at the most commonly used variation of the Java while loop.
Here is a simple Java while loop example:
int counter = 0;
while(counter < 10) {
System.out.println("counter: " + counter);
counter++;
}
This example shows a while loop that executes the body of the loop as long as
the counter variable is less than 10. Inside the while loop body the counter
is incremented. Eventually the counter variable will no longer be less than 10,
and the while loop will stop executing.
Here is another while example that uses a boolean to make the comparison:
boolean shouldContinue = true;
while(shouldContinue == true) {
System.out.println("running");
double random = Math.random() * 10D;
if(random > 5) {
shouldContinue = true;
} else {
shouldContinue = false;
}
}
This Java while example tests the boolean variable shouldContinue to check if the while loop should
be executed or not. If the shouldContinue variable has the value true, the while loop
body is executed one more time. If the shouldContinue variable has the value false, the while
loop stops, and execution continues at the next statement after the while loop.
Inside the while loop body a random number between 0 and 10 is generated. If the random number is larger than 5, then
the shouldContinue variable will be set to true. If the random number is 5 or less,
the shouldContinue variable will be set to false.
Like with for loops, the curly braces are optional around the while loop body. If you omit the curly braces
then the while loop body will consist of only the first following Java statement. Here is a Java while example
illustrating that:
while(iterator.hasNext())
System.out.println("next: " + iterator.next()); // executed in loop
System.out.println("second line"); // executed after loop
In this example, only the first System.out.println() statement is executed inside the while
loop. The second System.out.println() statement is not executed until after the while
loop is finished.
Forgetting the curly braces around the while loop body is a common mistake. Therefore it can be a
good habit to just always put them around the while loop body.
The Java do while Loop
The second variation of the Java while loop is the do while construct. Here is a
Java do while example:
InputStream inputStream =
new FileInputStream("data/text.xml");
int data;
do {
data = inputStream.read();
} while(data != -1);
Notice the while loop condition is now moved to after the do while loop body.
The do while loop body is always executed at least once, and is then executed repeatedly while
the while loop condition is true.
The main difference between the two while loop variations is exactly that the do while
loop is always executed at least once before the while loop condition is tested. This is not the
case with the normal while loop variation explained in the beginning of this text.
The two variations of the Java while loop come in handy in different situations. I mostly used the first
while loop variation, but there are situations where I have used the second variation.
The continue Command
Java contains a continue command which can be used inside Java while (and for)
loops. The continue command is placed inside the body of the while loop. When the continue
command is met, the Java Virtual Machine jumps to the next iteration of the loop without executing more of the
while loop body. The next iteration of the while loop body will proceed as any other. If that
iteration also meets the continue command, that iteration too will skip to the next iteration, and so forth.
Here is a simple continue example inside a while loop:
String[] strings = {
"John", "Jack", "Abraham", "Jennifer", "Ann" };
int wordsStartingWithJ = 0;
int i=0;
while( i < strings.length ) {
if(! strings[i].toLowerCase().startsWith("j")) {
i++;
continue;
}
wordsStartingWithJ++;
i++;
}
Notice the if statement inside the while loop. This if statement checks
if each String in the strings array does not start with with the letter j.
If not, then the continue command is executed, and the while loop continues to the next
iteration.
If the current String in the strings array does start with the letter j,
then the next Java statement after the if statement is executed, and the variable wordsStartingWithJ
is incremented by 1.
The continue command also works inside do while loops. Here is a do while
version of the previous example:
String[] strings = {
"John", "Jack", "Abraham", "Jennifer", "Ann" };
int wordsStartingWithJ = 0;
int i=0;
do {
if(! strings[i].toLowerCase().startsWith("j")) {
i++;
continue;
}
wordsStartingWithJ++;
i++;
} while( i < strings.length );
Notice that this version of the while loop requires that the strings array has a least
one element, or it will fail when trying to index into the array at position 0 during the first iteration of
the do while loop.
The break Command
The break command is a command that works inside Java while (and for) loops.
When the break command is met, the Java Virtual Machine breaks out of the while loop,
even if the loop condition is still true. No more iterations of the while loop are executed once
a break command is met. Here is a break command example inside a while loop:
String[] strings = {
"John", "Jack", "Abraham", "Jennifer", "Ann" };
int wordsStartingWithJ = 0;
int i=0;
while(i < strings.length; ) {
if(strings[i].toLowerCase().startsWith("j")) {
wordsStartingWithJ++;
}
if(wordsStartingWithJ >= 3) {
break;
}
i++;
}
Notice the second if statement inside the while loop. This if statement
checks if 3 words or more have been found which starts with the letter j. If yes, the break
command is executed, and the program breaks out of the while loop.
Like with the continue command, the break command also works inside do while
loops. Here is the example from before, using a do while loop instead:
String[] strings = {
"John", "Jack", "Abraham", "Jennifer", "Ann" };
int wordsStartingWithJ = 0;
int i=0;
do {
if(strings[i].toLowerCase().startsWith("j")) {
wordsStartingWithJ++;
}
if(wordsStartingWithJ >= 3) {
break;
}
i++;
} while( i < strings.length; )
Note that this variation of the while loop requires that the strings array always has
at least one element. If not, it will fail during the first iteration of the do while loop, when
it tries to access the first element (with index 0) of the array.
Variable Visibility in while Loops
Variables declared inside a Java while loop are only visible inside the while loop body.
Look at this while loop example:
int count = 0;
while(count < 10) {
String name = employees[i].getName();
count++;
}
After the while loop body has finished executing the count variable is still visible,
since the count variable is declared outside the while loop body. But the name
variable is not visible because it is declared inside the while loop body. Therefore the name
variable is only visible inside the while loop body.
while Loops in Other Languages
For the curious reader, here are links to tutorials about while loop instructions in all the programming languages covered here at Jenkov.com:
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Jakob Jenkov | |
