3.2. Loops

3.2.1. for Loop

In Java we write a definite loop (aka a for loop) as:

public static void main(String [] args) {
   for (int i = 0; i < 10; i++ ) {
      System.out.println(i);
   }
}

Output:

0
1
2
3
4
5
6
7
8
9

Note

You may not be familiar with the expression i++ since it is not found in all languages. The ++ is an increment operator that has the same effect as i += 1. In this example, since the ++ comes after i, we call it a postfix increment operator. There is also a -- decrement operator in Java. For more information, see Increment and Decrement Operators.

The Java for loop gives you explicit control over the starting, stopping, and stepping of the loop variable inside the parentheses. You can think of it this way:

for (start clause; stop clause; step clause) {
   statement1
   statement2
   ...
}

If you want to start at 100, stop at 0 and count backward by 5, the loop is written as:

for (int i = 100; i >= 0; i -= 5) {
   System.out.println(i);
}

Output:

100
95
90
...

3.2.2. for-each Loop

Java also provides a syntax to iterate over any sequence or collection, such as an Array:

public static void main(String [] args) {
   int nums[] = {1, 1, 2, 3, 5, 8, 13, 21};

   for (int i : nums) {
      System.out.println(i);
   }
}

Here, the loop variable moves through the items in the Array of integers, nums[]. The syntax here uses a colon symbol, :. This type of loop is known as a for-each loop.

Tip

When considering this structure, it can be helpful to read the code sample above to yourself as “For each integer in Array nums…”.

This loop version also works with a String, where we can convert the String to an Array of characters:

String msg = "Hello World";

for (char c : msg.toCharArray()) {
   System.out.println(c);
}

As you see, to iterate through a String in this way, Java requires an extra String method, .toCharArray(), to convert the String to an Array of characters.

3.2.3. while Loop

Java also supports the while loop, or indefinite loop. A while loop in Java:

public static void main(String [] args) {
   int i = 0;
   while (i < 3) {
      i++;
   }
}

3.2.4. do-while Loop

Java adds an additional, if seldom used, variation of the while loop called the do-while loop. The do-while loop is very similar to while except that the condition is evaluated at the end of the loop rather than the beginning. This ensures that a loop will be executed at least one time. Some programmers prefer this loop in some situations because it avoids an additional assignment prior to the loop.

For example:

public static void main(String [] args) {
   do {
      System.out.println("Hello, World");
   } while (false);
}

Output:

Hello, World

Above, the message prints despite the condition never being met.

3.2.5. Break Statements in Loops

There are instances where you may want to terminate a loop if a given condition is met. In these instances, the break statement comes in handy. For example, say you want to loop through an Array of integers to search for a given value. Once that number is found, you want to quit the loop. You can do the following:

public class testBreak {

   public static void main(String [] args) {
      int[] someInts = {1, 10, 2, 3, 5, 8, 10};
      int searchTerm = 10;
      for (int oneInt : someInts) {
         if (oneInt == searchTerm) {
            System.out.println("Found it!");
            break;
         }
      }
   }
}

In the code above, instead of the for loop iterating through all the integers in the array, it will stop after it finds the first matching instance. So once it finds the first 10 in the array, it prints “Found it!” and then terminates the loop. If the break statement weren’t there, the loop would continue and when it found the second 10, it would print “Found it!” a second time.

Note that the break statement terminates the innermost loop that it is contained within. So if you have nested loops and use a break statement within the innermost loop, then it will only terminate that loop and not the outer one. If a break is present in the outer loop, it — and any other block nested within it — is terminated when the break runs.

3.2.6. Continue Statements in Loops

The continue statement is similar to, but importantly different from, the break statement. Like break, it interrupts the normal flow of control of the loop. But unlike break, the continue statement only terminates the current iteration of the loop. So the loop will continue to run from the top (as long as the boolean expression that controls the loop is still true) after a continue statement. Here is an example:

public class testContinue {

   public static void main(String [] args) {
      int[] someInts = {1, 10, 2, 3, 5, 8, 10};
      int searchTerm = 10;
      for (int oneInt : someInts) {
         if (oneInt == searchTerm) {
            System.out.println("Found it!");
            continue;
         }
         System.out.println("Not here");
      }
   }
}

The above program will print “Not here” on every iteration of the for loop except where the number has been found. So the output looks like this:

Not here
Found it!
Not here
Not here
Not here
Not here
Found it!

Because of the continue statement, the final print statement in the for loop is skipped. If the continue statement weren’t there, the output would look like this instead (notice the extra “Not here” printouts):

Not here
Found it!
Not here
Not here
Not here
Not here
Not here
Found it!
Not here

3.2.7. References

• The for statement (docs.oracle.com)

• The while and do-while Statements (docs.oracle.com)

• Break and Continue Statements (docs.oracle.com)

• Summary of Control Flow Statements (docs.oracle.com)

3.2.8. Check Your Understanding

Question

char[] chars = {'p', 'l', 'r', 's', 't'};

for (<loop-statement>) {
   System.out.println(i);
}

What does the missing <loop-statement> need to be to print each item in chars?

1. char i : chars

2. char i : chars[]

3. char i in chars

4. char i in chars[]

Question

do {
   System.out.println("Hello world!");
} while (3 < 2);

How many times does the message print and why?

1. 0 — The while condition is never true.

2. 1 — The print statement is evaluated before the conditional.

3. infinite times — 3 is less than 2, and the condition is never changed in the loop.