Interrupting a loop

1. break statement

Let's take a look at an example from the previous lesson:

Scanner console = new Scanner(System.in);
boolean isExit = false;
while (!isExit)
{
   String s = console.nextLine();
   isExit = s.equals("exit");
}

The program reads lines from the console until the word exit is entered. If you enter this word, then the isExit variable becomes true, the loop condition !isExit will be false, and the loop will end."

Java has a special break statement that lets you simplify such logic. If a break statement is executed inside a loop, then the loop ends immediately. The program will start executing the statement that follows the loop. The statement is very brief:

break;

Here's how you can use the break statement to rewrite the example we just discussed:

Scanner console = new Scanner(System.in);
while (true)
{
   String s = console.nextLine();
   if (s.equals("exit"))
     break;
}

2. continue statement

But break isn't the only Java statement that lets you control a loop's behavior. Java also has the continue statement. If you execute a continue statement inside a loop, the current iteration of the loop will end ahead of schedule.

Executing the loop body once is called an iteration of the loop. The continue statement interrupts the current iteration of the loop, but unlike the break statement, it doesn't end the loop itself. The statement is also brief:

continue;

The continue statement is super convenient in a loop if we want to 'skip' execution of the loop body in certain situations.

Task: We want to write a program that prints numbers from 1 to 20 but skips numbers that are divisible by 7. This is what this code might look like.

int i = 1;
while (i <= 20)
{
   if ( (i % 7) == 0 )
     continue;
   System.out.println(i);
   i++;
}

Actually, this code will not work, because i will be forever stuck at the number 7. After all, the continue statement skips two other statements: System.out.println(i) and i++. As a result, once we reach the value 7, the variable i will stop changing and we'll be in an infinite loop.

We wrote the code this way on purpose to illustrate this very common mistake. How do we fix it?

There are two options here:

Option 1: change i before executing continue, but after i % 7

Option 2: always increment i at the beginning of the loop. But then i's starting value must be 0.

int i = 1;
while (i <= 20)
{
   if ( (i % 7) == 0 )
   {
     i++;
     continue;
   }
   
   System.out.println(i);
   i++;
}

int i = 0;
while (i < 20)
{
   i++;
   if ( (i % 7) == 0)
     continue;
   System.out.println(i);
}