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;
}

4

Task

New Java Syntax, level 4, lesson 4

Locked

10 numbers

Display the numbers from 1 to 10 using a while loop. Each value should be on a new line.

Open

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);
}

4

Task

New Java Syntax, level 4, lesson 4

Locked

From 10 to 1

Display the numbers from 10 to 1 using a while loop. Each value should be on a new line.

Open

4

Task

New Java Syntax, level 4, lesson 4

Locked

You can't have too much of a good thing

Use the keyboard to enter a string and a number N greater than 0. Use a while loop to display the string N times. Each value should be on a new line. Example input: abc 2 Example output: abc abc

Open

4

Task

New Java Syntax, level 4, lesson 4

Locked

Seeing dollars in your future

Use a while loop to display a 10x10 square of dollar signs. Don't separate the symbols in each line. Example output: $$$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$ $$$$$$$$$$

Open