Loops

The concept of loops arises in need of executing a specific block of code iteratively.

Suppose I want to print Hello, World! 5 times. Instead of using print method multiple times we can use a loop to reduce the lines of codes.

In java there are multiple types of loops.

• while loop
• do...while loop
• for loop
• for...each loop

while loop

while (condition) {...}

After using while statement, it takes a boolean value as condition. As long as the condition is true the java compiler will repeat the lines of codes inside the code block. For example,

// Do not execute this code
public class Loops {
    public static void main(String[] args) {
        //while loop
        while (true) {
            System.out.println("Hello, World!");
        }
    }
}

Have you identified the issue that might arise in this code?

Because we are using true inside the (condition) block, the java compiler will repeat the code inside the block infinite amount of times and eventually the code will crash. Can you think of any solution that we can use to solve this issue?

We can use a variable to store the the boolean value and once we are done with our repetition, we can simply assign the variable to false value. Also, to check whether we are done with our repetition, we can use condition statements.

Now, to print Hello, World! 5 times we can write the following code.

public class Loops {
    public static void main(String[] args) {
        int iteration = 1;
        boolean flag = true;
        //while loop
        while (flag) {
            System.out.println("Hello, World!");
            iteration++;
            if (iteration > 5) {
                flag = false;
            }
        }
        System.out.println("End");
    }
}

Here, multiple things are happening. First we assigned an int value to keep track how many times we want to repeat the code. And we stored the boolean value true initially.

Now, in every iteration first we are printing Hello, World!. Second, we are increasing the iteration value to keep track of the times we want to print Hello, World!. Third, we are checking whether we are done with our repetition. Once we are done, we are assigning the boolean value to false. When while loop condition is false it will skip the block of code and go to the next line.

Output

Hello, World!
Hello, World!
Hello, World!
Hello, World!
Hello, World!
End

This is how the code is working. We can make this code much more simpler in the following way,

public class Loops {
    public static void main(String[] args) {
        int iteration = 1;
        while (iteration <= 5) {
            System.out.println("Hello, World!");
            iteration++;
        }
        System.out.println("End");
    }
}

Here we don't need to store a boolean value manually. We know that relational operators give us boolean value. So here we are checking whether the condition is true or false. When the condition is false, the loop will terminate. This is how the iteration is working,

do...while loop

do {...} while (condition);

This works same as while loop. However, here first the code in the do block executes, then checks the condition and repeat accordingly. If we want to print numbers 1 to 5 we can write the following code,

int i = 1;
do {
    System.out.println(i);
    i++;
} while (i <= 5);

Output

1
2
3
4
5

Find out the different use cases of while loop and do...while loop.

Happy Coding