Java for Loop

Loop is used in programming to repeat a specific block of code until certain condition is met (test expression is false).

Loops are what makes computers interesting machines. Imagine you need to print a sentence 50 times on your screen. Well, you can do it by using print statement 50 times (without using loops). How about you need to print a sentence one million times? You need to use loops.

It's just a simple example. You will learn to use for loop to write some interesting programs in this article.

Java for Loop

The syntax of for Loop in Java is:

for (initialization; testExpression; update)
{
    // codes inside for loop's body
}

How for loop works?

1. The initialization expression is executed only once.
2. Then, the test expression is evaluated. Here, test expression is a boolean expression.
3. If the test expression is evaluated to true,
   • Codes inside the body of for loop is executed.
   • Then the update expression is executed.
   • Again, the test expression is evaluated.
   • If the test expression is true, codes inside the body of for loop is executed and update expression is executed.
   • This process goes on until the test expression is evaluated to false.
4. If the test expression is evaluated to false, for loop terminates.

for Loop Flowchart

Example 1: for Loop

// Program to print a sentence 10 times

class Loop {
   public static void main(String[] args) {
      
      for (int i = 1; i <= 10; ++i) {
         System.out.println("Line " + i);
      }
   }
}

When you run the program, the output will be:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

Here, the variable i is declared and initialized to 1.

Then, the test expression i <= 10 is evaluated. Since, it's true, the body of for loop is executed which prints Line 1 on the screen.

Then, the update expression ++i is executed. Now, the value of i is increased to 2. Again the expression i <= 10 is evaluated which is true and the body of for loop is executed which prints Line 2 on the screen.

This iteration process goes on until i is 11. When i is 11, expression i <= 10 is false and for loop terminates.

Example 2: for Loop

// Program to find the sum of natural numbers from 1 to 1000.

class Number {
   public static void main(String[] args) {
      
      int sum = 0;
	   
      for (int i = 1; i <= 1000; ++i) {
         sum += i;     // sum = sum + i
      }
	   
      System.out.println("Sum = " + sum);
   }
}

When you run the program, the output will be:

Sum = 500500

Here, the variable sum is initialized to 0. Then, in each iteration of for loop, variable sum is assigned sum + i and the value of i is increased until i is greater than 1000. For better visualization,

1st iteration: sum = 0+1 = 1
2nd iteration: sum = 1+2 = 3
3rd iteration: sum = 3+3 = 6
4th iteration: sum = 6+4 = 10
... .. ...

999th iteration: sum = 498501 + 999 = 499500
1000th iteration: sum = 499500 + 1000 = 500500

To learn more about test expression and how it is evaluated, visit relational and logical operators.

infinite for Loop

If the test expression is never false, for loop will run forever. This is called infinite for loop. Let's take an example:

// Infinite for Loop

class Infinite {
   public static void main(String[] args) {
      
      int sum = 0;

      for (int i = 1; i <= 10; --i) {
         System.out.println("Hello");
      }
   }
}

Here, the test expression i <= 10 is never false and hello is printed infinite number to times (at least in theory).

The initialization, update and test expression used in for statement is optional. Here's an another example of infinite for loop.

for ( ; ; ) {

}

Java for-each Loop

In Java, there is an alternative syntax of for loop to work with arrays and collections (known as for-each loop).

To learn more, visit: Java for-each Loop