Java For Loop

In Java, a for loop is one of the most commonly used control flow statements. It allows you to repeatedly execute a block of code for a fixed number of iterations. This makes it an essential tool for performing repetitive tasks, especially when you know in advance how many times you need to execute a particular statement or block of code.

In this blog, we’ll explore how the for loop works, its syntax, and provide examples to help you understand its usage clearly.

What is a For Loop?#

The for loop is used when you know the number of iterations you need to execute. It is ideal for iterating through collections, arrays, or performing repetitive tasks.

Syntax of a For Loop#

for(initialization; condition; update) {
    // Code to be executed
}

• initialization: This step is executed once, before the loop starts. It is typically used to initialize loop control variables.
• condition: This is evaluated before each iteration. If the condition evaluates to true, the loop body is executed. If it evaluates to false, the loop terminates.
• update: After each iteration, the update statement is executed. It is typically used to modify the loop control variable (like incrementing or decrementing).

Flow Chart:

Basic Example of a For Loop#

Let’s start with a simple example to demonstrate the basic structure of a for loop.

Example 1: Print Numbers from 1 to 5#

class Main {
  public static void main(String[] args) {
    // Using a for loop to print numbers from 1 to 5
    for (int i = 1; i <= 5; i++) {
      System.out.println(i);
    }
  }
}

Explanation:

• Initialization: int i = 1 sets the starting value of i to 1.
• Condition: i <= 5 checks if i is less than or equal to 5.
• Update: i++ increments i by 1 after each iteration.
• This loop prints the numbers from 1 to 5.

Expected Output:

1
2
3
4
5

For Loop with Multiple Statements#

You can also have multiple statements in the initialization, condition, or update sections of a for loop. This is useful when you need to modify more than one variable.

Example 2: Multiple Statements in For Loop#

class Main {
  public static void main(String[] args) {
    // Using a for loop with multiple statements
    for (int i = 1, j = 10; i <= 5; i++, j -= 2) {
      System.out.println("i: " + i + ", j: " + j);
    }
  }
}

Explanation:

• Initialization: int i = 1, j = 10 initializes two variables, i and j.
• Condition: i <= 5 checks if i is less than or equal to 5.
• Update: i++ increments i by 1, and j -= 2 decrements j by 2 after each iteration.
• This loop prints the values of i and j in each iteration.

Expected Output:

i: 1, j: 10
i: 2, j: 8
i: 3, j: 6
i: 4, j: 4
i: 5, j: 2

For Loop with Arrays#

A for loop is frequently used to iterate through arrays. This is useful when you need to perform an operation on each element in the array.

Example 3: Iterate Through an Array#

class Main {
  public static void main(String[] args) {
    int[] numbers = {2, 4, 6, 8, 10};

    // Using a for loop to iterate through the array
    for (int i = 0; i < numbers.length; i++) {
      System.out.println("Element at index " + i + ": " + numbers[i]);
    }
  }
}

Explanation:

• Initialization: int i = 0 starts the loop at the first element of the array.
• Condition: i < numbers.length ensures the loop continues as long as i is less than the length of the array.
• Update: i++ increments i by 1 in each iteration.
• The loop prints each element in the numbers array.

Expected Output:

Element at index 0: 2
Element at index 1: 4
Element at index 2: 6
Element at index 3: 8
Element at index 4: 10

For Loop with a Decrementing Counter#

A for loop can also be used to count downwards, by using a decrementing counter. This is useful when you need to execute a block of code in reverse order.

Example 4: Count Downwards from 5#

class Main {
  public static void main(String[] args) {
    // Using a for loop to count down from 5 to 1
    for (int i = 5; i > 0; i--) {
      System.out.println(i);
    }
  }
}

Explanation:

• Initialization: int i = 5 starts the loop at 5.
• Condition: i > 0 ensures the loop runs while i is greater than 0.
• Update: i-- decrements i by 1 after each iteration.
• The loop prints numbers from 5 to 1 in reverse order.

Expected Output:

5
4
3
2
1

Infinite For Loop#

A for loop can be set to run infinitely by leaving out the condition, or setting it to always be true. This is generally used when you need a loop to run indefinitely until a break condition is met.

Example 5: Infinite For Loop#

class Main {
  public static void main(String[] args) {
    // Infinite for loop
    for (;;) {
      System.out.println("This will print forever!");
      break; // Breaking out of the infinite loop
    }
  }
}

Explanation:

• The for(;;) creates an infinite loop, since the condition is always true.
• The break statement immediately exits the loop after the first print statement.
• Without the break, the loop would run forever.

Expected Output:

This will print forever!

For-Each Loop (Enhanced For Loop)#

In addition to the traditional for loop, Java also provides a for-each loop to simplify the iteration over arrays or collections.

Example 6: For-Each Loop#

class Main {
  public static void main(String[] args) {
    int[] numbers = {1, 2, 3, 4, 5};

    // Using a for-each loop to iterate through the array
    for (int num : numbers) {
      System.out.println(num);
    }
  }
}

Explanation:

• The for-each loop simplifies iterating over an array.
• It automatically accesses each element in the array one by one, without needing an index.

Expected Output:

1
2
3
4
5

Conclusion#

In this blog, we learned:

• The for loop is a fundamental control flow statement in Java used for iterating over a fixed number of iterations.
• We explored how to use the for loop with initialization, conditions, and updates.
• We also learned how to use the for loop with arrays and collections, as well as for counting upwards or downwards.
• Additionally, we looked at the for-each loop as a simpler way to iterate over arrays.

Mastering the for loop is crucial for efficient coding, especially when you need to perform repetitive tasks in a concise and readable manner.