Java - for Loop

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In Java, a for loop is a repetition control structure used to execute a block of code a specific number of times. It is particularly useful when the number of iterations is known beforehand, making it an efficient way to automate repetitive tasks.

For loops are used in Java for tasks such as iterating over arrays, performing calculations, and handling repetitive operations.

Java for Loop

The Java for loop is an entry control loop, meaning it checks the given condition before executing the loop body. Its structured syntax makes it more readable and concise compared to other loop structures like the while loop.

Syntax

The syntax of a for loop is −

for(initialization; Boolean_expression; update) {
   // Statements
}

Example

The following example demonstrates a simple for loop that prints numbers from 1 to 5:

public class Test {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Number: " + i);
        }
    }
}

When the above code is compiled and executed, it produces the following result −

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Structure of for Loop

The Java for loop consists of four essential components:

1. Initialization

Defines and initializes the loop counter variable. This part executes only once at the start.

// Initialization example
int i = 1; // Loop variable declared and initialized

2. Boolean Expression (Condition)

Evaluates before each iteration. If the condition evaluates to true, the loop executes; if false, it terminates.

// Condition example
i <= 5; // Loop continues while i is less than or equal to 5

3. Body

Contains the statements that execute repeatedly as long as the Boolean condition remains true. This section may also include an update to the loop counter.

// Body of the loop
System.out.println("Iteration: " + i); // Executes in each iteration

4. Update

Modifies the loop counter at the end of each iteration.

// Update example
i++; // Increments i by 1 after each iteration

The following is the complete syntax of a for loop with all four components:

for(int i = 1; i <= 5; i++) {
    System.out.println("Iteration: " + i);
}

Execution Process of a for Loop

Here is the flow of control in a for loop −

• The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables and this step ends with a semi colon (;).

• Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop will not be executed and control jumps to the next statement past the for loop.

• After the body of the for loop gets executed, the control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank with a semicolon at the end.

• The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats (body of loop, then update step, then Boolean expression). After the Boolean expression is false, the for loop terminates.

Flow Diagram

The following diagram shows the flow diagram (execution process) of a for loop in Java -

Examples of for Loop

Below are various examples demonstrating the usage of the for loop in Java:

Example 1: Printing Numbers in a Range Using for Loop

In this example, we're showing the use of a for loop to print numbers starting from 10 to 19. Here we've initialized an int variable x with a value of 10 within initialization blook of for loop. Then in expression block, we're checking x as less than 20, and in the end under update block, we're incrementing x by 1. Within body of for loop, we're printing the value of x. For loop will run until x becomes 20. Once x is 20, loop will stop execution and program exits.

public class Test {

   public static void main(String args[]) {

      for(int x = 10; x < 20; x = x + 1) {
         System.out.print("value of x : " + x );
         System.out.print("\n");
      }
   }
}

When the above code is compiled and executed, it produces the following result −

value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19

Example 2: Printing Array Elements Using for Loop

In this example, we're showing the use of a for loop to print contents of an array. Here we're creating an array of integers as numbers and initialized it some values. We've created a variable named index to represent index of the array within for loop, check it against size of the array and incremented it by 1. Within for loop body, we're printing element of the array using index notation. Once index becomes same as array size, for loop exits and program quits.

public class Test {

   public static void main(String args[]) {
      int [] numbers = {10, 20, 30, 40, 50};

      for(int index = 0; index < numbers.length; index++) {
         System.out.print("value of item : " + numbers[index] );
         System.out.print("\n");
      }
   }
}

When the above code is compiled and executed, it produces the following result −

value of item : 10
value of item : 20
value of item : 30
value of item : 40
value of item : 50

Nested for Loops

A nested for loop is a for loop containing another for loop inside it.

Example

In this example, we are printing tables of the numbers from 1 to 10.

public class Main {
  public static void main(String[] args) {
    // Implementing nested for loop
    // Initializing loop counters 
    int num = 1;
    int i = 1;

    // outer for loop
    for (num = 1; num <= 10; num++) {
      //inner for loop
      System.out.print("Table of " + num + " is : ");
      for (i = 1; i <= 10; i++) {
        // printing table
        System.out.print(num * i + " ");
      }
      // printing a new line
      System.out.println();
    }
  }
}

When the above code is compiled and executed, it produces the following result −

Table of 1 is : 1 2 3 4 5 6 7 8 9 10 
Table of 2 is : 2 4 6 8 10 12 14 16 18 20 
Table of 3 is : 3 6 9 12 15 18 21 24 27 30 
Table of 4 is : 4 8 12 16 20 24 28 32 36 40 
Table of 5 is : 5 10 15 20 25 30 35 40 45 50 
Table of 6 is : 6 12 18 24 30 36 42 48 54 60 
Table of 7 is : 7 14 21 28 35 42 49 56 63 70 
Table of 8 is : 8 16 24 32 40 48 56 64 72 80 
Table of 9 is : 9 18 27 36 45 54 63 72 81 90 
Table of 10 is : 10 20 30 40 50 60 70 80 90 100 

Infinite for Loop

An infinite loop never ends unless you stop manually by pressing CTRL + C. To implement an infinite for loop either use such a condition that is always true or directly use true as the condition.

Example

In this example, we're showing the infinite loop using for loop. It will keep printing the numbers until you press ctrl+c to terminate the program.

public class Test {

   public static void main(String args[]) {
      int x = 10;

      for( ;; ) {
         System.out.print("value of x : " + x );
         x++;
         System.out.print("\n");
      }
   }
}

When the above code is compiled and executed, it produces the following result −

value of item : 10
value of item : 11
value of item : 12
value of item : 13
value of item : 14
...
ctrl+c

Use Cases of Java for Loop

The for loop in Java is widely used for repetitive tasks. Below are some common use cases along with complete examples:

1. Iterating Over a Fixed Range of Numbers

When the number of iterations is known, a for loop efficiently executes the required steps.

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

2. Traversing Arrays and Collections

The for loop is commonly used to iterate through arrays or lists.

public class ArrayTraversal {
    public static void main(String[] args) {
        int[] numbers = {10, 20, 30, 40, 50};
        for(int i = 0; i < numbers.length; i++) {
            System.out.println("Element: " + numbers[i]);
        }
    }
}

3. Implementing Nested Loops

It is useful for iterating over multi-dimensional structures such as matrices.

public class NestedLoopExample {
    public static void main(String[] args) {
        for(int i = 1; i <= 3; i++) {
            for(int j = 1; j <= 3; j++) {
                System.out.print(i + "," + j + " ");
            }
            System.out.println();
        }
    }
}

4. Repeating Tasks a Specific Number of Times

When a task needs to be repeated a set number of times, a for loop is ideal.

public class RepeatTaskExample {
    public static void main(String[] args) {
        for(int i = 0; i < 5; i++) {
            System.out.println("Task executed: " + (i+1));
        }
    }
}

5. Using Enhanced for Loop (for-each)

Java provides an enhanced for loop for iterating over arrays and collections.

public class EnhancedForLoopExample {
    public static void main(String[] args) {
        String[] fruits = {"Apple", "Banana", "Cherry"};
        for(String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}