How to Use Non-Final Loop Variables Inside a Lambda Expression in Java?

How can I use non-final loop variables within a lambda expression in Java?

for (int i = 0; i < 10; i++) {  
    final int number = i;  
    Runnable task = () -> System.out.println(number);  
    task.run();  
}

In Java, lambda expressions can sometimes create confusion when trying to access loop variables. While they can access final or effectively final variables from their enclosing scope, non-final variables pose a challenge. This guide explains how to work around this limitation effectively.

for (int i = 0; i < 10; i++) {  
    final int number = i;  
    Runnable task = () -> System.out.println(number);  
    task.run();  
} // This allows accessing the loop variable safely.

Causes

• Java lambda expressions require variables from their enclosing scope to be final or effectively final.
• A non-final loop variable cannot be accessed from within a lambda because it may change with each iteration.

Solutions

• Use a final variable to hold the loop variable's value inside the loop.
• Convert the loop structure to use a stream, allowing clearer access to such values.

Mistake: Attempting to use a non-final loop variable directly inside a lambda expression.

Solution: Define a final variable within the loop that captures the current value.

Mistake: Not understanding the concept of effectively final variables in Java.

Solution: Review Java's rules on variable scope and mutability.