Why Do Lambda Expressions in Java Require 'Final' or 'Effectively Final' Variables?

Lambda Expressions Require "Final" or "Effectively Final" Variables

The error message "Variable used in lambda expression should be final or effectively final" indicates that a variable used within a lambda expression must be declared as final or meet specific criteria to be considered effectively final.

In the code snippet provided:

private TimeZone extractCalendarTimeZoneComponent(Calendar cal, TimeZone calTz) {
    // Code missing
    cal.getComponents().getComponents("VTIMEZONE").forEach(component -> {
        // Code missing
        if (calTz == null) {
            calTz = TimeZone.getTimeZone(v.getTimeZoneId().getValue());
        }
    });
    // Code missing
}

The error is thrown because the calTz variable is not declared as final within the lambda expression. Java requires that variables used in lambda expressions be final or effectively final to prevent potential concurrency issues.

Why the Requirement Exists

The Java Language Specification (§15.27.2) states:

The restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems.

Essentially, Java aims to mitigate concurrency risks by preventing mutation of captured variables within lambda expressions. This helps ensure predictable and correct behavior in multithreaded environments.

Therefore, to ensure thread safety and prevent concurrency problems, variables used in lambda expressions should be declared as final or effectively final, meaning they cannot be modified within the lambda's scope.

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