Why Does Java Require Variables in Lambda Expressions to Be Final or Effectively Final?

Fixing "Variable used in lambda expression should be final or effectively final" Error

This error occurs when attempting to access a non-final variable within a lambda expression. To resolve it, the variable must be declared final or effectively final.

Effective Finality

Effective finality means the variable cannot be reassigned after being initialized. This can be achieved by:

• Declaring the variable as final in the outer scope
• Ensuring the variable is only assigned to once within the lambda

Why is this enforced?

The Java Language Specification (JLS) states that "the restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems." By enforcing this, Java prevents potential data inconsistencies and threading issues.

Example

Consider the following code:

private TimeZone extractCalendarTimeZoneComponent(Calendar cal, TimeZone calTz) {
    try {
        cal.getComponents().getComponents("VTIMEZONE").forEach(component -> {
            VTimeZone v = (VTimeZone) component;
            v.getTimeZoneId();
            if (calTz == null) {
                calTz = TimeZone.getTimeZone(v.getTimeZoneId().getValue());
            }
        });
    } catch (Exception e) {
        log.warn("Unable to determine ical timezone", e);
    }
    return null;
}

The calTz variable is not declared final, leading to the error. To fix it, we can modify the code as follows:

private TimeZone extractCalendarTimeZoneComponent(Calendar cal, final TimeZone calTz) {
    ...
}

Alternatively, we can ensure calTz is only assigned to once within the lambda:

private TimeZone extractCalendarTimeZoneComponent(Calendar cal, TimeZone calTz) {
    ...
    if (calTz == null) {
        calTz = TimeZone.getTimeZone(v.getTimeZoneId().getValue());
    }
    ...
}

Additional Considerations

This requirement also applies to anonymous inner classes. For example, the following code would also cause the error:

new Thread(() -> {
    int i = 0; // Effectively final as it's only assigned once
    i++; // Error: cannot mutate effectively final variable
}).start();

By enforcing these rules, Java helps prevent potential bugs and promotes concurrency safety in multithreaded applications.

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