lambda expression and global variables

Question

I just started working around lambda expression in Java 8. I'm bit confused about effectively final.

Local variable initialized once is effectively final but what about global variable, I can change value of global variable and use it in lambda expression. So what is the reason behind local variable should be effectively final. I couldn't find any articles or on javadoc about this.

import java.util.ArrayList;
import java.util.List;

public class Main {

    private String lastname = "Thakor";

    public static void main(String[] args) {
        Main objMain = new Main();
        objMain.test();
    }

    public void test(){
        List<String> listStrings = new ArrayList<String>();
        listStrings.add("Vicky");
        listStrings.add("Thakor");

        String middlename = "V";
        //Local variable middlename defined in an enclosing scope must be final or effectively final
        /* middlename = "T";*/ 

        /**
         * In case of global variable, why lambda expression not throwing error...
         * Local variable middlename defined in an enclosing scope must be final or effectively final
         */
        lastname = "T"; 

        listStrings.stream()
                        .forEach(firstname ->{
                            System.out.println("Firstname: " + firstname);
                            System.out.println("Middle: " + middlename);
                            System.out.println("Lastname: " + lastname);
                            System.out.println("--------------------------------------");
                        });
    }
}

Output

Firstname: Vicky
Middle: V
Lastname: T
--------------------------------------
Firstname: Thakor
Middle: V
Lastname: T
--------------------------------------

possible duplicate of Difference between final and effectively final — Jordi Castilla
– Jordi Castilla, Commented Jun 9, 2015 at 9:56
@JordiCastilla I know the difference but I'm asking about global variable. — Vicky Thakor
– Vicky Thakor, Commented Jun 9, 2015 at 9:58
There are no global variables in Java. The instance variable is not in the (immediately) enclosing scope, it's just being used there. It's being accessed through a reference to the object itself. — Stultuske
– Stultuske, Commented Jun 9, 2015 at 9:59
@Vicky I can't recall the exact reason behind requiring local variables to be effectively final but I assume it has got something to do with threads using the local variables from the stack. — Chetan Kinger
– Chetan Kinger, Commented Jun 9, 2015 at 10:00

shmosel · Accepted Answer · 2015-06-09 10:03:52Z

7

When you reference a "global" variable in a lambda, you're really capturing a reference to this, which is effectively final.

Consider the following snippet (untested):

class Main {

    class Box {
        int value;
    }

    void foo() {
        Box box = new Box();
        Runnable r = () -> box.value++;
    }
}

Even though you're changing the box.value, you're doing it through the box reference, which is effectively final. Similarly, you code is equivalent to

System.out.println("Lastname: " + this.lastname);

meaning you're accessing lastname through the this reference, which is effectively final.

edited Jun 9, 2015 at 10:03

answered Jun 9, 2015 at 9:56

shmosel

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5 Comments

Vicky Thakor Over a year ago

Sorry I'm not getting your answer. I'm still confused.

shmosel Over a year ago

@VickyThakor, which part are you stuck on? Do you understand the example snippet?

Vicky Thakor Over a year ago

accessing lastname through the this reference Why it refer the variable as effectively final when accessing through reference? What makes it different from local variables? Frankly I don't understand the logic behind it. Can you explain it how java treat it from compiler level to run time.

shmosel Over a year ago

@VickyThakor, I recently saw it put this way: Java closures close over values, not variables. box is a value. this is a value. final variables are effectively values. Allowing mutable variables to be referenced in a lambda would imply that the lambda captures the variable itself, not just its value.

Hemanth Peela Over a year ago

Global variable you can update, but cannot do for Final declared variable

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