The variable r may not be initialized.

Question

There is a very simple program:

public class A { public static void main(String[] p) { final Runnable r = new Runnable() { public void run() { System.out.println(r); } }; r.run(); } } 

And it gives:

 $ javac A.java A.java:6: variable r might not have been initialized System.out.println(r); ^ 1 error 

• Why?
• How does Runnable refer to a variable pointing to it?

(In the real code there is one more level (listener), and the link through this does not work)

Answer 1

To answer the second question:

 Runnable outer = new Object() { public final Runnable r = new Runnable() { public void run() { System.out.println(r); } }; }.r; outer.run(); 

must work.

Here, the compiler sees that r will be initialized through an implicit constructor.

For your tiny example, it would be enough to move final Runnable r outside the main method and say:

 new A().r.run(); 

Answer 2

In this case, you can use "this" to avoid a compilation error:

  final Runnable r = new Runnable() { public void run() { System.out.println(this); } }; 

Answer 3

In this case, your new Runnable is created before r , due to the order of operations before the assignment, everything on the right side of the assignment statement is executed. This means that r not in scope when Runnable is declared, and therefore it does not know what is inside the run method.

Answer 4

There is no r variable in the scope of your run() method, you are trying to print some value during its initialization ... That's why it is "might not have been initialized" .

Answer 5

just use "this" in Runnable instead of "r"))) works fine. Then you do not need to add an “external” Runnable.

Answer 6

Work code:

 public class A { public static void main(String[] p) { class RunnableHolder { Runnable r; }; final RunnableHolder r = new RunnableHolder(); rr = new Runnable() { public void run() { System.out.println(rr); } }; rrrun(); } } 

Thus, you can use the "incomplete" variable using an auxiliary object.

It looks like this construct (a helper object whose fields can be accessed from Runnable) is exactly what Java developers tried to ban. :)

Answer 7

 public class A { private static Runnable r; public static void main(String[] p) { r = new Runnable() { public void run() { System.out.println(r); } }; } } 

That should work! And r does not exist in the scope and therefore cannot be found in an anonymous implementation.