/* First of all, your program compiles and works fine because you are using Java 8. 
If using Java 7 or lower, it won't even compile. The reason is exactly as you cited. 
But I will try to explain it a bit more. Consider the following code: */

public void m1() {
    int k = 30;
    class inner {
        public void m2() {
            System.out.println(k); 
        }
    }
    inner o = new inner();
    k = 42;     // <= Note the reassignment here.
    o.m2();
}

/*
What should the method call o.m2() print? "30" or "42"? Both outputs could 
reasonably be argumented. At the time the method was declared and defined, 
the variable k had the value 30. At the time the method was called, the 
variable k had the value 42.

To prevent such ambiguities, the compiler does not allow assignment to a 
variable that is used in such inner classes (local and anonymous). So it 
must be final.

In Java 8 this was relaxed a bit. Java 8 introduced the concept of 
effectively final. A variable that is declared and initialized and not 
being assigned again is considered effectively final. And the compiler 
allows that code without declaring the variable final.

As a matter of fact, you also get a compiler error in Java 8 when trying 
to compile the above code.  
*/