Is it ok to use 'this' in final instance variable declaration?

Question

Is it OK to use the this keyword in a final instance variable declaration/initialization in Java?

Like this:

private final SomeClass foo = new SomeClass(this);

It worked when I tried it out. And since it is not a static variable I guess it should be referring to a particular instance. But I felt unsure if it is advisable or not so therefore I wanted to ask here.

Edit: The main class is an Android Activity class, and the SomeClass-instance needs this Activity as Context.

Answer 1

It is "technically valid" to do this. Indeed, this refers to a particular instance - namely, the instance that contains the instance of SomeClass.

But I would not recommend to do this in general. The exact behavior and state of the this that is passed to the constructor depends on subtle details. Consider the following example:

class SomeClass
{
    public SomeClass(DangerousSelfReference dangerousSelfReference)
    {
        System.out.println("State: ");
        System.out.println("   a: "+dangerousSelfReference.getA());
        System.out.println("   b: "+dangerousSelfReference.getB());
        System.out.println("   c: "+dangerousSelfReference.getC());
        System.out.println("   d: "+dangerousSelfReference.getD());
        System.out.println(" ref: "+dangerousSelfReference.getRef());
    }
}

public class DangerousSelfReference
{
    public static void main(String[] args)
    {
        DangerousSelfReference d = new DangerousSelfReference();
    }

    private String a;
    private String b = "b";
    private final SomeClass ref = new SomeClass(this);
    private final String c = "c";
    private String d = "d";

    DangerousSelfReference()
    {
        a = "a";
    }

    String getA()
    {
        return a;
    }
    String getB()
    {
        return b;
    }
    String getC()
    {
        return c;
    }
    String getD()
    {
        return d;
    }
    SomeClass getRef()
    {
        return ref;
    }
}

I assume that this could make a neat job interview question, because predicting the output is hard. Surprisingly, it prints

State: 
   a: null
   b: b
   c: c
   d: null
 ref: null

Notice that the final variable c is initialized, but the non-final variable d is not intialized yet. In contrast to that, the non-final variable b (that is declared before the SomeClass instance) is already intitialized.

Such subtelities are always questionable, and should be avoided if possible.