Using “this” with class name

Question

I am doing Android programming and was learning about Intents, when I saw a constructor that, to my C# trained mind, seemed funky. The call was:

Intent myIntent = new Intent(CurrentActivity.this, NextActivity.class);

Both of the parameters are new to me. How is there a static ".this" off of a Class Name? Is this a Java thing or an Android thing? I am assuming that it is the same as just saying "this", since I am in the context of CurrentActivity, but I don't get how the "this" can be called off of the Class name itself. Also. The ".class" looks like it is used for reflection, which I am familiar with in C#, but any insight into this would be welcomed as well.

Thanks.

Answer 1

ClassName.this is used to reference the current instance of an outerclass from an inner class.

Answer 2

Usually, you can use only this. But, sometimes this makes reference to an inner class... so, for example:

Button button = (Button)findViewById(R.id.ticket_details_sell_ticket);
button.setOnClickListener(new OnClickListener() {
    @Override
    public void onClick(View v) {
        // it will be wrong to use only "this", because it would
        // reference the just created OnClickListener object
        Intent login = new Intent(ClassName.this, Login.class);
        startActivityForResult(login, LOGIN_REQUEST);
    }
});

Answer 3

One at a time:

The first construct is called a qualified this. The purpose of the syntax is in the case where you are in an inner class (typically an anonymous inner class) and you want to reference the this of the outer class rather than the this of the (anonymous) inner class. The "qualified this" can only be used in a context where this would be ambiguous. The quote the JLS "It is a compile-time error if the expression occurs in a class or interface which is not an inner class of class T or T itself".

The second construct is called a class literal is the way to reference the Class object that represents that type. It can be used in any context.

Answer 4

<ClassName>.this

is used in nested classes to refer to the current instance of the enclosing class, since the `this' keyword refers to the nest class instance.

public class Siht {
    class NestedSiht {
        void demoThis() {
            System.err.println("this' is an instance of: " + 
                            this.getClass().getName());
            System.err.println("Siht.this' is an instance of: " +
                            Siht.this.getClass().getName());
        }
    }


void demoThis() {
    new java.lang.Object() {
        void demoThis() {
            System.err.println("`this' is an instance of: " + 
                            this.getClass().getName());
            System.err.println("`Siht.this' is an instance of: " +
                            Siht.this.getClass().getName());
        }
    }.demoThis();
    new NestedSiht().demoThis();
}

public static void main(String [] args) {
    new Siht().demoThis();
}


}

Answer 5

The syntax "Classname.this" is for inner classes. If you want to refer to the enclosing instance of type "Outerclass" then you do it as "Outerclass.this".

NextActivity.class is simply the Class object that describes class "NextActivity".

Answer 6

NextActivity.class in java means typeof(NextActivity) in C#