What is the difference between Class.this and this in Java

In this case, they are the same. The Class.this syntax is useful when you have a non-static nested class that needs to refer to its outer class's instance.

class Person{
    String name;

    public void setName(String name){
        this.name = name;
    }

    class Displayer {
        String getPersonName() { 
            return Person.this.name; 
        }

    }
}

This syntax only becomes relevant when you have nested classes:

class Outer{
    String data = "Out!";

    public class Inner{
        String data = "In!";

        public String getOuterData(){
            return Outer.this.data; // will return "Out!"
        }
    }
}

You only need to use className.this for inner classes. If you're not using them, don't worry about it.

Class.this is useful to reference a not static OuterClass.

To instantiate a nonstatic InnerClass, you must first instantiate the OuterClass. Hence a nonstatic InnerClass will always have a reference of its OuterClass and all the fields and methods of OuterClass is available to the InnerClass.

public static void main(String[] args) {

        OuterClass outer_instance = new OuterClass();
        OuterClass.InnerClass inner_instance1 = outer_instance.new InnerClass();
        OuterClass.InnerClass inner_instance2 = outer_instance.new InnerClass();
        ...
}

In this example both Innerclass are instantiated from the same Outerclass hence they both have the same reference to the Outerclass.