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

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.

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

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!"
        }
    }
}

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; 
        }

    }
}