Qualified this

Java's this keyword is quite a handy little feature. It lets you get a reference to the current instance of a particular object which is useful in a variety of cases.

Probably the most common use case of the this keyword is to disambiguate variable references in constructors:

public class MyClass {
    private String name;

    public MyClass(String name) {
        this.name = name;
    }
}

Without the this keyword Java would not be able to tell which name variable we are assigning.

A more complex example involves passing this as a parameter to a method.

class Builder {
    private String name;

    public Builder(String name) {
        this.name = name;
    }

    public Creation build() {
        return new Creation(this);
    }
}

class Creation {
    private Builder builder;

    public Creation(Builder builder) {
        this.builder = builder;
    }
}

Notice how in line 9 this is being passed to the Creation constructor. Because we are inside of the Builder class (referred to as the Outer Class) this is an instance of the Builder class. Thus, this is a valid paramter to pass to the Creation constructor because its type is Builder.

Now let's look at some even more complex code:

interface Builder {
    public Creation build();
}

class MasterBuilder {
    private final String name;

    public MasterBuilder(String name) {
        this.name = name;
    }

    private Builder builder = new Builder() {
        @Override
        public Creation build() {
            return new Creation(MasterBuilder.this);
        }
    };

    public Builder getBuilder() {
        return builder;
    }
}

class Creation {
    private MasterBuilder builder;

    public Creation(MasterBuilder builder) {
        this.builder = builder;
    }
}

The Builder instance created on line 12 is an Inner Class because it is defined inside of another class (the Outer Class). When we call the Creation constructor in line 15 we need to use this to get a reference to the current instance of MasterBuilder but inside of the Inner Class this is an instance of Builder which is the wrong type. To fix this we use a Qualified this to specify that we want the current instance of the Outer Class. This is as simple as using: [OuterClassName].this in your code.