Java Enum definition

Question

I thought I understood Java generics pretty well, but then I came across the following in java.lang.Enum:

class Enum>
         


        

Answer 1

This can be illustrated by a simple example and a technique which can be used to implement chained method calls for sub-classes. In an example below setName returns a Node so chaining won't work for the City:

class Node {
    String name;

    Node setName(String name) {
        this.name = name;
        return this;
    }
}

class City extends Node {
    int square;

    City setSquare(int square) {
        this.square = square;
        return this;
    }
}

public static void main(String[] args) {
    City city = new City()
        .setName("LA")
        .setSquare(100);    // won't compile, setName() returns Node
}

So we could reference a sub-class in a generic declaration, so that the City now returns the correct type:

abstract class Node>{
    String name;

    SELF setName(String name) {
        this.name = name;
        return self();
    }

    protected abstract SELF self();
}

class City extends Node {
    int square;

    City setSquare(int square) {
        this.square = square;
        return self();
    }

    @Override
    protected City self() {
        return this;
    }

    public static void main(String[] args) {
       City city = new City()
            .setName("LA")
            .setSquare(100);                 // ok!
    }
}