递归通用和流畅的接口

Question

TL;博士

尝试实现一个层次流畅的界面,这样我就可以组合节点子类,同时也可以独立于类,但是获取类型参数不在其绑定错误范围内.

细节

我正在尝试实现一个解决方案,以便我可以创建一些类似于以下内容的东西:

farm
    .animal()
        .cat()
            .meow()
            .findsHuman()
                .saysHello()
                .done()
            .done()
        .dog()
            .bark()
            .chacesCar()
            .findsHuman()
                .saysHello()
                .done()
            .done()
        .done()
    .human()
        .saysHello()
        .done();

同时也能做到:

Human human = new Human()
    .saysHello()

我已经接近使用各种策略,但未能获得所描述的灵活性.

我当前的尝试使用以下类:

abstract class Base<T extends Base<T>>{

    private T parent;

    Base(){

    }

    Base( T parent ){
        this.parent = parent;
    }

    public T done() throws NullPointerException{
        if ( parent != null ){
            return (T) parent;
        }

        throw new NullPointerException();
    }   
}

class Farm<T extends Base<T>> extends Base{

    private Animal<Farm<T>> animal;
    private Human<Farm<T>> human;

    public Farm(){
        super();
        this.animal = new Animal( this );
        this.human = new Human( this );
    }

    public Animal<Farm> animal(){
        return this.animal;
    }

    public Human<Farm<T>> human(){
        return this.human;
    }
}

class Animal <T extends Base<T>> extends Base{

    private Cat<Animal<T>> cat;
    private Dog<Animal<T>> dog;

    public Animal(){
        super();
        init();
    }

    public Animal( T parent ){
        super( parent );
        init();
    }

    private void init(){
        this.cat = new Cat(this);
        this.dog = new Dog(this);
    }

    public Cat<Animal<T>> cat(){
        return cat;
    }

    public Dog<Animal<T>> dog(){
        return dog;
    }
}

class Human<T extends Base<T>> extends Base{

    public Human<T> saysHello(){
        System.out.println("human says hi");
        return this;
    }
}

class Cat <T extends Base<T>> extends Base{

    private Human<Cat> human;

    public Cat(){
        super();
        init();
    }

    public Cat( T parent ){
        super( parent );
        init();
    }

    private void init(){
        this.human = new Human();
    }

    public Cat<T> meow(){
        System.out.println("cat says meow");
        return this;
    }

    public Human<Cat<T>> findsHuman(){
        return this.human;
    }
}


class Dog <T extends Base<T>> extends Base{

    private Human<Dog> human;

    public Dog(){
        super();
        init();
    }

    public Dog( T parent ){
        super( parent );
        init();
    }

    private void init(){
        this.human = new Human();
    }


    public Dog<T> bark(){
        System.out.println("dog says woof");
        return this;
    }

    public Dog<T> chacesCar(){
        System.out.println("cat drinks milk");
        return this;
    }

    public Human<Dog<T>> findsHuman(){
        return this.human;
    }

}

我看到的错误通常是:

Animal.java:4:类型参数Animal不在其绑定的私有Cat cat中; Animal.java:5:类型参数Animal不在其绑定的私人狗狗中;

适用于所有类似的参考文献,也适用于我的示例所需案例:

找不到符号符号:方法dog()位置:类Base.dog()

我尝试使用以下解决方案似乎解决类似的问题,但无济于事,所以欢迎任何和所有的支持.

参考

• 有没有办法用类型变量引用当前类型？

• http://vyazelenko.com/2012/03/02/recursive-generics-to-the-rescue/

Answer 1

下面的代码似乎工作正常，不需要任何@SuppressWarnings. 要掌握的关键概念是，您的T参数实际上是对象父对象的类，但T的父对象可以是任何东西。所以而不是T extends Base<T>你想要的T extends Base<?>。

输出是：

cat says meow
human says hi
dog says woof
cat drinks milk
human says hi
human says hi

...我相信这是正确的，尽管您可能想更改您的Dog.chacesCar()方法，使其不输出cat drinks milk！也应该chases不是chaces。

希望这可以帮助！

abstract class Base<T extends Base<?>> {

    private final T parent;

    Base() {
        this.parent = null;
    }

    Base(T parent) {
        this.parent = parent;
    }

    public T done() throws NullPointerException {
        if (parent != null) {
            return parent;
        }

        throw new NullPointerException();
    }
}

class Farm<T extends Base<?>> extends Base<T> {

    private final Animal<Farm<T>> animal;
    private final Human<Farm<T>> human;

    public Farm() {
        super();
        this.animal = new Animal<>(this);
        this.human = new Human<>(this);
    }

    public Animal<Farm<T>> animal() {
        return this.animal;
    }

    public Human<Farm<T>> human() {
        return this.human;
    }
}

class Animal<T extends Base<?>> extends Base<T> {

    private Cat<Animal<T>> cat;
    private Dog<Animal<T>> dog;

    public Animal() {
        super();
        init();
    }

    public Animal(T parent) {
        super(parent);
        init();
    }

    private void init() {
        this.cat = new Cat<>(this);
        this.dog = new Dog<>(this);
    }

    public Cat<Animal<T>> cat() {
        return cat;
    }

    public Dog<Animal<T>> dog() {
        return dog;
    }
}

class Human<T extends Base<?>> extends Base<T> {
    public Human() {
        super();
    }

    public Human(T parent) {
        super(parent);
    }

    public Human<T> saysHello() {
        System.out.println("human says hi");
        return this;
    }
}

class Cat<T extends Base<?>> extends Base<T> {

    private Human<Cat<T>> human;

    public Cat() {
        super();
        init();
    }

    public Cat(T parent) {
        super(parent);
        init();
    }

    private void init() {
        this.human = new Human<>(this);
    }

    public Cat<T> meow() {
        System.out.println("cat says meow");
        return this;
    }

    public Human<Cat<T>> findsHuman() {
        return this.human;
    }
}

class Dog<T extends Base<?>> extends Base<T> {

    private Human<Dog<T>> human;

    public Dog() {
        super();
        init();
    }

    public Dog(T parent) {
        super(parent);
        init();
    }

    private void init() {
        this.human = new Human<>(this);
    }

    public Dog<T> bark() {
        System.out.println("dog says woof");
        return this;
    }

    public Dog<T> chacesCar() {
        System.out.println("cat drinks milk");
        return this;
    }

    public Human<Dog<T>> findsHuman() {
        return this.human;
    }

}

测试代码：

public static void main(String[] args) {
    Farm<?> farm = new Farm<>();
    farm
        .animal()
            .cat()
                .meow()
                .findsHuman()
                    .saysHello()
                    .done()
                .done()
            .dog()
                .bark()
                .chacesCar()
                .findsHuman()
                    .saysHello()
                    .done()
                .done()
            .done()
        .human()
            .saysHello()
            .done();

    Human human = new Human()
            .saysHello();
}