你還在new對象嗎？Java8通用Builder瞭解一下？

程序員經常會遇到靈魂拷問：你有對象嗎？

沒有，但我可以 new 一個！

public class GirlFriend {
    private String name;
    private int age;
    // 省略 getter & setter ...
    public static void main(String[] args) {
        GirlFriend myGirlFriend = new GirlFriend();
        myGirlFriend.setName("小美");
        myGirlFriend.setAge(18);
    }
}

沒問題，老鐵！但如果對象的屬性太多，咋辦？

public class GirlFriend {
    private String name;
    private int age;
    private int bust;
    private int waist;
    private int hips;
    private List<String> hobby;
    private String birthday;
    private String address;
    private String mobile;
    private String email;
    private String hairColor;
    private Map<String, String> gift;
    // 等等等等 ...
    // 省略 getter & setter ...
    public static void main(String[] args) {
        GirlFriend myGirlFriend = new GirlFriend();
        myGirlFriend.setName("小美");
        myGirlFriend.setAge(18);
        myGirlFriend.setBust(33);
        myGirlFriend.setWaist(23);
        myGirlFriend.setHips(33);
        myGirlFriend.setBirthday("2001-10-26");
        myGirlFriend.setAddress("上海浦東");
        myGirlFriend.setMobile("18688888888");
        myGirlFriend.setEmail("[email protected]");
        myGirlFriend.setHairColor("淺棕色帶點微卷");
        List<String> hobby = new ArrayList<>();
        hobby.add("逛街");
        hobby.add("購物");
        hobby.add("買東西");
        myGirlFriend.setHobby(hobby);
        Map<String, String> gift = new HashMap<>();
        gift.put("情人節禮物", "LBR 1912女王時代");
        gift.put("生日禮物", "迪奧烈焰藍金");
        gift.put("紀念日禮物", "阿瑪尼紅管脣釉");
        myGirlFriend.setGift(gift);
        // 等等等等 ...
    }
}

GirlFriend{name='小美'
, age=18
, bust=33
, waist=23
, hips=33
, hobby=[逛街, 購物, 買東西]
, birthday='2001-10-26'
, address='上海浦東'
, mobile='18688888888'
, email='[email protected]'
, hairColor='淺棕色帶點微卷'
, gift={情人節禮物=LBR 1912女王時代, 生日禮物=迪奧烈焰藍金, 紀念日禮物=阿瑪尼紅管脣釉}
}

GirlFriend 是很美，但寫起來也太麻煩了吧。

說說缺點：實例化和設置屬性分開，不好維護；變量名重複寫。

莫慌，看法寶~

這裏不再介紹其他 Builder 實現方式，直接祭出最實用的通用Builder：

適用於所有類，不需要改造原來類，不需要 lombok 插件支持。

先看看使用姿勢：

public class GirlFriend {
    // 省略屬性 ...
    // 省略 getter & setter ...

    // 爲了演示方便，加幾個聚合方法
    public void addHobby(String hobby) {
        this.hobby = Optional.ofNullable(this.hobby).orElse(new ArrayList<>());
        this.hobby.add(hobby);
    }
    public void addGift(String day, String gift) {
        this.gift = Optional.ofNullable(this.gift).orElse(new HashMap<>());
        this.gift.put(day, gift);
    }
    public void setVitalStatistics(int bust, int waist, int hips) {
        this.bust = bust;
        this.waist = waist;
        this.hips = hips;
    }
    public static void main(String[] args) {
        GirlFriend myGirlFriend = Builder.of(GirlFriend::new)
                .with(GirlFriend::setName, "小美")
                .with(GirlFriend::setAge, 18)
                .with(GirlFriend::setVitalStatistics, 33, 23, 33)
                .with(GirlFriend::setBirthday, "2001-10-26")
                .with(GirlFriend::setAddress, "上海浦東")
                .with(GirlFriend::setMobile, "18688888888")
                .with(GirlFriend::setEmail, "[email protected]")
                .with(GirlFriend::setHairColor, "淺棕色帶點微卷")
                .with(GirlFriend::addHobby, "逛街")
                .with(GirlFriend::addHobby, "購物")
                .with(GirlFriend::addHobby, "買東西")
                .with(GirlFriend::addGift, "情人節禮物", "LBR 1912女王時代")
                .with(GirlFriend::addGift, "生日禮物", "迪奧烈焰藍金")
                .with(GirlFriend::addGift, "紀念日禮物", "阿瑪尼紅管脣釉")
                // 等等等等 ...
                .build();
    }
}

看到了嗎！實例化和屬性設置在同一條語句執行，鏈式操作，一路點點點，清爽！

Talk is cheap, show me the code：

/**
 * 通用的 Builder 模式構建器
 *
 * @author: CipherCui
 * @since 2019/8/29
 */
public class Builder<T> {
    private final Supplier<T> instantiator;
    private List<Consumer<T>> modifiers = new ArrayList<>();
    public Builder(Supplier<T> instantiator) {
        this.instantiator = instantiator;
    }
    public static <T> Builder<T> of(Supplier<T> instantiator) {
        return new Builder<>(instantiator);
    }
    public <P1> Builder<T> with(Consumer1<T, P1> consumer, P1 p1) {
        Consumer<T> c = instance -> consumer.accept(instance, p1);
        modifiers.add(c);
        return this;
    }
    public <P1, P2> Builder<T> with(Consumer2<T, P1, P2> consumer, P1 p1, P2 p2) {
        Consumer<T> c = instance -> consumer.accept(instance, p1, p2);
        modifiers.add(c);
        return this;
    }
    public <P1, P2, P3> Builder<T> with(Consumer3<T, P1, P2, P3> consumer, P1 p1, P2 p2, P3 p3) {
        Consumer<T> c = instance -> consumer.accept(instance, p1, p2, p3);
        modifiers.add(c);
        return this;
    }
    public T build() {
        T value = instantiator.get();
        modifiers.forEach(modifier -> modifier.accept(value));
        modifiers.clear();
        return value;
    }
    /**
     * 1 參數 Consumer
     */
    @FunctionalInterface
    public interface Consumer1<T, P1> {
        void accept(T t, P1 p1);
    }
    /**
     * 2 參數 Consumer
     */
    @FunctionalInterface
    public interface Consumer2<T, P1, P2> {
        void accept(T t, P1 p1, P2 p2);
    }
    /**
     * 3 參數 Consumer
     */
    @FunctionalInterface
    public interface Consumer3<T, P1, P2, P3> {
        void accept(T t, P1 p1, P2 p2, P3 p3);
    }
}

這個示例最多支持三個參數的設置屬性方法，也完全夠用了。如果要擴展也很容易，依葫蘆畫瓢，添加多個參數的Consumer。

快用你的 Builder 建個對象吧