Java 算法珠玑

多个构造函数时，考虑使用构造器

一个对象，具有多个成员变量可能需要初始化，常规方法，需要提供大量构造函数。例如：

public class AlertDialog {
    private int width;
    private int height;
    private String title;
    private String confirmText;
    private String denyText;

    private AlertDialog(){}
    public AlertDialog(int width, int height){ // 空白的警告框
         AlertDialog(width,height,null);
    }

    // 带标题的警告框
    public AlertDialog(int width, int height, String title){ // 带标题的警告框
        AlertDialog(width, height, title, "确定");
    }

    // 带标题的警告框，有确定按钮
    public AlertDialog(int width, int height, String title, String confirm){
        AlertDialog(width, height, title, confirm, null);
    }

    // 带标题的警告框，有确定按钮，取消按钮
    public AlertDialog(int width, int height, String title, String confirm, String denyText){
        // set every thing.
    }
}

有多种样式的警告框，为了调用方便，必须提供多个构造函数，不灵活。如果采用另外一种方式，则可以解决，但会花费很多经历处理并发的情况:

public class AlertDialog {
    private int width;
    private int height;
    private String title;
    private String confirmText;
    private String denyText;

    public AlertDialog(){}// 空白的构造函数

    public void setWidth(int width){
        this.width = width;
    }
    // 其他set方法
}

调用时，通过调用各个参数的set方法进行设置，但是并发解决不了，且无法进行参数校验。此时，可以使用构造器来解决：

public class AlertDialog {
    private int width;
    private int height;
    private String title;
    private String confirmText;
    private String denyText;

    // private
    private AlertDialog(){}

    // Builder中使用
    protected AlertDialog(Builder b){
        width = b.width;
        height = b.height;
        // .....
        if(width==0||height==0) throws new Exception("size must be set");
    }

    // 构造器
    public static class Builder {
        private int width;
        private int height;
        private String title;
        private String confirmText;
        private String denyText;

        // 注意：返回的Builder。
        public Builder setTitle(String title) {
            this.title = title;
            return this;
        }
        // 其他set...

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

于是，可以根据相应需求，进行相应设置，并在AlertDialog真正构造时，进行参数校验。如下：

new AlertDialog.Builder().setTitle("提示").build();

通用的 Builder 模式构建器

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 参数 Consumer
     */
    @FunctionalInterface
    public interface Consumer1<T, P1> {
        void accept(T t, P1 p1);
    }
    /**
     * 2 参数 Consumer
     */
    @FunctionalInterface
    public interface Consumer2<T, P1, P2> {
        void accept(T t, P1 p1, P2 p2);
    }
    /**
     * 3 参数 Consumer
     */
    @FunctionalInterface
    public interface Consumer3<T, P1, P2, P3> {
        void accept(T t, P1 p1, P2 p2, P3 p3);
    }
}

使用方法

public class GirlFriend {
    // 省略属性 ...
    // 省略 getter & setter ...

    // 为了演示方便，加几个聚合方法
    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, "上海浦东")
                .with(GirlFriend::setMobile, "18688888888")
                .with(GirlFriend::setEmail, "pretty-xiaomei@qq.com")
                .with(GirlFriend::setHairColor, "浅棕色带点微卷")
                .with(GirlFriend::addHobby, "逛街")
                .with(GirlFriend::addHobby, "购物")
                .with(GirlFriend::addHobby, "买东西")
                .with(GirlFriend::addGift, "情人节礼物", "LBR 1912女王时代")
                .with(GirlFriend::addGift, "生日礼物", "迪奥烈焰蓝金")
                .with(GirlFriend::addGift, "纪念日礼物", "阿玛尼红管唇釉")
                // 等等等等 ...
                .build();
    }
}