Java類設(shè)計(jì)過程中，如果類的構(gòu)造器或者靜態(tài)工廠中具有多個(gè)參數(shù)，并且其中有大量的可選參數(shù)時(shí)，我們應(yīng)該怎么辦？

1.Telescoping Constructor模式(重疊構(gòu)造器)

我們首先想到的方法肯定是傳統(tǒng)的構(gòu)造器

/**
 * Created by itbird on 2017/3/23
 */

public class Person {
    private String name;
    private String sex;
    private int year;

    public Person(String name, String sex, int year) {
        this.name = name;
        this.sex = sex;
        this.year = year;
    }
}

但是設(shè)計(jì)以及使用過程中我們發(fā)現(xiàn)以下幾點(diǎn)問題：
（1）屬性參數(shù)逐漸變多時(shí)
屬性參數(shù)逐漸變多時(shí)，由于要對(duì)之前的代碼做兼容，所以不可以直接在現(xiàn)有構(gòu)造器后面追加屬性，只能不斷新增構(gòu)造器

/**
 * Created by itbird on 2017/3/23
 */

public class Person {
    private String name;
    private String sex;
    private int year;
    private String city;

    public Person(String name, String sex, int year) {
        this.name = name;
        this.sex = sex;
        this.year = year;
    }

    public Person(String name, String sex, int year, String city) {
        this.name = name;
        this.sex = sex;
        this.year = year;
        this.city = city;
    }
}

（2）屬性參數(shù)中有大量的可選參數(shù)

/**
 * Created by itbird on 2017/3/23
 */

public class Person {
    private String name;
    private String sex;
    private int year;
    private String city;
    private String state;
    private boolean isFemale;
    private boolean isEmployed;
    private boolean isHomewOwner;

    public Person(String name, String sex, int year) {
        this.name = name;
        this.sex = sex;
        this.year = year;
    }

    public Person(String name, String sex, int year, String city) {
        this.name = name;
        this.sex = sex;
        this.year = year;
        this.city = city;
    }

    public Person(String name, String sex, int year, String city, String newState,
                  boolean newIsFemale, boolean newIsEmployed, boolean newIsHomeOwner) {
        this.name = name;
        this.sex = sex;
        this.year = year;
        this.city = city;
        this.state = newState;
        this.isFemale = newIsFemale;
        this.isEmployed = newIsEmployed;
        this.isHomewOwner = newIsHomeOwner;
    }
}

顯而易見，這樣寫的類構(gòu)造器雖然無可厚非，但是當(dāng)有許多參數(shù)的時(shí)候，客戶端代碼會(huì)很難編寫，并且難以閱讀。如果讀者想知道那些值是什么意思，必須很仔細(xì)的數(shù)著這些參數(shù)來探個(gè)究竟。

2.JavaBeans模式

在這種模式下，調(diào)用一個(gè)無參構(gòu)造器來創(chuàng)建對(duì)象，然后調(diào)用setter方法來設(shè)置每個(gè)必要的參數(shù)，以及每個(gè)相關(guān)的可選參數(shù)。

/**
 * Created by itbird on 2017/3/23
 */

public class Person {
    private String name;
    private String sex;
    private int year;
    private String city;
    private String state;
    private boolean isFemale;
    private boolean isEmployed;
    private boolean isHomewOwner;

    public Person() {
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getState() {
        return state;
    }

    public void setState(String state) {
        this.state = state;
    }

    public boolean isFemale() {
        return isFemale;
    }

    public void setFemale(boolean female) {
        isFemale = female;
    }

    public boolean isEmployed() {
        return isEmployed;
    }

    public void setEmployed(boolean employed) {
        isEmployed = employed;
    }

    public boolean isHomewOwner() {
        return isHomewOwner;
    }

    public void setHomewOwner(boolean homewOwner) {
        isHomewOwner = homewOwner;
    }
}

這種模式彌補(bǔ)重疊構(gòu)造器模式的不足。說的明白一點(diǎn)，就是創(chuàng)建實(shí)例很容易，這樣產(chǎn)生的代碼讀起來也很容易:

 Person person = new Person();
 person.setCity("重慶");
 person.setYear(12);
 person.setSex("男");
 person.setName("itbird");

遺憾的是，JavaBeans模式自身有著很嚴(yán)重的缺點(diǎn)。因?yàn)闃?gòu)造過程被分到幾個(gè)調(diào)用中，在構(gòu)造過程中JavaBean可能處于非一致的狀態(tài)。JavaBeans模式阻止了把類做成不可變的可能，這就需要程序員付出額外的努力來確保他的線程安全。

3.Builder模式

/**
 * Created by itbird on 2017/3/23
 */

public class Person {
    private String name;
    private String sex;
    private int year;
    private String city;
    private String state;
    private boolean isFemale;
    private boolean isEmployed;
    private boolean isHomewOwner;

    public Person() {
    }

    public static class PersonBuilder {
        // 必要參數(shù)
        private String name;
        // 可選參數(shù)
        private String sex;
        private int year;
        private String city;
        private String state;
        private boolean isFemale;
        private boolean isEmployed;
        private boolean isHomewOwner;

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

        public PersonBuilder setSex(String sex) {
            this.sex = sex;
            return this;
        }

        public PersonBuilder setYear(int year) {
            this.year = year;
            return this;
        }

        public PersonBuilder setCity(String city) {
            this.city = city;
            return this;
        }

        public PersonBuilder setState(String state) {
            this.state = state;
            return this;
        }

        public PersonBuilder setFemale(boolean female) {
            isFemale = female;
            return this;
        }

        public PersonBuilder setEmployed(boolean employed) {
            isEmployed = employed;
            return this;
        }

        public PersonBuilder setHomewOwner(boolean homewOwner) {
            isHomewOwner = homewOwner;
            return this;
        }

        public Person build() {
            Person person = new Person();
            person.name = name;
            person.sex = sex;
            person.city = city;
            person.isEmployed = isEmployed;
            person.isFemale = isFemale;
            person.isHomewOwner = isHomewOwner;
            person.state = state;
            person.year = year;
            return person;
        }
    }
}

調(diào)用的實(shí)例:

 Person person = new Person.PersonBuilder("itbird")
                    .setCity("重慶").setYear(15).build();

顯然，使用Builder模式解決了上訴的難題，達(dá)到了“以不變(Builder)應(yīng)萬變(參數(shù))”的目的。

總結(jié)：

Java類設(shè)計(jì)過程中，如果類的構(gòu)造器或者靜態(tài)工廠中具有多個(gè)參數(shù)，并且其中有大量的可選參數(shù)時(shí)，我們應(yīng)該考慮使用構(gòu)建器，與傳統(tǒng)的重疊構(gòu)造器模式相比，使用Builder模式的代碼更易于閱讀和編寫，有更好的可擴(kuò)展性，同時(shí)構(gòu)建器也比JavaBeans更加安全。
從最后的實(shí)例演練中我們知道，在實(shí)際開發(fā)過程中，我們往往是需要幾種模式混合使用，這樣才能在保證代碼健壯性、可讀性的同時(shí)，去保證代碼的可擴(kuò)展性以及線程安全性等，這才是代碼設(shè)計(jì)開發(fā)過程中真正的“以不變應(yīng)萬變”之道。