為什么使用Builder模式

1、傳統(tǒng)的創(chuàng)建對(duì)象可以使用構(gòu)造器創(chuàng)建，但是當(dāng)我們調(diào)用構(gòu)造器時(shí)通常需要許多你不想要設(shè)置的參數(shù)，但是又不得不為它們傳值。這個(gè)時(shí)候如果我們使用重疊構(gòu)造器可行，但是當(dāng)有許多參數(shù)的時(shí)候，客戶端的代碼就會(huì)非常難寫，并且很難閱讀。

重疊構(gòu)造器代碼如下

public class BuilderPerson {
    //必選參數(shù)
    private final String name;
    private final int age;
    private final int height;
    private final int weight;
    //可選參數(shù)
    private final boolean isMarraying;
    private final boolean isHasChildren;

    
    public BuilderPerson(String name, int age) {
        this(name,age,0);
    }

    public BuilderPerson(String name, int age, int height) {
        this(name,age,height,0);
    }

    public BuilderPerson(String name, int age, int height, int weight) {
        this(name,age,height,weight,false);
    }

    public BuilderPerson(String name, int age, int height, int weight, boolean isMarraying) {
        this(name, age,height,weight,isMarraying,false);
    }

    public BuilderPerson(String name, int age, int height, int weight, boolean isMarraying, boolean isHasChildren) {
        this.name = name;
        this.age = age;
        this.height = height;
        this.weight = weight;
        this.isMarraying = isMarraying;
        this.isHasChildren = isHasChildren;
    }
}

2、當(dāng)遇到許多構(gòu)造器參數(shù)的時(shí)候，還有第二中代替的辦法，即使用JavaBean模式，在這種模式下，調(diào)用一個(gè)無參的構(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)。

JavaBean代碼如下

public class BuilderPerson {
    //必選參數(shù)
    private  String name;
    private  int age;
    private  int height;
    private  int weight;
    //可選參數(shù)
    private  boolean isMarraying;
    private  boolean isHasChildren;

    public BuilderPerson() {
    }

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

    public void setAge(int age) {
        this.age = age;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }

    public void setMarraying(boolean marraying) {
        isMarraying = marraying;
    }

    public void setHasChildren(boolean hasChildren) {
        isHasChildren = hasChildren;
    }
}

但是JavaBean模式自身有著嚴(yán)重的缺點(diǎn)，會(huì)導(dǎo)致類處于不一致（你new了Test類的兩個(gè)實(shí)例，一個(gè)只set了A屬性，一個(gè)只設(shè)置了B屬性，這兩個(gè)實(shí)例不一致，你不能保證通過該類的同一個(gè)構(gòu)造器保證構(gòu)造出來的對(duì)象是屬性相同的。）的狀態(tài)。

這時(shí)就出現(xiàn)了第三種替代的方法既能像重疊構(gòu)造器那樣的安全性，也能保證代碼的可讀性即Builder模式。

代碼如下

public static class Builder{
        //必選參數(shù)
        private  String name;
        private  int age;
        private  int height;
        private  int weight;
        //可選參數(shù)
        private final boolean isMarraying;
        private final boolean isHasChildren;

        public Builder(boolean isMarraying, boolean isHasChildren) {
            this.isMarraying = isMarraying;
            this.isHasChildren = isHasChildren;
        }
        public Builder name(String name) {
            name = name;
            return this;
        }

        public Builder age(int age) {
            age = age;
            return this;
        }

        public Builder height(int height) {
            height = height;
            return this;
        }

        public Builder weight(int weight) {
            weight = weight;
            return this;
        }
        public BuilderPerson builderPerson() {
            return new BuilderPerson(this);
        }
    }
}
調(diào)用如下
BuilderPerson person = new BuilderPerson.Builder(false,false).
name("張三").age(20).height(180).weight(120).buildPerson();

總結(jié)

簡(jiǎn)而言之，如果類的構(gòu)造器或者靜態(tài)工廠中具有多個(gè)參數(shù)，設(shè)計(jì)這種類時(shí)，Builder模式就是種不錯(cuò)的選擇，特別是當(dāng)大多數(shù)參數(shù)都是可選的時(shí)候，與使用傳統(tǒng)的重疊構(gòu)造器模式相比，使用Builder模式的客戶端代碼將更易于閱讀和編寫，構(gòu)建器也比JavaBeans更加安全。

歡迎加入學(xué)習(xí)交流群569772982，大家一起學(xué)習(xí)交流。