Builder 模式

引入

这个设计模式有什么用，解决什么问题，为什么要用

• 一个类有很多的field，构造函数很复杂
• 处理对象的不可变性

例子

• StringBuilder
• DocumentBuiler
• Locale.Builder

详解

例1

现有一个“订餐类”LunchOrderBean，其中有属性面包，调料，肉等等属性

实现一：默认构造函数和setter方法

public class LunchOrderBean {

    private String bread;
    private String condiments;
    private String dressing;
    private String meat;

    public LunchOrderBean() {

    }

    public String getBread() {
        return bread;
    }

    public String getCondiments() {
        return condiments;
    }

    public String getDressing() {
        return dressing;
    }

    public String getMeat() {
        return meat;
    }

    public void setBread(String bread) {
        this.bread = bread;
    }

    public void setCondiments(String condiments) {
        this.condiments = condiments;
    }

    public void setDressing(String dressing) {
        this.dressing = dressing;
    }

    public void setMeat(String meat) {
        this.meat = meat;
    }

}

我们要使用这个类，也就是获取这个类的对象，如何做？
很简单，就像下面这样

        LunchOrderBean lunchOrderBean = new LunchOrderBean();

        lunchOrderBean.setBread("Wheat");
        lunchOrderBean.setCondiments("Lettuce");
        lunchOrderBean.setDressing("Mustard");
        lunchOrderBean.setMeat("Ham");

        System.out.println(lunchOrderBean.getBread());
        System.out.println(lunchOrderBean.getCondiments());
        System.out.println(lunchOrderBean.getDressing());
        System.out.println(lunchOrderBean.getMeat());

我们看到这个类有一个默认的无参数的构造函数，且对每个属性都有setter方法。
但是这样的问题是：

1. 类LunchOrderBean不是不可变的（immutable），这意味着在创建它的对象后，我们可以通过setter方法来改变对象的状态
2. 从类本身看不出类的使用规范或说明，也就是说我们看不出对一个合法的order来说，哪些属性是必须的，哪些是可选的。

实现二 层叠（telescoping）构造函数

public class LunchOrderTele {

    private String bread;
    private String condiments;
    private String dressing;
    private String meat;

    public LunchOrderTele(String bread) {
        this.bread = bread;
    }

    public LunchOrderTele(String bread, String condiments) {
        this(bread);
        this.condiments = condiments;
    }

    public LunchOrderTele(String bread, String condiments, String dressing) {
        this(bread, condiments);
        this.dressing = dressing;
    }

    public LunchOrderTele(String bread, String condiments, String dressing, String meat) {
        this(bread, condiments, dressing);
        this.meat = meat;
    }

    public String getBread() {
        return bread;
    }

    public String getCondiments() {
        return condiments;
    }

    public String getDressing() {
        return dressing;
    }

    public String getMeat() {
        return meat;
    }
}

这个实现消除了setter方法，解决了“不可变”的问题。
并且使用了很多构造函数，我们称为层叠（telescoping）构造函数，让我们可以创建我们想要的order，只有面包的，有面包和调料的，有面包和调料和肉的等等。
但是它没有列举出所有情况，假如我们想要一个只有面包和肉的，它就无能为力了。

实现三 builder模式

public class LunchOrder {

    public static class Builder {
        private String bread;
        private String condiments;
        private String dressing;
        private String meat;
        
        public Builder() {
            
        }
        
        public LunchOrder build() {
            return new LunchOrder(this);
        }
        
        public Builder bread(String bread) {
            this.bread = bread;
            return this;
        }
        
        public Builder condiments(String condiments) {
            this.condiments = condiments;
            return this;
        }
        
        public Builder dressing(String dressing) {
            this.dressing = dressing;
            return this;
        }
        
        public Builder meat(String meat) {
            this.meat = meat;
            return this;
        } 
        
    }
    
    private final String bread;
    private final String condiments;
    private final String dressing;
    private final String meat;

    private LunchOrder(Builder builder) {
        this.bread = builder.bread;
        this.condiments = builder.condiments;
        this.dressing = builder.dressing;
        this.meat = builder.meat;
    }

    public String getBread() {
        return bread;
    }

    public String getCondiments() {
        return condiments;
    }
    
    public String getDressing() {
        return dressing;
    }
    
    public String getMeat() {
        return meat;
    }
    
}