设计模式之构建型模式（四）：建造者模式

1. 建造者模式概述

• 概念

  • 用于创建过程稳定，但配置多变的对象，主要通过链式调用生成不同的配置
  • 《设计模式》一书中的定义：将一个复杂的构建与其表示相分离，使得同样的构建过程可以创建不同的表示

• 特点

  • 将类的构造方法设为私有，所以外部不能通过 new 构建出类的实例，只能通过 Builder 构建
  • 对于必须配置的属性，通过 Builder 的构造方法传入；可选的属性通过 Builder 的链式调用方法传入。如果不配置，将使用默认配置

• 优点

  • 不用担心忘了指定某个配置，保证了构建过程是稳定的（必须配置的属性通过 Builder 的构造方法传入）
  • 在 OkHttp、Retrofit 等著名框架的源码中都使用到了建造者模式

• Demo

  • 珍珠奶茶类

    	public class MilkTea {
    	    private final String type;
    	    private final String size;
    	    private final boolean pearl;
    	    private final boolean ice;
    	
    		private MilkTea() {}
    		
    	    private MilkTea(Builder builder) {
    	        this.type = builder.type;
    	        this.size = builder.size;
    	        this.pearl = builder.pearl;
    	        this.ice = builder.ice;
    	    }
    	
    	    public String getType() {
    	        return type;
    	    }
    	
    	    public String getSize() {
    	        return size;
    	    }
    	
    	    public boolean isPearl() {
    	        return pearl;
    	    }
    	    public boolean isIce() {
    	        return ice;
    	    }
    	
    	    public static class Builder {
    	
    	        private final String type;
    	        private String size = "中杯";
    	        private boolean pearl = true;
    	        private boolean ice = false;
    	
    	        public Builder(String type) {
    	            this.type = type;
    	        }
    	
    	        public Builder size(String size) {
    	            this.size = size;
    	            return this;
    	        }
    	
    	        public Builder pearl(boolean pearl) {
    	            this.pearl = pearl;
    	            return this;
    	        }
    	
    	        public Builder ice(boolean cold) {
    	            this.ice = cold;
    	            return this;
    	        }
    	
    	        public MilkTea build() {
    	            return new MilkTea(this);
    	        }
    	    }
    }

  • 调用者

    public class User {
        private void buyMilkTea() {
            MilkTea milkTea = new MilkTea.Builder("原味").build();
            show(milkTea);
    
            MilkTea chocolate =new MilkTea.Builder("巧克力味")
                    .ice(false)
                    .build();
            show(chocolate);
            
            MilkTea strawberry = new MilkTea.Builder("草莓味")
                    .size("大杯")
                    .pearl(false)
                    .ice(true)
                    .build();
            show(strawberry);
        }
    
        private void show(MilkTea milkTea) {
            String pearl;
            if (milkTea.isPearl())
                pearl = "加珍珠";
            else
                pearl = "不加珍珠";
            String ice;
            if (milkTea.isIce()) {
                ice = "加冰";
            } else {
                ice = "不加冰";
            }
            System.out.println("一份" + milkTea.getSize() + "、"
                    + pearl + "、"
                    + ice + "的"
                    + milkTea.getType() + "奶茶");
        }
    }