在平時的項目開發中，我們會通過構造方法傳參來實例化對象。

　　但在需要多個參數時，如果繼續使用構造方法實例，編寫代碼會非常麻煩，而且在其他項目成員傳參時特別容易出現傳參錯誤的情況，這時我們不妨來使用Builder模式進行編寫。

　　在使用Builder模式之前，我們先看下重疊構造器模式和JavaBeans模式：

1、 重疊構造器模式

 1 package com.effectiveJava;
 2 
 3 public class MultiContrucProduct {
 4     private String name;
 5     private float
 
 price;
 6     private float weight;
 7     private String introduction;
 8     private String category;
 9     
10     public MultiContrucProduct(String name,float price){
11         this(name,price,(float)0);
12     }
13     
14     public MultiContrucProduct(String name,float price, float weight){
 
15         this(name, price,weight,null);
16     }
17     
18     public MultiContrucProduct(String name,float price,float weight,String introduction){
19         this(name,price,weight,introduction,null);
20     }
21     
22     public MultiContrucProduct(String name,float price,float weight,String introduction,String category){
 
23         this.name = name;
24         this.price = price;
25         this.weight = weight;
26         this.introduction = introduction;
27         this.category = category;
28     }
29 
30     public String getName() {
31         return name;
32     }
33 
34     public void setName(String name) {
35         this.name = name;
36     }
37 
38     public float getPrice() {
39         return price;
40     }
41 
42     public void setPrice(float price) {
43         this.price = price;
44     }
45 
46     public float getWeight() {
47         return weight;
48     }
49 
50     public void setWeight(float weight) {
51         this.weight = weight;
52     }
53 
54     public String getIntroduction() {
55         return introduction;
56     }
57 
58     public void setIntroduction(String introduction) {
59         this.introduction = introduction;
60     }
61 
62     public String getCategory() {
63         return category;
64     }
65 
66     public void setCategory(String category) {
67         this.category = category;
68     }
69     
70     
71 }

實例化對象：

1 MultiContrucProduct multiContrucProduct = 2 new MultiContrucProduct("multiProduct", 10, 10, null, "cloth");

使用重疊構造器模式在參數較多時，比較繁雜，冗余代碼多，而且在傳參時容易出現參數類型，參數順序不對，造成錯誤。

2、JavaBeans模式

 1 package com.effectiveJava;
 2 
 3 public class JavaBeanProduct {
 4     private String name;
 5     private float price;
 6     private float weight;
 7     private String introduction;
 8     private String category;
 9     
10     public JavaBeanProduct(){
11         
12     }
13 
14     public String getName() {
15         return name;
16     }
17 
18     public void setName(String name) {
19         this.name = name;
20     }
21 
22     public float getPrice() {
23         return price;
24     }
25 
26     public void setPrice(float price) {
27         this.price = price;
28     }
29 
30     public float getWeight() {
31         return weight;
32     }
33 
34     public void setWeight(float weight) {
35         this.weight = weight;
36     }
37 
38     public String getIntroduction() {
39         return introduction;
40     }
41 
42     public void setIntroduction(String introduction) {
43         this.introduction = introduction;
44     }
45 
46     public String getCategory() {
47         return category;
48     }
49 
50     public void setCategory(String category) {
51         this.category = category;
52     }
53     
54     
55 }

實例化對象：

1 JavaBeanProduct javaBeanProduct = new JavaBeanProduct();
2         javaBeanProduct.setName("javaBeanProduct");
3         javaBeanProduct.setPrice(10);
4         javaBeanProduct.setWeight(10);
5         javaBeanProduct.setIntroduction(null);
6         javaBeanProduct.setCategory("cloth");

使用JavaBeans模式編寫較為簡單，通過set方式也比較容易區分參數。但在構造過程中JavaBean可能處於不一致的狀態，這時使用該實例將會導致失敗。此外JavaBeans模式阻止了把類構造為不可變對象的

可能性，影響多線程中安全性處理。

3、Builder模式

 1 package com.effectiveJava;
 2 
 3 public class BuilderProduct {
 4     private String name;
 5     private float price;
 6     private float weight;
 7     private String introduction;
 8     private String category;
 9     
10     private BuilderProduct(Builder builder){
11         this.name = builder.name;
12         this.price = builder.price;
13         this.weight = builder.weight;
14         this.introduction = builder.introduction;
15         this.category = builder.category;
16     }
17     
18     public static class Builder{
19         private final String name;
20         private final float price;
21         private float weight;
22         private String introduction;
23         private String category;
24 
25         public float getWeight() {
26             return weight;
27         }
28 
29 
30 
31         public Builder setWeight(float weight) {
32             this.weight = weight;
33             return this;
34         }
35 
36 
37 
38         public String getIntroduction() {
39             return introduction;
40         }
41 
42 
43 
44         public Builder setIntroduction(String introduction) {
45             this.introduction = introduction;
46             return this;
47         }
48 
49 
50 
51         public String getCategory() {
52             return category;
53         }
54 
55 
56 
57         public Builder setCategory(String category) {
58             this.category = category;
59             return this;
60         }
61 
62 
63 
64         public BuilderProduct builder(){
65             return new BuilderProduct(this);
66         }
67         
68         public Builder(String name,float price){
69             this.name = name;
70             this.price = price;
71         }
72     }
73 }

實例化對象：

1         BuilderProduct builderProduct =  new BuilderProduct.Builder("builderProduct",0)
2                 .setCategory("cloth")
3                 .setIntroduction(null)
4                 .setWeight(10)
5                 .builder();

Builder模式是綜合了以上兩種模式的優點，即保證了重疊構造器模式的安全性，也能兼顧JavaBeans模式的可讀性。當然Builder模式還有其他的使用方式，本文只是展示了其中的一種使用。

設計模式之Builder模式