设计模式之Builder模式

　　在平时的项目开发中，我们会通过构造方法传参来实例化对象。

　　但在需要多个参数时，如果继续使用构造方法实例，编写代码会非常麻烦，而且在其他项目成员传参时特别容易出现传参错误的情况，这时我们不妨来使用Builder模式进行编写。

　　在使用Builder模式之前，我们先看下重叠构造器模式和JavaBeans模式：

1、 重叠构造器模式

package com.effectiveJava;

public class MultiContrucProduct {
    private String name;
    private float price;
    private float weight;
    private String introduction;
    private String category;
    
    public MultiContrucProduct(String name,float price){
        this(name,price,(float)0);
    }
    
    public MultiContrucProduct(String name,float price, float weight){
        this(name, price,weight,null);
    }
    
    public MultiContrucProduct(String name,float price,float weight,String introduction){
        this(name,price,weight,introduction,null);
    }
    
    public MultiContrucProduct(String name,float price,float weight,String introduction,String category){
        this.name = name;
        this.price = price;
        this.weight = weight;
        this.introduction = introduction;
        this.category = category;
    }

    public String getName() {
        return name;
    }

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

    public float getPrice() {
        return price;
    }

    public void setPrice(float price) {
        this.price = price;
    }

    public float getWeight() {
        return weight;
    }

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

    public String getIntroduction() {
        return introduction;
    }

    public void setIntroduction(String introduction) {
        this.introduction = introduction;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }
    
    
}

实例化对象：

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

使用重叠构造器模式在参数较多时，比较繁杂，冗余代码多，而且在传参时容易出现参数类型，参数顺序不对，造成错误。

2、JavaBeans模式

package com.effectiveJava;

public class JavaBeanProduct {
    private String name;
    private float price;
    private float weight;
    private String introduction;
    private String category;
    
    public JavaBeanProduct(){
        
    }

    public String getName() {
        return name;
    }

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

    public float getPrice() {
        return price;
    }

    public void setPrice(float price) {
        this.price = price;
    }

    public float getWeight() {
        return weight;
    }

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

    public String getIntroduction() {
        return introduction;
    }

    public void setIntroduction(String introduction) {
        this.introduction = introduction;
    }

    public String getCategory() {
        return category;
    }

    public void setCategory(String category) {
        this.category = category;
    }
    
    
}

实例化对象：

JavaBeanProduct javaBeanProduct = new JavaBeanProduct();
        javaBeanProduct.setName("javaBeanProduct");
        javaBeanProduct.setPrice(10);
        javaBeanProduct.setWeight(10);
        javaBeanProduct.setIntroduction(null);
        javaBeanProduct.setCategory("cloth");

使用JavaBeans模式编写较为简单，通过set方式也比较容易区分参数。但在构造过程中JavaBean可能处于不一致的状态，这时使用该实例将会导致失败。此外JavaBeans模式阻止了把类构造为不可变对象的

可能性，影响多线程中安全性处理。

3、Builder模式

package com.effectiveJava;

public class BuilderProduct {
    private String name;
    private float price;
    private float weight;
    private String introduction;
    private String category;
    
    private BuilderProduct(Builder builder){
        this.name = builder.name;
        this.price = builder.price;
        this.weight = builder.weight;
        this.introduction = builder.introduction;
        this.category = builder.category;
    }
    
    public static class Builder{
        private final String name;
        private final float price;
        private float weight;
        private String introduction;
        private String category;

        public float getWeight() {
            return weight;
        }



        public Builder setWeight(float weight) {
            this.weight = weight;
            return this;
        }



        public String getIntroduction() {
            return introduction;
        }



        public Builder setIntroduction(String introduction) {
            this.introduction = introduction;
            return this;
        }



        public String getCategory() {
            return category;
        }



        public Builder setCategory(String category) {
            this.category = category;
            return this;
        }



        public BuilderProduct builder(){
            return new BuilderProduct(this);
        }
        
        public Builder(String name,float price){
            this.name = name;
            this.price = price;
        }
    }
}

实例化对象：

        BuilderProduct builderProduct =  new BuilderProduct.Builder("builderProduct",0)
                .setCategory("cloth")
                .setIntroduction(null)
                .setWeight(10)
                .builder();

Builder模式是综合了以上两种模式的优点，即保证了重叠构造器模式的安全性，也能兼顾JavaBeans模式的可读性。当然Builder模式还有其他的使用方式，本文只是展示了其中的一种使用。

 

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