【读书笔记】《Effective Java》第二章第2条：遇到多个构造器参数时要考虑使用Builder

一、前言

《Effective Java》读书笔记系列
第二章第1条：创建和销毁对象
第二章第2条：遇到多个构造器参数时要考虑使用Builder

二、介绍

我们开发中偶尔会遇到一些需要设置10个或以上大量参数的情况，通常情况有几种处理方式：多构造函数、setter、Builder模式。

1、多构造函数：这种方式虽然最方便，但也最繁琐，假设有一个方法需要设置6个int型的参数，但多数情况下，这6个中总有你不想传的参数，这时候就需要约定一个规则：例如传0，但是传0又会有一个问题就是传入顺序问题，传入顺序不同并不会导致程序编译不通过，却会导致程序运行结果不正确。所以这并不是一种好方式
2、setter：
虽然它可以解决多构造带来的传入顺序问题，但是它还是略显繁琐了。所以我们通常只会在JavaBean中使用setter
3、Builder模式：
本文要介绍的一种方式，可以完美解决上述两种方式带来的弊端，让代码更加符合设计模式。参考Glide和Retrofit的使用方式，它们都是通过链式调用一个一个参数连接起来的

三、定义

Builder的定义是：将一个复杂对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示

四、使用场景

主要强调两点：调用顺序、多参数

1、相同的方法，不同的执行顺序，需要产生不同的事件结果时
2、多个部件或零件，都可以装配到一个对象中，但是产生的运行结果又不相同的时候
3、产品类非常复杂，或者产品类中的调用顺序不同产生了不同的作用时
4、当初始化一个对象特别复杂，如参数多，且很多参数都具有默认值时

四、UML图

背景：我要建造一个xsic
在这个背景下，Product是xsic的抽象，也就是人，ConcreteBuilder是一个具体Builder类，继承了抽象Builder类Builder，而Director可以是一个高层客户端模块，也可以是一个为高层客户端模块提供构建功能的中间模块

四、举例

1、抽象产品类，在UML中没有具体体现

 public abstract class Person {protected String mHead;protected int mBody;protected String mArm;protected String mLeg;protected String mHair;public abstract void setHead(String head);public abstract void setBody(int body);public void setmArm(String mArm) {this.mArm = mArm;}public void setmLeg(String mLeg) {this.mLeg = mLeg;}public void setmHair(String mHair) {this.mHair = mHair * mBody;}}

2、具体产品类，对应UML中的Product

 public class Xsic extends Person {public Xsic() {}@Overridepublic void setHead(String head) {mHead = "默认头";}@Overridepublic void setBody(int body) {mBody = 1;}}

3、抽象Builder类，对应UML中的Builder

 public abstract class Builder {public abstract Builder buildHead(String head);public abstract Builder buildBody(int body);public abstract Builder buildArm(String arm);public abstract Builder buildLeg(String leg);public abstract Builder buildHair(String hair);public abstract Person build();}

4、具体Builder类，对应UML中的ConcreteBuilder

 public class XsicBuilder extends Builder {private Xsic xsic = new Xsic();@Overridepublic Builder buildHead(String head) {xsic.setHead(head);return this;}@Overridepublic Builder buildBody(int body) {xsic.setBody(body);return this;}@Overridepublic Builder buildArm(String arm) {xsic.setmArm(arm);return this;}@Overridepublic Builder buildLeg(String leg) {xsic.setmLeg(leg);return this;}@Overridepublic Builder buildHair(String hair) {xsic.setmHair(hair);return this;}@Overridepublic Person build() {return xsic;}}

5、Director类，可以理解为为高层客户端模块提供构造方式的类，对应UML中的Director，但是通常都不会写这个类，而是直接给高层客户端模块调用。

 public class Director {private Builder mBuilder = null;public Director(Builder mBuilder) {this.mBuilder = mBuilder;}public void construct(String head, int body, String arm, String leg, String hair){mBuilder.buildHead(head).buildBody(body).buildArm(arm).buildLeg(leg).buildHair(hair).build();}}

6、客户端，在UML中没有具体体现

 public class Client {private void main(){Builder mBuilder = new XsicBuilder();Person xsic = mBuilder.buildArm("手").buildLeg("脚").buildBody("身体").buildHead("头").build();}}

这个例子里面除了明显的多参数外，还加入了调用顺序产生不同结果：buildBody调用的先后顺序会影响buildHair的结果

五、实际使用

实际使用的时候通常情况下并不会有这么多类（除非真的有抽象Builder的必要，例如要build一系列类似的事物），通常都会将Director去掉，毕竟Director的写法跟setter没什么差别。因此通常都会只写一个外部类，将Builder作为这个外部类的静态内部类。
因为使用Glide经常需要写RequestBuilder，我们这里以二次封装一个Glide工具类为例子：

public enum ImagLoader2 {INSTANCE;private static Builder mBuilder = null;public Builder getBuilder(ImageView view){mBuilder = newBuilder(view,view.getContext());return mBuilder;}private Builder newBuilder(ImageView view, Context context){return new Builder(view,context);}public static final class Builder{/* 参数 *///字符串型数据源private String mUrl = "";//资源型数据源private @DrawableRes int mRes = -1;//drawable型数据源private Drawable mDrawable = null;//目标viewprivate ImageView mView;//默认图，只取第一次设置private @DrawableRes int mIntPlaceHolder = -1;private Drawable mDrawablePlaceHolder = null;//错误图，只取第一次设置private @DrawableRes int mIntError = -1;private Drawable mDrawableError = null;//默认图private @DrawableRes int mDefault = -1;//圆角，按left、top、right、bottom顺序，只取第一次设置private int mLTRadius = 0;private int mRTRadius = 0;private int mRBRadius = 0;private int mLBRadius = 0;//全圆角，只取第一次设置private int mRadius = 0;//预览private float mThumnail = 0f;//模糊半径private int mBlurRadius = -1;//是否gif图private boolean mIsGif = false;//是否Bitmapprivate boolean mIsBmp = false;/* 参数 *///是否已经设置数据源，只取第一次设置的值private boolean mIsSourceSet = false;private boolean mIsPlaceHolderSet = false;private boolean mIsErrorSet = false;private RequestOptions mErrorRQ, mPlaceHolderRQ, mRoundCornerRQ, mPartCornerRQ;private RequestManager mGlide;private RequestBuilder mGlideBuilder;private PartCornerTransform2 mPartCornerTransform;private RoundedCorners mRoundedCorners;//四个角的圆角值public enum DIRETION{LEFT_TOP,RIGHT_TOP,RIGHT_BOTTOM,LEFT_BOTTOM}public Builder(ImageView view, Context context) {if (view == null){throw new IllegalArgumentException("目标view必须有效！");}mView = view;mGlide = Glide.with(context);}public Builder setUrl(String url){if (mIsSourceSet) return this;mUrl = url;mIsSourceSet = true;return this;}public Builder setRes(@DrawableRes int res){if (mIsSourceSet) return this;mRes = res;mIsSourceSet = true;return this;}public Builder setDrawable(Drawable drawable){if (mIsSourceSet) return this;mDrawable = drawable;mIsSourceSet = true;return this;}public Builder setDefault(@DrawableRes int res){mDefault = res;return this;}public Builder setPlaceHolder(@DrawableRes int placeHolder){if (mIsPlaceHolderSet) return this;mIntPlaceHolder = placeHolder;mIsPlaceHolderSet = true;return this;}public Builder setPlaceHolder(Drawable placeHolder){if (mIsPlaceHolderSet) return this;mDrawable = placeHolder;mIsPlaceHolderSet = true;return this;}public Builder setError(@DrawableRes int error){if (mIsErrorSet) return this;mIntError = error;mIsErrorSet = true;return this;}public Builder setError(Drawable error){if (mIsErrorSet) return this;mDrawableError = error;mIsErrorSet = true;return this;}public Builder setRadius(int radius){if (radius <= 0){throw new IllegalArgumentException("圆角值必须大于0！圆角值为："+radius);}mRadius = radius;return this;}/*** 设置各自角的圆角值* @param radius 圆角值* @param diretion 四个角* @return Builder*/public Builder setPartRadius(int radius, DIRETION diretion){if (radius <= 0){throw new IllegalArgumentException("圆角值必须大于0！圆角值为："+radius);}switch (diretion){case LEFT_TOP:mLTRadius = radius;break;case RIGHT_TOP:mRTRadius = radius;break;case RIGHT_BOTTOM:mRBRadius = radius;break;case LEFT_BOTTOM:mLBRadius = radius;break;}return this;}public Builder setBlurRadius(int blurRadius){if (blurRadius <= 0) {throw new IllegalArgumentException("模糊值必须大于0！模糊值为："+blurRadius);}mBlurRadius = blurRadius;return this;}public Builder setThumnail(float thumnail){if (thumnail <= 0f){throw new IllegalArgumentException("预览图之于原图的比例不能小于0！当前为："+thumnail);}mThumnail = thumnail;return this;}public Builder asGif(){mIsGif = true;return this;}public Builder asBitmap(){mIsBmp = true;return this;}public void build(){mGlideBuilder = null;mErrorRQ = new RequestOptions();mPlaceHolderRQ = new RequestOptions();mRoundCornerRQ = new RequestOptions();mPartCornerRQ = new RequestOptions();//数据源if (!mUrl.equals("")){mGlideBuilder = mGlide.load(mUrl);}else if (mRes != -1){mGlideBuilder = mGlide.load(mRes);}else if (mDrawable != null){mGlideBuilder = mGlide.load(mDrawable);}else {if (mDefault != -1){mGlideBuilder = mGlide.load(mDefault);}else {mGlideBuilder = mGlide.load("");}}//错误图if (mIntError != -1){mErrorRQ = mErrorRQ.error(mIntError);mGlideBuilder = mGlideBuilder.apply(mErrorRQ);}else if (mDrawableError != null){mErrorRQ = mErrorRQ.error(mDrawableError);mGlideBuilder = mGlideBuilder.apply(mErrorRQ);}//默认图if (mIntPlaceHolder != -1){mPlaceHolderRQ = mPlaceHolderRQ.placeholder(mIntPlaceHolder);mGlideBuilder = mGlideBuilder.apply(mPlaceHolderRQ);}else if (mDrawablePlaceHolder != null){mPlaceHolderRQ = mPlaceHolderRQ.placeholder(mDrawablePlaceHolder);mGlideBuilder = mGlideBuilder.apply(mPlaceHolderRQ);}//圆角if (mRadius != 0){mRoundedCorners = new RoundedCorners(mRadius);mRoundCornerRQ = RequestOptions.bitmapTransform(mRoundedCorners);mGlideBuilder = mGlideBuilder.apply(mRoundCornerRQ);}//各自圆角if (mLTRadius != 0 || mRTRadius != 0 || mRBRadius != 0 || mLBRadius != 0){mPartCornerTransform = new PartCornerTransform2(mLTRadius,mRTRadius,mRBRadius,mLBRadius);mPartCornerRQ = RequestOptions.bitmapTransform(mPartCornerTransform);mGlideBuilder = mGlideBuilder.apply(mPartCornerRQ);}//预览if (mThumnail > 0f){mGlideBuilder = mGlideBuilder.thumbnail(mThumnail);}//是否gifif (mIsGif){mGlideBuilder = mGlide.asGif();}//是否bmpif (mIsBmp){mGlideBuilder = mGlide.asBitmap();}mGlideBuilder.into(mView);}}
}

六、总结

Builder模式如果真的要吹毛求疵找出缺点的话，那就是会比普通setter或者多构造函数方式多了一个Builder对象的创建，这对于对性能要求苛刻的应用来说可能不是一个最好的选择，但是对于大多数应用来说，Builder的优势还是显而易见的。