作為java/Android開發者, 如果你想設計一個通用的庫, Builder模式幾乎肯定是會用到的, 我們需要提供良好的入口/接口來使用者可以彈性地構造他們需要的對象.

像一些優秀的開源庫, 例如Glide, OkHttp等都在構造Glide, OkHttpClient時用到Builder模式, 例如OkHttpClient的創建, 如下節選OkHttpClient.java的代碼:

public class OkHttpClient implements Cloneable, Call.Factory {    public OkHttpClient() {        this(new Builder());    }    PRivate OkHttpClient(Builder builder) {        this.dispatcher = builder.dispatcher;        this.proxy = builder.proxy;        this.protocols = builder.protocols;        this.connectionSpecs = builder.connectionSpecs;        this.interceptors = Util.immutableList(builder.interceptors);        this.networkInterceptors = Util.immutableList(builder.networkInterceptors);        this.proxySelector = builder.proxySelector;        this.cookieJar = builder.cookieJar;        this.cache = builder.cache;        this.internalCache = builder.internalCache;        this.socketFactory = builder.socketFactory;        // more code...    }    public static final class Builder {        public Builder() {            ...        }        public Builder cache(Cache cache) {            ...        }        public Builder dispatcher(Dispatcher dispatcher) {            ...        }        public Builder protocols(List protocols) {            ...        }        public List networkInterceptors() {            ...        }        public Builder addNetworkInterceptor(Interceptor interceptor) {            ...        }        public OkHttpClient build() {            return new OkHttpClient(this);        }    }}
Buidler模式, 是一種創建型的設計模式.
通常用來將一個復雜的對象的構造過程分離, 讓使用者可以根據需要選擇創建過程.
另外, 當這個復雜的對象的構造包含很多可選參數時, 那Builder模式可以說是不二之選.
所以今天寫了一個Builer模式的一個demo  
一個餃子的類  然后顧客根據自己的喜好自助添加作料
public class Dumpling {    private boolean soy;    private boolean vinegar;    private boolean garlic;    private boolean chili;    public Dumpling(Builder builder) {        this.soy = builder.soy;        this.chili = builder.chili;        this.garlic = builder.garlic;        this.vinegar = builder.vinegar;    }    @Override    public String toString() {        StringBuilder builder = new StringBuilder("來一盤餃子放：");        if (this.soy){            builder.append("醬油");        }          if (this.chili){            builder.append("辣椒");        }          if (this.vinegar){            builder.append("醋");        }          if (this.garlic){            builder.append("蒜");        }        return builder.toString();    }    public static class Builder {        private boolean soy;        private boolean vinegar;        private boolean garlic;        private boolean chili;        public Builder() {        }        public Builder withSoy() {            this.soy = true;            return this;        }        public Builder withVinegar() {            this.vinegar = true;            return this;        }        public Builder withGarlic() {            this.garlic = true;            return this;        }        public Builder withChili() {            this.chili = true;            return this;        }        public Dumpling build() {            return new Dumpling(this);        }    }
一般來說Builder常常作為靜態內部類（提高內聚性）
下面是顧客自助活動
public class BuilderDemo {    public static void main(String[] args) {        Dumpling customer1 = new Dumpling.Builder()                .withChili()                .withGarlic()                .withSoy()                .build();        System.out.println("第一個顧客"+customer1.toString());        Dumpling customer2 = new Dumpling.Builder()                .withSoy()                .withVinegar()                .build();        System.out.println("第二個顧客"+customer2.toString());        Dumpling customer3 = new Dumpling.Builder()                .withVinegar()                .withChili()                .withGarlic()                .build();        System.out.println("第三個顧客"+customer3.toString());    }}運行結果
第一個顧客來一盤餃子放：醬油辣椒蒜第二個顧客來一盤餃子放：醬油醋第三個顧客來一盤餃子放：辣椒醋蒜