构建者模式：主要用于对外提供统一的参数封装。

案例：Request 参数封装。

public class Request {
    
    private String url;
    private String method;
    //私有化构造函数
    private Request(Builder builder) {
        this.url = builder.url;
        this.method = builder.method;
    }
    //提供静态的Buidler构建器封装参数
    public static Request.Builder builder() {
        return new Builder();
    } 
    
    @Override
    public String toString() {
        return "Request [url=" + url + ", method=" + method + "]";
    }

    //内部创建Builder构建器类，提供给外界设置参数
    final static class Builder{

        private String url;
        private String method;
        
        public Builder setUrl(String url) {
            this.url = url;
            return this;
        }
        public Builder setMethod(String method) {
            this.method = method;
            return this;
        }
        
        public Request build() {
            return new Request(this);
        }
    }
    //调用案例
    public static void main(String[] args) {
        Request request = Request.builder().
                setMethod("POST")
                .setUrl("http://baidu.com/") 
                .build();
        System.out.println(request.toString());
    }
}