java Builder模式创建不可变类

package com.geostar.gfstack.operationcenter.logger.manager.common;

/**
 * Created by Nihaorz on 2017/8/14.
 */
public class DBConfig {

    private String driver;
    private String url;
    private String username;
    private String password;

    public static class Builder {

        private String driver;
        private String url;
        private String username;
        private String password;

        public Builder(){
            super();
        }

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

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

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

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

        public DBConfig build(){
            return new DBConfig(this);
        }

    }

    private DBConfig(Builder builder) {
        driver = builder.driver;
        url = builder.url;
        username = builder.username;
        password = builder.password;
    }

    public String getDriver() {
        return driver;
    }

    public String getUrl() {
        return url;
    }

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    @Override
    public String toString() {
        return "DBConfig{" +
                "driver='" + driver + '\'' +
                ", url='" + url + '\'' +
                ", username='" + username + '\'' +
                ", password='" + password + '\'' +
                '}';
    }
}

　　

DBConfig类只有一个私有构造函数，没有setter方法