《effective java》读书笔记——遇到多个构造器参数时

在日常工作中，业务场景会非常复杂。为了满足对应的业务场景，我们需要同时兼容多种情况。

业务背景

账号体系支持多种用户身份，对于这种需求，本应该生成一个全局唯一的id。但是由于原先的设计不太优雅，我们需要适配原先的业务场景。

1. 手机号身份，这是内部注册的账号，buyerId。
2. 其他app账号绑定，例如微信、qq、微博，fansType和fansId。

用户类

public class User {
    private long buyerId;
    private int fansType;
    private long fansId;

    public long getBuyerId() {
        return buyerId;
    }

    public void setBuyerId(long buyerId) {
        this.buyerId = buyerId;
    }

    public int getFansType() {
        return fansType;
    }

    public void setFansType(int fansType) {
        this.fansType = fansType;
    }

    public long getFansId() {
        return fansId;
    }

    public void setFansId(long fansId) {
        this.fansId = fansId;
    }
}

问题：需要了解业务细节，没有规则约束，容易遗漏字段

多个构造器参数类

public class User {
    private final long buyerId;
    private final int fansType;
    private final long fansId;

    public User(long buyerId) {
        this.buyerId = buyerId;
        this.fansType = 0;
        this.fansId = 0;
    }

    public User(int fansType, long fansId) {
        this.buyerId = 0;
        this.fansType = fansType;
        this.fansId = fansId;
    }

    public User(long buyerId, int fansType, long fansId) {
        this.buyerId = buyerId;
        this.fansType = fansType;
        this.fansId = fansId;
    }
}

问题：构造方法太多，不利于后续维护

业务需求变化

产品说，我们需要支持匿名用户。构造器参数又得增加一倍，程序员的恶梦开始了。

用构建器改造

public class Builder {
    private long buyerId;
    private int fansType;
    private long fansId;

    public long getBuyerId() {
        return buyerId;
    }

    public void setBuyerId(long buyerId) {
        this.buyerId = buyerId;
    }

    public int getFansType() {
        return fansType;
    }

    public void setFansType(int fansType) {
        this.fansType = fansType;
    }

    public long getFansId() {
        return fansId;
    }

    public void setFansId(long fansId) {
        this.fansId = fansId;
    }

    public User build() {
        if (buyerId == 0 && fansType == 0 && fansId == 0) {
            throw new RuntimeException("please init user data");
        }
        if (fansId != 0 && fansType == 0) {
            throw new RuntimeException("invalid fans type");
        }
        if (fansType != 0 && fansId == 0) {
            throw new RuntimeException("invalid fans id");
        }
        return new User(this);
    }

}
public class User {
    private final long buyerId;
    private final int fansType;
    private final long fansId;

    User(Builder builder) {
        buyerId = builder.getBuyerId();
        fansType = builder.getFansType();
        fansId = builder.getFansId();
    }
}

问题：不够内聚

内部类

public class User {
    private final long buyerId;
    private final int fansType;
    private final long fansId;

    private User(Builder builder) {
        buyerId = builder.buyerId;
        fansType = builder.fansType;
        fansId = builder.fansId;
    }

    public static class Builder {
        private long buyerId;
        private int fansType;
        private long fansId;

        public Builder buyerId(long buyerId) {
            this.buyerId = buyerId;
            return this;
        }

        public Builder fansType(int fansType) {
            this.fansType = fansType;
            return this;
        }

        public Builder fansId(long fansId) {
            this.fansId = fansId;
            return this;
        }

        public User build() {
            if (buyerId == 0 && fansType == 0 && fansId == 0) {
                throw new RuntimeException("please init user data");
            }
            if (fansId != 0 && fansType == 0) {
                throw new RuntimeException("invalid fans type");
            }
            if (fansType != 0 && fansId == 0) {
                throw new RuntimeException("invalid fans id");
            }
            return new User(this);
        }

    }
}

优点：灵活控制入参，便于后续扩展。
这就是设计模式中的builder模式。

使用构建器

public class Main {

    public static void main(String[] args) throws Exception {
        User buyer = new User.Builder().buyerId(1L).build();
        User fans = new User.Builder().fansType(1).fansId(1L).build();
    }
}

适应变化

这个时候，如果产品提出支持匿名用户的需求，我们就可以从容应对了：

public class User {
    private final long buyerId;
    private final int fansType;
    private final long fansId;
    private final String nobody;

    private User(Builder builder) {
        buyerId = builder.buyerId;
        fansType = builder.fansType;
        fansId = builder.fansId;
        nobody = builder.nobody;
    }

    public static class Builder {
        private long buyerId;
        private int fansType;
        private long fansId;
        private String nobody;

        public Builder buyerId(long buyerId) {
            this.buyerId = buyerId;
            return this;
        }

        public Builder fansType(int fansType) {
            this.fansType = fansType;
            return this;
        }

        public Builder fansId(long fansId) {
            this.fansId = fansId;
            return this;
        }

        public Builder nobody(String nobody) {
            this.nobody = nobody;
            return this;
        }

        public User build() {
            if (buyerId == 0 && fansType == 0 && fansId == 0 && nobody.equals("")) {
                throw new RuntimeException("please init user data");
            }
            if (fansId != 0 && fansType == 0) {
                throw new RuntimeException("invalid fans type");
            }
            if (fansType != 0 && fansId == 0) {
                throw new RuntimeException("invalid fans id");
            }
            return new User(this);
        }

    }
}

参考资料

《effective java》