Firebase结合RecyclerView使用时出现无参构造函数缺失错误

从你给出的错误日志可以直接定位到问题核心：MyItem$Sellm这个内部类没有定义无参构造函数，而Firebase Realtime Database在将数据库快照转换为Java对象时，必须依赖无参构造函数来通过反射创建实例。

问题原因 #

Firebase的序列化/反序列化机制工作流程是这样的：

1. 先调用类的无参构造函数创建对象实例
2. 再通过Setter方法或者直接访问字段（如果字段是public的）来填充数据
   如果你的类只定义了带参构造函数，没有显式添加无参构造，Firebase就无法完成对象实例化，从而抛出DatabaseException。

解决方案 #

你需要给Sellm内部类添加无参构造函数，同时为了避免后续出现同样的问题，也要给Buy内部类和MyItem主类都补上无参构造，并且确保所有字段都有对应的Getter/Setter方法（或者将字段设为public）。

修正后的MyItem类示例

public class MyItem {
    // 主类字段
    private Buy buy;
    private Sellm sell;
    private String title;

    // 主类无参构造（必须添加）
    public MyItem() {}

    // 主类带参构造（可选，根据你的业务需求）
    public MyItem(Buy buy, Sellm sell, String title) {
        this.buy = buy;
        this.sell = sell;
        this.title = title;
    }

    // 主类Getter/Setter
    public Buy getBuy() { return buy; }
    public void setBuy(Buy buy) { this.buy = buy; }

    public Sellm getSell() { return sell; }
    public void setSell(Sellm sell) { this.sell = sell; }

    public String getTitle() { return title; }
    public void setTitle(String title) { this.title = title; }

    // ------------------------------
    // Buy内部类
    public static class Buy {
        private long buyBellow;
        private String date;
        private long stoploss;
        private long target1;
        private long target2;

        // Buy类无参构造
        public Buy() {}

        // Buy类带参构造
        public Buy(long buyBellow, String date, long stoploss, long target1, long target2) {
            this.buyBellow = buyBellow;
            this.date = date;
            this.stoploss = stoploss;
            this.target1 = target1;
            this.target2 = target2;
        }

        // Buy类Getter/Setter
        public long getBuyBellow() { return buyBellow; }
        public void setBuyBellow(long buyBellow) { this.buyBellow = buyBellow; }

        public String getDate() { return date; }
        public void setDate(String date) { this.date = date; }

        public long getStoploss() { return stoploss; }
        public void setStoploss(long stoploss) { this.stoploss = stoploss; }

        public long getTarget1() { return target1; }
        public void setTarget1(long target1) { this.target1 = target1; }

        public long getTarget2() { return target2; }
        public void setTarget2(long target2) { this.target2 = target2; }
    }

    // ------------------------------
    // Sellm内部类
    public static class Sellm {
        private String date;
        private long sellAbove;
        private long stoploss;
        private long target1;
        private long target2;

        // Sellm类无参构造（关键！之前缺失的就是这个）
        public Sellm() {}

        // Sellm类带参构造
        public Sellm(String date, long sellAbove, long stoploss, long target1, long target2) {
            this.date = date;
            this.sellAbove = sellAbove;
            this.stoploss = stoploss;
            this.target1 = target1;
            this.target2 = target2;
        }

        // Sellm类Getter/Setter
        public String getDate() { return date; }
        public void setDate(String date) { this.date = date; }

        public long getSellAbove() { return sellAbove; }
        public void setSellAbove(long sellAbove) { this.sellAbove = sellAbove; }

        public long getStoploss() { return stoploss; }
        public void setStoploss(long stoploss) { this.stoploss = stoploss; }

        public long getTarget1() { return target1; }
        public void setTarget1(long target1) { this.target1 = target1; }

        public long getTarget2() { return target2; }
        public void setTarget2(long target2) { this.target2 = target2; }
    }
}

额外注意：ProGuard配置 #

如果你开启了ProGuard代码混淆，一定要在proguard-rules.pro中添加规则，防止Firebase需要的类、构造函数和字段被混淆移除：

# 保留MyItem及其内部类的所有成员
-keepclassmembers class com.google.example.adinaranayaragh.MyItem {
    *;
}
-keepclassmembers class com.google.example.adinaranayaragh.MyItem$Buy {
    *;
}
-keepclassmembers class com.google.example.adinaranayaragh.MyItem$Sellm {
    *;
}

总结 #

补上缺失的无参构造函数后，Firebase就能正常将数据库中的items数据转换为MyItem对象，你的RecyclerView也就能正确加载并展示这些数据了。

内容的提问来源于stack exchange，提问作者mihalca bogdan