无法将 java.lang.String 类型的对象转换为数据类

Question

我试图从 firebase 获取数据。我尝试了 2 种方法，第一种方法给了我无法将 java.lang.String 类型的对象转换为数据类错误。尽管数据快照具有值，但第二个返回 null。 第一种方法：

database.getReference("Customers").child(custID).get().addOnSuccessListener {
            it.children.forEach() {
                var cust = it.getValue(DataClassCustomer::class.java)//Error
                custName = cust?.firstName + " " + cust?.lastName
            }
        }

错误：

2021-03-12 00:25:39.634 28929-28929/com.example.goldenbeachhotelmanagementsystem E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.goldenbeachhotelmanagementsystem, PID: 28929
    com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.goldenbeachhoteldataclasses.DataClassCustomer
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(CustomClassMapper.java:436)
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(CustomClassMapper.java:232)
        at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(CustomClassMapper.java:80)
        at com.google.firebase.database.DataSnapshot.getValue(DataSnapshot.java:203)
        at com.example.goldenbeachhotelmanagementsystem.RoomManagement$readCustData$1.onSuccess(RoomManagement.kt:243)
        at com.example.goldenbeachhotelmanagementsystem.RoomManagement$readCustData$1.onSuccess(RoomManagement.kt:12)
        at com.google.android.gms.tasks.zzn.run(Unknown Source:4)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

这是数据类：

data class DataClassCustomer(
    var email: String? = "", var firstName:String? = "", var ic:String? = "", var lastName:String? ="", var phone:String?= ""){

}

第二种方法： 我尝试不使用循环，但 cust 值为 null

database.getReference("Customers").child(custID).get().addOnSuccessListener {
                var cust = it.getValue(DataClassCustomer::class.java)
                custName = cust?.firstName + " " + cust?.lastName
                println(custName)
        }

调试输出： enter image description here

Firebase 客户： enter image description here

Answer 1

您收到以下错误：

com.google.firebase.database.DatabaseException：无法将 java.lang.String 类型的对象转换为 com.example.goldenbeachhoteldataclasses.DataClassCustomer 类型

因为您试图将 String 对象转换为 DataClassCustomer 对象，这在 Java 中实际上是不可能的。发生这种情况是因为您使用的是以下参考：

database.getReference("Customers").child(custID)

指向 C0001 节点。使用 forEach() 循环时，您将获得该对象中的所有子对象，这些子对象实际上是字符串和 not "DataClassCustomer" 对象，因此会出现该错误。如果您想获取“客户”节点中存在的所有“DataClassCustomer”对象，只需删除对.child(custID) 的调用，您的代码就可以正常工作。如果要获取“email”、“firstName”、“ic”、“lastName”和“phone”的值，则替换以下行：

var cust = it.getValue(DataClassCustomer::class.java)

与：

var cust = it.getValue(String::class.java)

Answer 2

尝试使用Gson 库来解码客户值：

database.getReference("Customers").child(custID).get().addOnSuccessListener {
    it.children.forEachIndexed() { index, element ->
        try {
            val dataClassCustomerType = object : TypeToken<DataClassCustomer>() {}.type
            val customerJson = element.value.getJSONObject(custID)
            val cust = Gson().fromJson<DataClassCustomer>(customerJson, dataClassCustomerType)
            val custData = cust.firstName + " " + cust.lastName
        } catch (e: Exception) {
            e.printStackTrace()
        }
    }
}