Android Firebase获取子节点值问题：Receipt对象返回null

Hey there! Let's walk through why your Receipt instance is coming up null when fetching data, and how to fix it. These are the most common culprits:

1. Mismatched Field Names (Most Likely) #

Firebase relies on exact name matching when serializing/deserializing objects. If your Receipt class has fields named differently than the keys in your database (e.g., orderId in the DB vs order_id in your class), the mapping will fail silently, resulting in a null object.

Fix:
Either rename your class fields to match the database keys exactly, or use the @PropertyName annotation to explicitly link them:

import com.google.firebase.database.PropertyName;

public class Receipt {
    @PropertyName("order_id") // Match the key in your database
    private String orderId;

    // Don't forget to annotate getters/setters too
    @PropertyName("order_id")
    public String getOrderId() {
        return orderId;
    }

    @PropertyName("order_id")
    public void setOrderId(String orderId) {
        this.orderId = orderId;
    }
}

2. Missing No-Argument Constructor #

Firebase requires a public no-argument constructor to instantiate your Receipt class. If you only have constructors with parameters, Firebase can't create an instance of the class, leading to null.

Fix:
Add an empty constructor to your Receipt class:

public class Receipt {
    // Required for Firebase deserialization
    public Receipt() {}

    // Your existing parameterized constructors, fields, etc.
}

3. Inaccessible Fields or Missing Getters/Setters #

Firebase needs to access your class's fields to populate them. If fields are private and you don't have corresponding getter/setter methods, Firebase can't assign values to them.

Fix:
Either make your fields public, or add standard getter and setter methods for every field you want to map from the database.

4. Data Type Mismatch #

If a field in your Receipt class has a different data type than the corresponding value in Firebase (e.g., a String in your class vs an integer in the DB), the mapping will fail.

Fix:
Double-check the data types of each key in your database and ensure they match the fields in your Receipt class exactly.

5. Incorrect Database Path #

Double-confirm that the path shop/orders is pointing directly to the parent node of your red-region data. If the red region is nested deeper (e.g., shop/orders/{someId}/receipt), your current code isn't targeting the right node.

Debug Tip:
Add a print statement to see the raw data you're fetching—this will help you confirm if the issue is with the path or the object mapping:

@Override
public void onDataChange(DataSnapshot dataSnapshot) {
    if (dataSnapshot.getValue() != null) {
        for (DataSnapshot temp : dataSnapshot.getChildren()) {
            // Print raw data to verify
            System.out.println("Fetched raw data: " + temp.getValue().toString());
            Receipt temp2 = temp.getValue(Receipt.class);
        }
    }
}

内容的提问来源于stack exchange，提问作者Hoàng Thái