空对象引用上的 Firebase 读取数据错误 onDataChange 方法

Question

我在从 firebase DB 读取数据时遇到错误，我正在尝试允许使用 MAC 地址登录：

错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

数据：

这是我的代码：

 DatabaseReference users = FirebaseDatabase.getInstance().getReference("users");
Query query = users.orderByChild("macAddress").equalTo(getMacAddr());
query.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        if(dataSnapshot.exists()) {
            User user = dataSnapshot.getValue(User.class);
            if(!(user.getEmail().equals(edtEmail.getText().toString()))){
                Snackbar.make(rootLayout, "Please Login With Phone You Registered With", Snackbar.LENGTH_SHORT).show();
                waitingdialog.dismiss();
                return;
            }
        }
        else{
            Snackbar.make(rootLayout, "Please Login With Phone You Registered With", Snackbar.LENGTH_SHORT).show();
            waitingdialog.dismiss();
            return;
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
    }
});

getMacAddr() 是一个以字符串形式返回设备mac地址的函数

上面代码中使用的User.class代码：

public class User {

private  String email,password,name,phone,macAddress;
private int isSecured;

public User(String email, String name, String password,String phone,String macAddress,int isSecured){

    this.email=email;
    this.name=name;
    this.password=password;
    this.phone=phone;
    this.macAddress=macAddress;
    this.isSecured=isSecured;
}
public User() {}

public int getIsSecured() {
    return isSecured;
}

public void setIsSecured(int isSecured) {
    this.isSecured = isSecured;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getMacAddress() {
    return macAddress;
}

public void setMacAddress(String macAddress) {
    this.macAddress = macAddress;
}

public String getPhone() {
    return phone;
}

public void setPhone(String phone) {
    this.phone = phone;
}
}

这是用于创建用户对象的 User.class

Answer 1

我刚刚发现错误，我改变了这一行：

Query query = users.orderByChild("macAddress").equalTo(getMacAddr());

到

Query query=users.child(auth.getCurrentUser().getUid()).orderByChild("email");

auth 是 FirebaseAuth 对象

Answer 2

假设问题确实在您提供并给出错误消息的代码中

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

我认为问题出在这一行：

Query query = users.orderByChild("macAddress").equalTo(getMacAddr());

更具体地说，users.orderByChild("macAddress") 似乎正在返回null。我将研究如何在 Firebase 数据库中定义 users 条目，并确保其结构正确。也许您可以尝试在问题行上放置一个断点并检查 users 对象以验证 users.orderByChild("macAddress") 没有返回 null 并根据需要对 Firebase 数据库进行更正。