访问Firebase实时数据库值

Question

I'm making app for my developers team and I want to add app version editor for all logged in users with firebase realtime database, I use code

appIdEndPoint = mDatabase.child("users").child(user.getUid()).child("app").child("id");
        appVersionEndPoint = mDatabase.child("users").child(user.getUid()).child("app").child("latest_version");
        uid_text.setText("User firebase ID: " + user.getUid());

        appIdEndPoint.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                appDetails address = dataSnapshot.getValue(appDetails.class);
                if (address.getID().equals("")){
                    appIdEndPoint.setValue(getRandom(1000, 9999));
                }
                appid_text.setText("App ID: " + address.getID());
                appid_edittext.setText(address.getID());
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.d("AppID", databaseError.getMessage());
                Toast.makeText(AppDetailsEditor.this, "Failed to fetch data from database", Toast.LENGTH_LONG);
                finish();
            }
        });

        appVersionEndPoint.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                appDetails address = dataSnapshot.getValue(appDetails.class);
                if (address.getVersion().equals("")){
                    appVersionEndPoint.setValue("Waiting for developers input");
                }
                appversion_text.setText("App version: " + address.getVersion());
                appversion_edittext.setText(address.getVersion());
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.d("AppVersion", databaseError.getMessage());
                Toast.makeText(AppDetailsEditor.this, "Failed to fetch data from database", Toast.LENGTH_LONG);
                finish();
            }
        });

I tried to use "String appID = dataSnapshot.getValue().toString();" but it produced java.lang.NullPointerException error. Please can someone edit my code to propertly get database values? Here is "appDetails.java" class:

public class appDetails {

public String ID = "";
public String version = "";

public appDetails(){

}

public appDetails(String ID, String version){
    this.ID = ID;
    this.version = version;
}

public String getID (){
    return ID;
}
public String getVersion (){
    return version;
}

}

EDIT:

Code showed above too produce java.lang.NullPointerException error

Answer 1

If your data is not saved in your Firbease database, then you need to change the rules like this:

{
  "rules": {
    ".read": true,
    ".write": true
  }
}

But change your rules to this just temporarily, please don't leave it that way.

Answer 2

Your code is not null safe. First of all address can be null. Second the getID can be null. Anyway this is better:

if (address.getID() == null || "".equals(address.getID())){
    appIdEndPoint.setValue(getRandom(1000, 9999));
}

Answer 3

Since your database is currently empty, the ValueListener.onDataChange() will fire with an empty snapshot. Your code needs to check for this:

appIdEndPoint.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot dataSnapshot) {
    if (dataSnapshot.exists()) {
      appDetails address = dataSnapshot.getValue(appDetails.class);
      ...

You'll need such a check in any ValueEventListener that listens for data that might not exist.