How can I read values from Firebase Realtime Database by using a Java method in AndroidStudio?

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My Firebase Realtime Database has been built by loading an object of the Java class HashMap. In my Android Studio app I'm trying to write a method that takes a String (the key) as input, searches through the database and if the string is found it returns the associated Float (the value), otherwise it returns 0. How can I do this? Any help would be really appreciated!

EDIT: I've tried to follow the suggestions, adapting them to my particular case, but I didn't manage to solve the problem yet.
I wrote the following code in MainActivity: 

DatabaseReference myRef;
Float tempValue;

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    myRef = FirebaseDatabase.getInstance().getReference("myRoot");
    tempValue=0f;
    ...
}

public void retrieveValueFromDatabase(String childName, final MainActivity activity){
    myRef.child(childName).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            Float value=dataSnapshot.getValue(Float.class);
            if (value==null){
                value=0f;
            }
            activity.tempValue=value;
            //First Toast
            //Toast.makeText(activity,"tempValue = "+tempValue.toString(), Toast.LENGTH_LONG).show();
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            throw databaseError.toException();
        }
    });
}

public void useValues(){
    retrieveValueFromDatabase(childName,this);
    //Second Toast
    //Toast.makeText(this,"tempValue = "+tempValue.toString(), Toast.LENGTH_LONG).show();
    //code using tempValue from here
    ...
}

If I uncomment the first toast, the correct value inside tempValue is shown, but if I uncomment the second toast, the value of tempValue shown is the default one (0.0). What am I missing?

firebase-realtime-database firebase hashmap java
1 Answers

You need to use addValueEventListener to retrieve data from the database:

DatabaseReference ref = FirebaseDatabase.getInstance().getReference("myRoot").orderByChild("name").equalTo("peter");
ref.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        Log.i("Database", dataSnapshot.child("floatValue").getValue(Long.class));
    }

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

Here, you add a reference to the root node, then query using equalTo() to check if name = peter exists in the database and return the float value.

You should read the guide:

https://firebase.google.com/docs/database/android/read-and-write

user_70154000
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