MassStrike MassStrike - 1 month ago 17
Java Question

Best way to retrieve/format data using Firebase Java API

I'm using

Firebase
for data storage on an
Android
project, and using the
Firebase Java API
to deal with data. I'm not sure I'm doing it as efficiently as possible, though, and I'd like some advice on best practices for retrieving and formatting data. My
Firebase
repository looks something like this....

-POLLS
NUMPOLLS - 5
(pollskey) - NAME - Poll1
NUMELECTIONS - 2
ELECTIONS
(electionskey) - NAME - Election1
NUMNOMINATIONS - 2
NUMVOTERS - 2
NUMBERTOELECT - 1
VOTERS - (votesrkey) - NAME - Charles
NUMBER - (678) 333-4444
.
.
.
(voterskey) - ...
NOMINATIONS - (nominationskey) - NAME - Richard Nixon
NUMBEROFVOTES - 2
.
.
.
(nominationskey) - ...
.
.
.
(electionskey) - ...
.
.
.
(pollskey) - ...


So, for example here I'm trying to get all data out of a poll to list poll name, it's election names, and the candidate names and number of votes for each election. I get the POLLS level
DataSnapshot
during the
OnCreate()
function of my main activity like this...

private static final Firebase polls = pollsFirebase.child("Polls");
protected void onCreate(Bundle savedInstanceState) {

polls.addListenerForSingleValueEvent(new ValueEventListener() {

@Override
public void onDataChange(DataSnapshot snapshot) {
for (DataSnapshot child : snapshot.getChildren()) {
if (!child.getName().equals("NumPolls")) {
createPollTableAndHeaders(child);
}
}

}
});
}


Then I proceed to read out the individual pieces of data I need by successively calling
getValue()
on
DataSnapshots
, and checking the keys of the resulting
HashMaps
...

private void createPollTableAndHeaders(DataSnapshot poll) {
String pollName = "";
int numPolls;
Object p = poll.getValue();
if (p instanceof HashMap) {
HashMap pollHash = (HashMap) p;
if (pollHash.containsKey("Name")) {
pollName = (String) pollHash.get("Name");
}
if (pollHash.containsKey("Elections")) {
HashMap election = (HashMap) pollHash.get("Elections");
Iterator electionIterator = election.values().iterator();
while (electionIterator.hasNext()) {
Object electionObj = electionIterator.next();
if (electionObj instanceof HashMap) {
HashMap electionHash = (HashMap) electionObj;
if (electionHash.containsKey("Name")) {
String electionName = (String) electionHash.get("Name");
}
}
};
}
}


This seems like a pretty tedious way to drill down through the data structure, and I'm wondering if there's a better way.

I've seen the
getValue(java.lang.Class<T> valueType)
method in the documentation, but haven't been able to get it to work in my case, since I'm working with composed objects and not just containers for primitive types. How does the function know what
Firebase
data to assign to which member variables of a model object? Does it match
Firebase
key names with member variables, and therefore do these have to be exactly the same, with case sensitivity? How would that deal with
Firebase
generated key names like produced when pushing to a
List
? How to you construct model objects for composed objects?

Answer

The getValue(java.lang.Class valueType) method follows the same rules as the jackson object mapping library (it's what we use internally: http://wiki.fasterxml.com/JacksonInFiveMinutes). So, your Java classes must have default constructors (no arguments) and getters for the properties that you want assigned (https://www.firebase.com/docs/java-api/javadoc/com/firebase/client/DataSnapshot.html#getValue(java.lang.Class)). In short, yes, the key names in Firebase must match the member variables.

For an example using composite objects, including a list, see AndroidDrawing. Specifically, the Segment class contains a list of Point instances. There is one catch using lists of data generated with the .push() method. Since the key names generated are Strings so that they can be unique across clients, they deserialize the Maps rather than Lists. However, if you iterate over dataSnapshot.getChildren() they will be returned in order.

In addition, if you don't want to deserialize into a HashMap, you can use the child() method on DataSnapshot. For instance:


String pollName = poll.child("Name").getValue(String.class); 
DataSnapshot elections = poll.child("Elections");
for (DataSnapshot election : elections.getChildren()) {
    String electionName = election.child("Name").getValue(String.class);
}

In this example, any values that don't exist will be returned as null.

Hope that helps!

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