如何从 POJO class 中检索列表

Question

I am unable to fetch the data from Firestore to Android. I am using the POJO (Java object class) method to get the values. How to proceed for this?

Pojo class:

public class Documents {
    String documentID;
    String documentName;
    String documentDate;
    String inspectorName;
    String marketLocation;

public Documents() {

}

public Documents(String documentID, String documentName, String documentDate, String inspectorName, String marketLocation) {
    this.documentID = documentID;
    this.documentName = documentName;
    this.documentDate = documentDate;
    this.inspectorName = inspectorName;
    this.marketLocation = marketLocation;
}

public String getDocumentID() {
    return documentID;
}

public String getDocumentName() {
    return documentName;
}

public String getDocumentDate() {
    return documentDate;
}

public String getInspectorName() {
    return inspectorName;
}

public String getMarketLocation() {
    return marketLocation;
}

}

MainActivity.java:

List<Documents> documentsList = new ArrayList<>();

Documents documents = documentsList.get(); // Error to retrieve all data

primaryLayout(documents.getDocumentID(), documents.getDocumentName(), documents.getDocumentDate(), documents.getInspectorName(), documents.getMarketLocation());

Answer 1

This is a convention mistake as much as it is a blunder. You should name your POJO classes singular. That is, instead of naming it Documents you should have named it Document so that List<Document> is documentsList and Document document = documentsList.get(<index>) makes sense.

Line Documents documents = documentsList.get(); is wrong since you are trying to get one Document from the documentsList by calling get() method but you are not passing an index value to the get() method. Also the documentsList variable is initialized just above this line which means, the list is most probably empty.

What I suggest you do is:

1. First off Refactor> Rename your POJO class from Documents to Document .
2. Second, after initializing the list of documents, add the instances of Document to the list by calling: documentsList.add(new Document("documentID","documentName","documentDate","inspectorName","marketLocation")); as many times as you have the Document s from your firebase by using an iterator like foreach()

Answer 2

you have only initialized the ArrayList....Not it's objects....You have to initialize each of it's object and then you can access it by calling documentList.get(position).. However this will not solve your problem of getting data from Firebase FireStore.. You can follow this link adding fireStore to android

Answer 3

More code from MainActivity class

MainActivity.java:

    List<Documents> documentsList = new ArrayList<>();

    Documents documents = documentsList.get(); // Error to retrieve all data

    primaryLayout(documents.getDocumentID(), documents.getDocumentName(), documents.getDocumentDate(), documents.getInspectorName(), documents.getMarketLocation());

public void primaryLayout(String documentID, String documentName, String documentDate, String inspectorName, String marketLocation) {
    progressBar = findViewById(R.id.progressLoader);
    progressBar.setVisibility(View.VISIBLE);

    Documents documents = new Documents(documentID, documentName, documentDate, inspectorName, marketLocation);

    collectionReference.document(documents.getDocumentID()).collection("Products").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if(task.isSuccessful()) {

                itemsList.clear();

                for(DocumentSnapshot documentSnapshot : task.getResult()) {
                    Items items = documentSnapshot.toObject(Items.class);
                    itemsList.add(items);

                    productListAdapter.notifyDataSetChanged();

                    progressBar.setVisibility(View.GONE);
                }
            }
        }
    });
}