Reading data

Understanding which data referenced by getReference()

In this example, we use this database:

"your-project-name" : {
    "users" : {
        "randomUserId1" : {
            "display-name" : "John Doe",
            "gender" : "male"
        }
        "randomUserId2" : {
            "display-name" : "Jane Dae",
            "gender" : "female"
        }
    },
    "books" {
        "bookId1" : {
            "title" : "Adventure of Someone"
        },
        "bookId1" : {
            "title" : "Harry Potter"
        },
        "bookId1" : {
            "title" : "Game of Throne"
        }
    }
}

If you use above database then:

FirebaseDatabase.getInstance().getReference()

will point at your project’s parent, "your-project-name" data. So the dataSnapshot you acquired will contain all of data inside it, including all of "users" data and "books" data.
FirebaseDatabase.getInstance().getReference("users") and FirebaseDatabase.getInstance().getReference().child("users")

will have the same result, pointing at "your-project-name/users"
FirebaseDatabase.getInstance().getReference("users/randomUserId1") and FirebaseDatabase.getInstance().getReference().child("users/randomUserId1") and FirebaseDatabase.getInstance().getReference().child("users").child("randomUserId1")

will have the same result, pointing at "your-project-name/users/randomUserId1"

Note: this example is needed to fully understand which data is inside dataSnapshot object

Understanding which data is inside dataSnapshot object

Note: You need to know which data referenced by getReference() first before you can completely understand this example.

There are three common method to get your data from Firebase Realtime Database:

addValueEventListener()
addListenerForSingleValueEvent()
addChildEventListener()

When we talk about which data is inside dataSnapshot object, then addValueEventListener() and addListenerForSingleValueEvent() is basically the same. The only difference is addValueEventListener() keep listen to changes made in the referenced data while addListenerForSingleValueEvent() is not.

So consider we have this database:

"your-project-name" : {
    "users" : {
        "randomUserId1" : {
            "display-name" : "John Doe",
            "gender" : "male"
        }
        "randomUserId2" : {
            "display-name" : "Jane Dae",
            "gender" : "female"
        }
    },
    "books" {
        "bookId1" : {
            "title" : "Adventure of Someone"
        },
        "bookId1" : {
            "title" : "Harry Potter"
        },
        "bookId1" : {
            "title" : "Game of Throne"
        }
    }
}

DataSnapshot produced by addValueEventListener and addListenerForSingleValueEvent

dataSnapshot produced by addValueEventListener() and addListenerForSingleValueEvent() will contain value(s) of the exact data it is referenced into. Like when ref is point to "your-project-name" then dataSnapshot should be :

... onDataChange(DataSnapshot dataSnapshot) {
    dataSnapshot.getKey(); // will have value of String: "your-project-name"
    for (DataSnapshot snapshot : dataSnapshot) {
        snapshot.getKey(); // will have value of String: "users", then "books"
        for (DataSnapshot deeperSnapshot : dataSnapshot) {
            snapshot.getKey();
            // if snapshot.getKey() is "users", this will have value of String: "randomUserId1", then "randomUserId2"
            // If snapshot.getKey() is "books", this will have value of String: "bookId1", then "bookId2"
        }
    }
}

DataSnapshot produced by addChildEventListener

dataSnapshot produced by addChildEventListener() will contain value(s) of data one level deeper inside the data it is referenced into. Like in these cases:

When ref is point to "your-project-name" then dataSnapshot should be :

... onChildAdded(DataSnapshot dataSnapshot, String s) {
    dataSnapshot.getKey(); // will have value of String: "users", then "books"
    for (DataSnapshot snapshot : dataSnapshot) {
        snapshot.getKey();
        // if dataSnapshot.getKey() is "users", this will have value of String: "randomUserId1", then "randomUserId2"
        // If dataSnapshot.getKey() is "books", this will have value of String: "bookId1", then "bookId2"
        for (DataSnapshot deeperSnapshot : dataSnapshot) {
            snapshot.getKey();
            // if snapshot.getKey() is "randomUserId1" or "randomUserId1", this will have value of String: "display-name", then "gender"
            // But the value will be different based on key
            // If snapshot.getKey() is "books", this will have value of String: "title", but the value will be different based on key
        }
    }
}
// dataSnapshot inside onChildChanged, onChildMoved, and onChildRemoved will have the same data as onChildAdded

I know most likely we will want to use .getValue() instead of getKey(). But in here we use getKey because it will always contain one String and no need to convert into custom object, or Map, or other. Basically, when you know which key dataSnapshot is pointing into, you can easily know which value it contains and parse it into your own custom obeject (or anything)