Variable not assigned even though I assigned it (Android Java)

There are currently two different thread executing in your Code. The first was your main Thread that executes from

String walletname = w.getName();
String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
final String[] WalletKey = new String[1];
return WalletKey[0];

and the asynchronous thread that fetching the Data from the firebase. By the time you returning the WalletKey[0] the value is still null, because the asynchronous process not finished yet.

I think the best way to overcome this is to change the getAWalletKey function to alter the wallet. Like this.

public void updateWalletString(){
     User user = (User)Homeactivity.getUser()
     wallet = user.getWallet()
     updateAWalletKey(wallet);
}

private void getAWalletKey(Wallet w) {
    String walletname = w.getName();
    String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
    final String[] WalletKey = new String[1];
    databaseReference.child("Users").child(uid).child("wallets").orderByChild("name")
            .equalTo(walletname).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot childSnapshot : dataSnapshot.getChildren()) {
                WalletKey[0] = childSnapshot.getKey();
                Log.v(TAG, walletKey);
                Log.d(TAG, "getWalletkey: "+ WalletKey[0]);
                doTheWallet(childSnapshot.getKey()); // whatever you want to do with the wallet.
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            Toast.makeText(HomeActivity.this, "Error retrieving walletId from database", Toast.LENGTH_SHORT);
        }
    });
    return WalletKey[0];
}