private void getClients() {
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("client");
    myRef.addValueEventListener(valueEventListener);
}

ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot){
        readClients(dataSnapshot);
    }
    @Override
    public void onCancelled(DatabaseError error) {}
};

private void readClients(DataSnapshot dataSnapshot) {
    for(DataSnapshot dss: dataSnapshot.getChildren()){
        Client client = dss.getValue(Client.class);
        listClients.add(client);
    }
}