"Customer" : {
"1537672911526" : {
  "email" : "user1@gmail.com",
  "firstName" : "user1",
  "lastName" : "user1",
  "password" : "user1",
  "username" : "user1"
},
"1537672951066" : {
  "email" : "user2@gmail.com",
  "firstName" : "user2",
  "lastName" : "user2",
  "password" : "user2",
  "username" : "user2"
}

//Init Firebase
    final FirebaseDatabase database = FirebaseDatabase.getInstance();
    final DatabaseReference table_user = database.getReference("Customer");

    btnLogIn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            final ProgressDialog mDialog = new ProgressDialog(Login.this);
            mDialog.setMessage("Please wait...");
            mDialog.show();

            table_user.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    //If user exists
                    if(dataSnapshot.child(username.getText().toString()).exists()){
                        //Get user information
                        mDialog.dismiss();
                        User user = dataSnapshot.child(username.getText().toString()).getValue(User.class);
                        if(user.getPassword().equals(user_pass.getText().toString())){
                            Intent showMenu = new Intent(Login.this, Home.class);
                            Common.currentUser = user;
                            startActivity(showMenu);
                            finish();
                        }
                        else{
                            Toast.makeText(Login.this, "Incorrect Email or Password", Toast.LENGTH_SHORT).show();
                        }
                    }
                    else{
                        mDialog.dismiss();
                        Toast.makeText(Login.this, "User does not exist", Toast.LENGTH_SHORT).show();
                    }
                }

                @Override
                public void onCancelled(@NonNull DatabaseError databaseError) {

                }
            });

        }
    });