@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_login);

    mAuth = FirebaseAuth.getInstance();
    username = (EditText) (findViewById(R.id.username));
    password = (EditText) (findViewById(R.id.password));
    signIn = (Button) (findViewById(R.id.signIn));
    register = (Button) (findViewById(R.id.register));
}

private void registerUser()
{
    loginUsernameString = username.getText().toString();
    loginPasswordString = password.getText().toString();

    mAuth.createUserWithEmailAndPassword(loginUsernameString, loginPasswordString)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    Log.d(TAG, "createUserWithEmail:onComplete:" + task.isSuccessful());

                    // If sign in fails, display a message to the user. If sign in succeeds
                    // the auth state listener will be notified and logic to handle the
                    // signed in user can be handled in the listener.
                    if (!task.isSuccessful()) {
                        Toast.makeText(LoginActivity.this, "Failed",
                                Toast.LENGTH_SHORT).show();
                    }

                    // ...
                }
            });
}
public void onClick(View view)
{
    if (view == signIn)
        registerUser();
}