EditText editTextSignUpUserName = (EditText) findViewById(R.id.sign_up_user_name);
EditText editTextSignUpPassword = (EditText) findViewById(R.id.sign_up_password);
EditText editTextSignUpEmailAddress = (EditText) findViewById(R.id.sign_up_email_address);

String messageUserName = intent.getStringExtra(SignUp.EXTRA_MESSAGE);
 String messagePassword = intent.getStringExtra(SignUp.EXTRA_MESSAGE);
 String messageEmailAddress = intent.getStringExtra(SignUp.EXTRA_MESSAGE);

Intent i = new Intent(getApplicationContext(), MainActivity.class);
i.putExtra("password",editTextSignUpPassword.getText().toString());
i.putExtra("username",editTextSignUpUserName.getText().toString());
startActivity(i);

Bundle extras = getIntent().getExtras();
if (extras != null) {
    String messageUserName = extras.getString("username");
    String messagePassword = extras.getString("password");
}

EditText editTextSignUpUserName;
  EditText editTextSignUpPassword;
  @Override
   public void onCreate(Bundle savedInstanceState)
   { super.onCreate(savedInstanceState);
  setContentView(R.layout.sign_up);
  editTextSignUpUserName = (EditText) findViewById(R.id.sign_up_user_name);
  editTextSignUpPassword = (EditText)findViewById(R.id.sign_up_password);
   }

   public void onClickSignUp(View view)
  { Intent intent = new Intent(this, SignIn.class);
  String signUpUserName = editTextSignUpUserName.getText().toString();
  String signUpPassword = editTextSignUpPassword.getText().toString();
  intent.putExtra("username", signUpUserName);
  intent.putExtra("password", signUpPassword);
  startActivity(intent);
  }

Bundle extras = getIntent().getExtras();
if (extras != null) {
    String messageUserName = extras.getString("username");
    String messagePassword = extras.getString("password");
}

String username = sign_up_user_name.getText().toString();
String password = editTextSignUpPassword.getText().toString();
String email = editTextSignUpEmailAddress.getText().toString();
Intent i = new Intent(getApplicationContext(), NextActivity.class);
i.putExtra("username",username);
i.putExtra("password",password);
i.putExtra("email",email);
this.getContext.startActivity(i);

String username = this.getIntent.getStringExtra("username");
String password = this.getIntent.getStringExtra("password");
String email = this.getIntent.getStringExtra("email");