Java stringBuilder值被清除

我有两个stringBuilder值，电子邮件和密码在某些地方被清除或删除，但如果其中一个是正确的，我想保留它。我认为我的逻辑是正确的，但是在if语句之后，值一直告诉我它是空的。。。else ifemail.toString.equalslogin\u test\u name&&password.toString.isEmpty ||password.toString.equalslogin\u test\u name&&email.toString.isEmpty

public class MainActivity extends ActionBarActivity {

    //variables
    String login_test_name = "t";
    String login_test_pass = "t";
    EditText username;
    EditText pass;
    private StringBuilder email = new StringBuilder();
    private StringBuilder password = new StringBuilder();
    //Remember password variables

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE); //Remove action bar from screen
        setContentView(R.layout.activity_main);
        //get EditText field data by id name
        username = (EditText)findViewById(R.id.emailTxt);
        pass = (EditText)findViewById(R.id.passTxt);

    }//end oncreate

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    //code to change activity with BTN click (to register user)
    public void registerUser(View v)
    {
        Intent i = new Intent(MainActivity.this, UserRegister.class );
        startActivity(i);
    }

    // code to change activity (to order menu if username and pass are ture)
    public void orderMenu(View v)
    {
        // change EditText type to String type
        //String u = username.getText().toString(); 
        //String p = pass.getText().toString();
        email.append(username.getText().toString());
        password.append(pass.getText().toString());
        //test login username and password
        if (!email.toString().equals(login_test_name) || !password.toString().equals(login_test_pass) ||
                email.toString().isEmpty() || password.toString().isEmpty())
        {
            if(!email.toString().equals(login_test_name) && !password.toString().equals(login_test_pass) ||
                    email.toString().isEmpty() && password.toString().isEmpty())
            {
                //if fields are not empty but are not the same
                if(!email.toString().equals(login_test_name) && !password.toString().equals(login_test_pass) &&
                        !email.toString().isEmpty() && !password.toString().isEmpty())
                {
                        Toast.makeText(this, "Invalid Email and Password", Toast.LENGTH_LONG).show();
                        //clear text fields
                        username.setText("");
                        pass.setText("");
                        email.delete(0, email.length());
                        password.delete(0, password.length());
                }
                //else the fields are empty
                else 
                {
                    Toast.makeText(this, "Email and Password fields are empty", Toast.LENGTH_LONG).show();
                }
            }
            //if one field is NOT correct but the other is and the field is NOT empty
            else if(!email.toString().equals(login_test_name) && password.toString().equals(login_test_name) && !email.toString().isEmpty() 
                    || !password.toString().equals(login_test_pass) && email.toString().equals(login_test_name) && !password.toString().isEmpty())
            {
                    if(!email.toString().equals(login_test_name) && password.toString().equals(login_test_name) &&
                            !email.toString().isEmpty())
                    {
                        Toast.makeText(this, "Invalid Email but \nPassword is correct", Toast.LENGTH_LONG).show();
                        email.delete(0, email.length());
                        username.setText("");
                    }
                    else
                    {
                        Toast.makeText(this, "Invalid Password but \nEmail is correct", Toast.LENGTH_LONG).show();
                        password.delete(0, password.length());
                        pass.setText("");
                    }
            }
            //if one fields is correct but the other is EMPTY
            else if(email.toString().equals(login_test_name) && password.toString().isEmpty() 
                    || password.toString().equals(login_test_name) && email.toString().isEmpty())
            {
                if(email.toString().equals(login_test_name) && password.toString().isEmpty())
                {
                    Toast.makeText(this, "Email is correct but \nPassword field is empty", 
                            Toast.LENGTH_LONG).show();
                }
                else
                {
                    Toast.makeText(this, "Password is correct but \nEmail field is empty",
                            Toast.LENGTH_LONG).show();
                }
            }
            // Add if statement for if text is missing in fields
            /*StringBuilder errorMessage = new StringBuilder();
            errorMessage.append("wrong username or password\n");
            errorMessage.append("username: " + u + "\n");
            errorMessage.append("pass: " + p + "\n");
            errorMessage.append(login_test_name + "\n");
            errorMessage.append(login_test_pass + "\n");

            Toast.makeText(this, errorMessage.toString(), Toast.LENGTH_LONG).show();*/
        }//end if for invalid and empty text fields
        //else credentials gain access
        else if (email.toString().equals(login_test_name) &&    password.toString().equals(login_test_pass))
        {
                // change to /order menu activity
                Intent i = new Intent(MainActivity.this, OrderMenu.class );
                startActivity(i);
                // message to user
                Toast.makeText(this, "logged in", Toast.LENGTH_SHORT).show();
                email.delete(0, email.length());
                password.delete(0, password.length());
                username.setText("");
                pass.setText("");
        }//end else
    }//end order menu method

    //cancel app button
    public void cancelApp(View v)
    {
        finish();
        username.setText("");
        pass.setText("");
        System.exit(0);
    }//end cancel app method

    //change activity to forgot password 
    public void forgotPassword(View v)
    {
         Intent i = new Intent(MainActivity.this, ForgotPassword.class );
         startActivity(i);

    }//end forgotPassword method

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    }//end main activity class

---

你可以使它简单得多

if (!email.toString().equals(login_test_name)) {
    Toast.makeText(this, "Invalid Email", Toast.LENGTH_LONG).show();
    email.setLength(0);
}

if (!password.toString().equals(login_test_pass)) {
    Toast.makeText(this, "Invalid Password", Toast.LENGTH_LONG).show();
    password.setLength(0);
}
if (email.length() > 0 && password.length > 0) {
// start your activity
}

---

为什么需要StringBuilder？只要使用字符串，我只是觉得编程更好。我在string中使用了它，它就像一个charmI一样工作。我认为您应该仔细查看您的许多if-else语句，因为它们没有多大意义。如果您要重复向现有字符串添加字符串以生成新字符串，StringBuilder/StringBuffer非常有用。