if-语句中的警报对话框不显示()

Question

我有以下代码:

   public void button_login(View view) {
    // Instantiate an AlertDialog.Builder with its constructor
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setPositiveButton(R.string.ok, new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int id) { /* User clicked OK button */ }
    });
    // Preserve EditText values.
    EditText ET_username = (EditText) findViewById(R.id.username);
    EditText ET_password = (EditText) findViewById(R.id.password);
    String str_username = ET_username.toString();
    String str_password = ET_password.toString();
    // Intercept missing username and password.
    if(str_username.length() == 0) {
        builder.setMessage(R.string.hint_username_empty);
        AlertDialog dialog = builder.create();
        dialog.show();
    }
    }

我有一个活动，其中包括两个EditText-Views和一个按钮。当我点击按钮时，所示的方法将被调用。

我的问题:AlertDialog没有显示!

当我像这样把create and show放在开头时:

 // Instantiate an AlertDialog.Builder with its constructor
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setPositiveButton(R.string.ok, new DialogInterface.OnClickListener() {
        public void onClick(DialogInterface dialog, int id) { /* User clicked OK button */ }
    });
    builder.setMessage(R.string.hint_username_empty);
    AlertDialog dialog = builder.create();
    dialog.show();
    // Preserve EditText values.
    EditText ET_username = (EditText) findViewById(R.id.username);
    EditText ET_password = (EditText) findViewById(R.id.password);
    String str_username = ET_username.toString();
    String str_password = ET_password.toString();
    // Intercept missing username and password.
    if(str_username.length() == 0) {
    }
    }

对话框出现。

你知道为什么对话框一开始没有出现吗?

Answer 1

这是因为EditText.toString()不返回文本。使用EditText.getText().toString()代替。您还应该在if语句之前和中添加一些日志语句，以便您可以更好地理解发生了什么。

Answer 2

问题出在以下几行:

  String str_username = ET_username.toString();//is never empty
  String str_password = ET_password.toString();//is never empty

试试下面的代码，它应该可以工作

 String str_username = ET_username.getText().toString();
 String str_password = ET_password.getText().toString();