Android对话框错误中的Edittext不合逻辑？

Question

当我在我的活动中按下对话框上的肯定按钮时，我的错误就出现了。最初 Edittext 应该将信息保存到 String 中，然后保存到 SharedPreferences 文件中，但经过很多努力无济于事，我修改了大部分代码并隔离了一行仍然给我一个错误。

spinnerClass.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        public void onItemSelected(AdapterView<?> parent, View view,
                int pos, long id) {
            // have each new entry be "insert"ed to arrayadapter so create is at end
            // the newer, the closer to the top
            if (id==1)
                // eventually uses as element value to check array or SharedPrefs if matches
                // "Create New+" or not < so doesn't prompt for no reason
            {
                build = new AlertDialog.Builder(MainActivity.this);
                LayoutInflater inflater = getLayoutInflater();

                // Pass null as the parent view because its going in the dialog layout
                build.setView(inflater.inflate(R.layout.create_class_dialog, null))
                       .setTitle("New Class Entry")
                       .setPositiveButton("Create", new DialogInterface.OnClickListener() {
                           @Override
                           public void onClick(DialogInterface dialog, int id) {
                               String farnsworth = inputClass.getText().toString();

为了测试可能给我带来问题的这一行，我硬编码了“Hello world”。执行没有错误。现在这个简单的改变让我的应用崩溃了！

/*                         
                              // classesPrefsList = getSharedPreferences(CLASSES_PREFS, 0);
                              //SharedPreferences.Editor editor = classesPrefsList.edit();
                               //editor.putString("ClassList"+(classesList.size()-1), enteredClass);
                               //editor.commit();
                               //classesList = getClassesArrayListSharedPreferences();
                              // classAdapter.notifyDataSetChanged();
                               * */
                               dialog.cancel();
                           }
                       })
                       .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                           public void onClick(DialogInterface dialog, int id) {
                               dialog.cancel();
                           }
                       });      
                AlertDialog alert = build.create();
                alert.show();
            }
        }
        public void onNothingSelected(AdapterView<?> parent) {
        }
    });

Answer 1

如果inputClass EditText 在Dialog 内部，则使用AlertDialog layout View 初始化为：

build = new AlertDialog.Builder(MainActivity.this);
LayoutInflater inflater =  LayoutInflater.from(MainActivity.this);
View alertview = inflater.inflate(R.layout.create_class_dialog, null);

/// initialize EditText here..
inputClass=(EditText)alertview.findViewById(R.id.edittextid);
build.setView(alertview)
.setTitle("New Class Entry")
....your code here...