安卓中如何获取对话框中Editext的输入

这是我的代码

    final View layout= (LinearLayout)
                               getLayoutInflater().inflate(R.layout.lable_input,null);

                       AlertDialog.Builder builder_lable=new AlertDialog.Builder(addAlarmActivity.this)
                               .setTitle("标签")
                               .setView(layout)
                               .setPositiveButton("确定", new DialogInterface.OnClickListener() {
                                   @Override
                                   public void onClick(DialogInterface dialogInterface, int i) {
                                       input=(EditText) findViewById(R.id.input);
                                       System.out.println(input.getText().toString());
                                   }
                               }).setNegativeButton("取消",null);
                       builder_lable.show();

报了错误

EditText tv=new EditText(this);
tv.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            Log.e("Tag",s.toString());
        }

        @Override
        public void afterTextChanged(Editable s) {

        }
    });

public class MainActivity extends Activity {
private Button button1;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    button1 = (Button) findViewById(R.id.button1);
    button1.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            input();
        }
    });
}

public void input() {
    final View layout = getLayoutInflater().inflate(R.layout.lable_input,
            null);
    final EditText input = (EditText) layout.findViewById(R.id.input);
    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this)
            .setTitle("标签").setView(layout)
            .setPositiveButton("确定", new DialogInterface.OnClickListener() {

                @Override
                public void onClick(DialogInterface dialog, int which) {
                    Toast.makeText(MainActivity.this, "消息:"
                            + input.getText().toString(),
                            Toast.LENGTH_SHORT).show();
                }
            }).setNegativeButton("取消", null);
    builder.create().show();
}

}