android AlertDialog设置点击button不退出

dialog = new AlertDialog.Builder(MainActivity.this);
		dialog.setTitle("输入用户名：");
		dialog.setPositiveButton("start",
				new DialogInterface.OnClickListener() {

					@Override
					public void onClick(DialogInterface dialogq, int which) {
						Field field;
						try {
							field = dialogq.getClass().getSuperclass()
									.getDeclaredField("mShowing");
							field.setAccessible(true);
							// 设置mShowing值，欺骗android系统
							field.set(dialogq, false);//如果为true则会推出
						} catch (NoSuchFieldException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						} catch (IllegalArgumentException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						} catch (IllegalAccessException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}

					}
				});
		dialog.setNegativeButton("stop", new DialogInterface.OnClickListener() {

			@Override
			public void onClick(DialogInterface dialog, int which) {
				dialog.cancel();

			}
		});
		dialog.setView(linear);