为什么在微调监听器中需要onNothingSelected方法？

Question

母语为英语的人，所以我对你们的英语水平不好对你们说抱歉。

我从5周前开始学习Android。我试图实现一个微调器，导师问为什么需要onNothingSelected方法。我无话可说

所以，为什么我需要那种方法？你可以回复吗？

以下代码是我的微调器。它可以正确执行我的预期。

 public class SpinnerViewPractice extends Activity {
    private Spinner spinner;
    private String spinner_value = "";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.spinnerviewpractice);

        spinner = (Spinner)findViewById(R.id.spinner1);

        String[] str = {"","good", "dislike", "like", "hate", "moderate"};
        spinner.setPrompt("Set Text");
        ArrayAdapter<String> list = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item, str); 
        spinner.setAdapter(list);
        spinner.setOnItemSelectedListener(new OnItemSelectedListener() {

            @Override
            public void onItemSelected(AdapterView<?> arg0, View arg1,
                    int arg2, long arg3) {
                TextView tv = (TextView)arg1;
                spinner_value = tv.getText().toString();
                if(spinner_value.length() == 0)
                {
                    spinner_value = "Nothing";                  
                }
                Toast.makeText(SpinnerViewPractice.this, spinner_value, Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onNothingSelected(AdapterView<?> arg0) {
                Toast.makeText(SpinnerViewPractice.this, "NothingSelected", Toast.LENGTH_SHORT).show();
            }           
        });
    }
}

Answer 1

如文档所述：

Answer 2

来自文档here。