[英]java.lang.ClassCastException: MainActivity cannot be cast?
[英]ClassCastException: MainActivity cannot be cast to Listener
我正在嘗試為我的 ScanFragment 實現一個 WifiScanner 監聽器,但我收到了這個錯誤: java.lang.ClassCastException: emilsoft.wifitest3.MainActivity cannot be cast to emilsoft.wifitest3.WifiScanner$Listener
我已經用正常的活動做了這個,並且現在我正在嘗試將其轉換為 Fragments,我目前正在了解它們。 我做了很多研究,但找不到可行的解決方案。 我已經評論了有錯誤的代碼
所以我的主要活動:
private SectionsPagerAdapter mSectionsPagerAdapter;
private ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.container);
mViewPager.setAdapter(mSectionsPagerAdapter);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(mViewPager);
}
我的SectionsPagerAdapter 類:
public class SectionsPagerAdapter extends FragmentPagerAdapter{
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0: return ScanFragment.newInstance();
}
return null;
}
我的掃描片段:
public class ScanFragment extends Fragment implements WifiScanner.Listener {
private ScanCollector sc;
private WifiManager wifi;
public ScanFragment() {}
public static ScanFragment newInstance() {
return new ScanFragment();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View result = inflater.inflate(R.layout.fragment_scan_results, container, false);
wifi = (WifiManager) getActivity().getSystemService(Context.WIFI_SERVICE);
sc = new ScanCollector(this.getContext()); //THE ERROR STARTS HERE
return result;
}
我的ScanCollector 類(處理添加到 WifiScanner 類的偵聽器):
public class ScanCollector {
// The context wrapper that we'll use for accessing system services, receiving
// broadcasts from the WifiManager
private final Context context;
private WifiScanner.Listener listener;
public ScanCollector(Context context) {
if (context == null)
throw new NullPointerException();
this.context = context;
this.listener = (WifiScanner.Listener)context; //THE ERROR IS HERE
}
問題是我無法將正確的上下文傳遞給我的 ScanCollector 類,然后該類會將其轉換為 WifiScanner.Listener。 可能是一個非常愚蠢的解決方案,但我找不到它。
提前致謝!
一件事是context
,另一件事是WifiScanner.Listener
。 您的ScanCollector
需要兩者,因此請同時通過它們:
public ScanCollector(Context context, WifiScanner.Listener listener) {
if (context == null)
throw new NullPointerException();
this.context = context;
this.listener = listener
}
當你創建它時:
sc = new ScanCollector(getActivity(), this);
改變
sc = new ScanCollector(this.getContext()); // The fragment's context
到
sc = new ScanCollector(getActivity()); // The activity's context
this.getContext()
指的是fragment的當前上下文,不同於真正實現了Listener
接口的宿主activity的上下文。
(確保MainActivity implements WifiScanner.Listener
)
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