ClassCastException：无法将MainActivity强制转换为Listener

Question

我正在尝试为我的ScanFragment实施WifiScanner侦听器，但我收到此错误：java.lang.ClassCastException: emilsoft.wifitest3.MainActivity cannot be cast to emilsoft.wifitest3.WifiScanner$Listener 我已经在正常的活动中做了这个，现在我试图将它转换为Fragments，我目前正在学习它们。 我做了很多研究，但我找不到合适的解决方案。我已经评论了存在错误的代码

所以主要活动：

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类：

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 ScanFragment ：

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类（处理添加到WifiScanner类的侦听器）：

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
}

问题是我无法将正确的Context传递给我的ScanCollector类，然后将其转换为WifiScanner.Listener。可能是一个非常愚蠢的解决方案，但我无法找到它。

提前致谢！

Answer 1

有一件事是context，另一件事是WifiScanner.Listener。你的ScanCollector需要两者都通过它们：

public ScanCollector(Context context, WifiScanner.Listener listener) {
    if (context == null)
        throw new NullPointerException();
    this.context = context;
    this.listener = listener
}

当你创建它时：

sc = new ScanCollector(getActivity(), this);

Answer 2

更改

sc = new ScanCollector(this.getContext()); // The fragment's context

到

sc = new ScanCollector(getActivity()); // The activity's context

this.getContext()指的是片段的当前上下文，它与真正实现Listener接口的主机活动的上下文不同。

（确保MainActivity implements WifiScanner.Listener）

Answer 3

将实现添加到实现监听器的类中（您必须从示例中找到您的监听器名称）...然后单击Alt + Enter并从右键单击选项中选择实现方法