android.view.View.findViewById(int) 空对象引用 (Android Lint)

Question

我有问题。我在 android 工作室运行 Lint 修复了我的应用程序的某些部分，但此应用程序无法启动。它写道：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cewe.martin.cewetracking/com.cewe.martin.cewetracking.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)

我也找不到，问题出在哪里。我无法撤消，因为有一些更改的文件，所以我尝试删除对我来说不常见的部分，但它仍然无法启动。这是 MainActivity 的 onCreate 方法：

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
    private DrawerLayout drawer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        drawer = findViewById(R.id.drawer_layout);

        NavigationView navigationView = findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);

        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();

        if (savedInstanceState == null) {
            Log.d("START","DO IFU");
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new ScanFragment()).commit();
            Log.d("START","ZA GETSUPP");
            navigationView.setCheckedItem(R.id.nav_scan);
        }
    }

可以看到Log.d。第一个 "DO IFU" 显示，第二个不显示，所以我认为它可能在 ScanFragment() 中。代码在这里：

public class ScanFragment extends Fragment {

    private final EditText rucneKod = (EditText) getView().findViewById(R.id.numberInput);

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        final View view = inflater.inflate(R.layout.fragment_scan, container, false);
        Button skenovaciTlacitko = view.findViewById(R.id.button2);
        skenovaciTlacitko.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                zapnoutSkener();
            }
        });

        Button rucne = view.findViewById(R.id.button3);

        View.OnFocusChangeListener ofcListener = new MyFocusChangeListener();
        rucneKod.setOnFocusChangeListener(ofcListener);

        final InputMethodManager imm = (InputMethodManager) getActivity().getSystemService(Context.INPUT_METHOD_SERVICE);

        rucne.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (rucneKod.getText().length() != 0) {
                    imm.hideSoftInputFromWindow(v.getWindowToken(), 0);
                    odeslat(rucneKod.getText().toString());
                } else {
                    imm.hideSoftInputFromWindow(v.getWindowToken(), 0);
                    Snackbar snackbar = Snackbar.make(view, R.string.pozadovanKod, Snackbar.LENGTH_LONG);
                    View sbView = snackbar.getView();
                    TextView textView = sbView.findViewById(android.support.design.R.id.snackbar_text);
                    textView.setTextColor(Color.YELLOW);
                    snackbar.show();
                }
            }
        });
        return view;
    }

有人可以看到有问题的代码吗？谢谢

Answer 1

private final EditText rucneKod = (EditText) 
getView().findViewById(R.id.numberInput);

导致 NPE。把它放在 OnCreateView 里面，用 view

替换 getView()

Answer 2

我认为问题出在这里：

private final EditText rucneKod = (EditText) getView().findViewById(R.id.numberInput);

您应该在 fragment 的布局膨胀之后立即在 onCreateView 中调用 findViewById。 getView() fragment构造时为null，调用onCreateView前初始化存储属性

Answer 3

getView() return the view returned by onCreateView(LayoutInflater, ViewGroup, Bundle))

这样做：private final EditText rucneKod = (EditText) getView().findViewById(R.id.numberInput);

它在onCreateView函数之前调用了getView，它会是null，这是正常的，

尝试在 onCreateView 函数中移动实例化

希望对您有所帮助

android.view.View.findViewById(int) 空对象引用 (Android Lint)

android.view.View.findViewById(int) on a null object reference (Android Lint)

android

android-fragments

android-view