设置 setOnItemSelectedListener 时微调器错误

Question

当我将微调器用于下拉列表或出现setOnItemSelectedListener 错误时：

nav_login.xml：

<Spinner
    android:layout_width="20dp"
    android:layout_height="12dp"
    android:id="@+id/spinner"
    android:layout_alignParentBottom="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_alignParentLeft="false"
    android:layout_alignParentTop="false"
    android:layout_alignWithParentIfMissing="false"
    android:layout_marginRight="30dp"
    android:layout_marginBottom="12dp"
    android:entries="@array/items" />

字符串数组：

<string-array name="items">
    <item>Log Out</item>
    <item>Setting</item>
</string-array>

主活动：

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


    Firebase.setAndroidContext(this);
    Firebase rootRef = new Firebase("https://intense-heat-4844.firebaseio.com/property");

    navigationView = (NavigationView) findViewById(R.id.nav_view);
    View mLoginButton = navigationView.inflateHeaderView(R.layout.nav_header);
    mLoginButton.findViewById(R.id.Login);
    if (!myBooleanVariable) {
        mLoginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //Calling activity LoginRegister
                Intent intent = new Intent(getApplication(), LoginRegister.class);
                startActivity(intent);
            }
        });
    }

    Intent intent = getIntent();
    if (null != intent) {
        myBooleanVariable = intent.getBooleanExtra("my_boolean_key", myBooleanVariable);
        if (myBooleanVariable) {
            navigationView.getHeaderView(0).setVisibility(View.GONE);
            navigationView = (NavigationView) findViewById(R.id.nav_view);
            View mUserLoggedIn = navigationView.inflateHeaderView(R.layout.nav_header_login);


            final Spinner spinner;
            spinner = (Spinner) findViewById(R.id.spinner);
            spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
                @Override
                public void onItemSelected(AdapterView<?> arg0, View arg1,
                                           int arg2, long arg3) {
                    String msupplier = spinner.getSelectedItem().toString();

                    Log.e("Selected item : ", msupplier);

                    Intent intent = new Intent(getApplication(), Spinner.class);
                    startActivity(intent);
                }

                @Override
                public void onNothingSelected(AdapterView<?> arg0) {

                }
            });



        }
    }

    ePager = (ViewPager) findViewById(R.id.Pager);
    ePager.setAdapter(new MyPagerAdapter(getSupportFragmentManager()));
    eTabs = (SlidingTabLayout) findViewById(R.id.TabLayout);
    eTabs.setViewPager(ePager);

    Toolbartext = (TextView) findViewById(R.id.toolbar_title);
    toolbar = (Toolbar) findViewById(R.id.app_bar);
    setSupportActionBar(toolbar);
    Toolbartext.setText("Residential Search");

    getSupportActionBar().setDisplayShowHomeEnabled(true);
    Navigation1 drawerfragment = (Navigation1)
            getSupportFragmentManager().findFragmentById(R.id.fragment_navigation1);
    drawerfragment.setup(R.id.fragment_navigation1, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar);


}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        Toast.makeText(this, "hitted item" + item.getTitle(), Toast.LENGTH_SHORT).show();
        return true;
    }

    return super.onOptionsItemSelected(item);
}

例外：

03-01 13:17:47.039    8587-8587/com.example.syedsaadali.myapplication I/System.out﹕ User ID: 4310e794-0b9b-4fad-afd0-e3f73c5f7412, Provider: password
03-01 13:17:47.074    8587-8587/com.example.syedsaadali.myapplication D/AndroidRuntime﹕ Shutting down VM
03-01 13:17:47.074    8587-8587/com.example.syedsaadali.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.syedsaadali.myapplication, PID: 8587
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.syedsaadali.myapplication/com.example.syedsaadali.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setOnItemSelectedListener(android.widget.AdapterView$OnItemSelectedListener)' on a null object reference
            at com.example.syedsaadali.myapplication.MainActivity.onCreate(MainActivity.java:72)

Answer 1

如果你这样使用，
spinner = (Spinner) findViewById(R.id.spinner); 它将在布局 activity_main 中搜索微调器（因为您在 onCreate 中将 activity_main 设置为内容视图），但它不是 activity_main 的子级。所以它抛出 NullPointerException。

改为这样使用，

spinner = (Spinner) mUserLoggedIn.findViewById(R.id.spinner); （如果微调器在 nav_header_login 布局内）

Answer 2

这就是问题所在。您将 OnCreate 中的 contentView 设置为 R.layout.activity_main 但 Spinner 位于 nav_login.xml 中，因此程序正在 activity_main 中寻找 spinner 但找不到它。

2 解决方案：

1. 将微调器从 nav_login.xml 移动到 activity_main.xml

或

1. 将内容视图设置为 setContentView(R.layout.nav_login);