Question

我是Android新手。对不起，我找不到我的错。我在模拟器上打开应用程序时遇到NullPointerException。日志显示“无法启动组件信息”。请帮忙。

    package com.myworks.converter;

    import android.os.Bundle;
    import android.support.v4.app.Fragment;
    import android.support.v7.app.ActionBarActivity;
    import android.view.LayoutInflater;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.AdapterView;
    import android.widget.AdapterView.OnItemSelectedListener;
    import android.widget.ArrayAdapter;
    import android.widget.Spinner;

    public class MainActivity extends ActionBarActivity implements OnItemSelectedListener 
    {

    private Spinner spinner;
    private static final String items[] = {"Length", "Weight", "Temperature"};

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

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

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(MainActivity.this,
                android.R.layout.simple_spinner_item,items);

        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setAdapter(adapter);
        spinner.setOnItemSelectedListener(this);

        if (savedInstanceState == null) 
        {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.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();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }

    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position,long id)
    {
        switch (position) 
        {
            case 0:
            // Whatever you want to happen when the first item gets selected
                break;
            case 1:
            // Whatever you want to happen when the second item gets selected
                break;
            case 2:
            // Whatever you want to happen when the thrid item gets selected
       }

    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) 
    {
        // TODO Auto-generated method stub

    }

    }

fragment_main.xml

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.myworks.converter.MainActivity$PlaceholderFragment" >

   <Spinner
       android:id="@+id/spinner"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content" />
  </LinearLayout>

错误记录

07-12 03:14:08.603: D/AndroidRuntime(1862): Shutting down VM
07-12 03:14:08.603: W/dalvikvm(1862): threadid=1: thread exiting with uncaught exception (group=0xb1d1eb20)
07-12 03:14:08.713: E/AndroidRuntime(1862): FATAL EXCEPTION: main
07-12 03:14:08.713: E/AndroidRuntime(1862): Process: com.myworks.converter, PID: 1862
07-12 03:14:08.713: E/AndroidRuntime(1862): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.myworks.converter/com.myworks.converter.MainActivity}: java.lang.NullPointerException
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.os.Handler.dispatchMessage(Handler.java:102)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.os.Looper.loop(Looper.java:136)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at java.lang.reflect.Method.invokeNative(Native Method)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at java.lang.reflect.Method.invoke(Method.java:515)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at dalvik.system.NativeStart.main(Native Method)
07-12 03:14:08.713: E/AndroidRuntime(1862): Caused by: java.lang.NullPointerException
07-12 03:14:08.713: E/AndroidRuntime(1862):     at com.myworks.converter.MainActivity.onCreate(MainActivity.java:34)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.Activity.performCreate(Activity.java:5231)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-12 03:14:08.713: E/AndroidRuntime(1862):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-12 03:14:08.713: E/AndroidRuntime(1862):     ... 11 more
07-12 03:14:17.303: I/Process(1862): Sending signal. PID: 1862 SIG: 9

Answer 1

崩溃的原因在于：

@Override
protected void onCreate(Bundle savedInstanceState) 
{
    spinner = (Spinner)findViewById(R.id.spinner); // <------------- returns null!
}

为什么？

因为您正在尝试访问片段视图（R.id.spinner在片段布局xml中声明）在其活动的创建中，即 - 在片段完全膨胀之前。

要修复，请更改逻辑，以便从片段本身引用片段视图：

class PlaceHolderFragment {
   .....

   @Override
   public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
         View view = inflater.inflate(R.layout.event_log, container, false);
         spinner = (Spinner)findViewById(R.id.spinner);
         // here spinner != null  <------------------------
         return view;
   }

}

使用Spinner时无法启动组件信息

1 个答案: