Question

我是android程序员的新手。最近，我试图制作在Android应用程序中使用谷歌地图v2的项目，但我收到了很多错误（我已经添加了谷歌播放服务到我的库，并在这里找到类似的主题，但没有解决我的问题）。请看我的日志猫吼：

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03-22 13:52:29.331: E/AndroidRuntime(17708): FATAL EXCEPTION: main
03-22 13:52:29.331: E/AndroidRuntime(17708): java.lang.RuntimeException: Unable to resume activity {com.yai.testmap/com.yai.testmap.MainActivity}: java.lang.NullPointerException
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2613)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2641)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2127)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.access$600(ActivityThread.java:140)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1228)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.os.Looper.loop(Looper.java:137)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.main(ActivityThread.java:4895)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at java.lang.reflect.Method.invokeNative(Native Method)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at java.lang.reflect.Method.invoke(Method.java:511)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:994)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:761)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at dalvik.system.NativeStart.main(Native Method)
03-22 13:52:29.331: E/AndroidRuntime(17708): Caused by: java.lang.NullPointerException
03-22 13:52:29.331: E/AndroidRuntime(17708):    at com.yai.testmap.MainActivity.setUpMapIfNeeded(MainActivity.java:93)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at com.yai.testmap.MainActivity.onResume(MainActivity.java:187)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1199)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.Activity.performResume(Activity.java:5237)
03-22 13:52:29.331: E/AndroidRuntime(17708):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2603)
03-22 13:52:29.331: E/AndroidRuntime(17708):    ... 12 more

更新

我的MainActivity.java：

package com.yai.testmap;

import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

/* FOR ERROR REPPORT */
import android.util.Log;

/* IMPORT FOR DROPDOWNLIST USED THIS PROJECT */
import android.widget.CheckBox;
import android.widget.Spinner;
import android.widget.Toast;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.AdapterView.OnItemSelectedListener;

/* IMPORT FOR GOOGLE MAP */
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.SupportMapFragment;

/* FOR GOOGLE MAP TYPE */
import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_HYBRID;
import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_NONE;
import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_NORMAL;
import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_SATELLITE;
import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_TERRAIN;

class PlaceholderFragment extends Fragment implements OnItemSelectedListener{
    private GoogleMap mGoogleMap;
    private CheckBox mTrafficCheckbox, mLocationCheckbox, mBuildingCheckbox, mIndoorCheckbox;

    public PlaceholderFragment() {
    }

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

    Spinner spinner = (Spinner) rootView.findViewById(R.id.spinnerOption);
    ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(), R.array.spinner_option_string, android.R.layout.simple_spinner_item);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner.setAdapter(adapter);
    spinner.setOnItemSelectedListener((OnItemSelectedListener) getActivity());

    mTrafficCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_traffic);
    mLocationCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_location);
    mBuildingCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_building);
    mIndoorCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_indoor);

        return rootView;
    }

    @Override
    public void onItemSelected(AdapterView parent, View view, int position,
            long id) {
        // TODO Auto-generated method stub

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

    }
}


public class MainActivity extends ActionBarActivity implements OnItemSelectedListener {
    private GoogleMap mGoogleMap;
    private CheckBox mTrafficCheckbox, mLocationCheckbox, mBuildingCheckbox, mIndoorCheckbox;

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

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

    private void setUpMapIfNeeded(){
        if(mGoogleMap == null){
            mGoogleMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
        }
    }

    private boolean checkReady(){
        if(mGoogleMap == null){
            Toast.makeText(this, R.string.map_not_ready, Toast.LENGTH_SHORT).show();
            return false;
        }
        return true;
    }

    private void updateTraffic(){
        if(!checkReady()){
            return;
        }
        mGoogleMap.setTrafficEnabled(mTrafficCheckbox.isChecked());
    }

    public void onTrafficToggled(View view){
        updateTraffic();
    }

    private void updateLocation(){
        if(!checkReady()){
            return;
        }
        mGoogleMap.setMyLocationEnabled(mLocationCheckbox.isChecked());
    }

    public void onLocationToggled(View view){
        updateLocation();
    }

    private void updateBuilding(){
        if(!checkReady()){
            return;
        }
        mGoogleMap.setBuildingsEnabled(mBuildingCheckbox.isChecked());
    }

    public void onBuildingToggled(View view){
        updateBuilding();
    }

    private void updateIndoor(){
        if(!checkReady()){
            return;
        }
        mGoogleMap.setIndoorEnabled(mIndoorCheckbox.isChecked());
    }

    public void onIndoorToggled(View view){
        updateIndoor();
    }

    @Override
    public void onItemSelected(AdapterView parent, View view, int position, long id){
         setLayer((String) parent.getItemAtPosition(position));
    }

    private void setLayer(String layerName){
        if(!checkReady()){
            return;
        }

        if(layerName.equals(getString(R.string.normal))){
            mGoogleMap.setMapType(MAP_TYPE_NORMAL);
        }
        else if(layerName.equals(getString(R.string.hybrid))){
            mGoogleMap.setMapType(MAP_TYPE_HYBRID);
        }
        else if(layerName.equals(getString(R.string.satellite))){
            mGoogleMap.setMapType(MAP_TYPE_SATELLITE);
        }
        else if(layerName.equals(getString(R.string.terrain))){
            mGoogleMap.setMapType(MAP_TYPE_TERRAIN);
        }
        else if(layerName.equals(getString(R.string.none_map))){
            mGoogleMap.setMapType(MAP_TYPE_NONE);
        }
        else{
            Log.i("LDA", "Error setting layer with name " + layerName);
        }
    }

    @Override
    public void onNothingSelected(AdapterView parent){

    }

    @Override
    protected void onResume() {
        super.onResume();
        setUpMapIfNeeded();
        if (mGoogleMap != null) {
            updateTraffic();
            updateLocation();
            updateBuilding();
            updateIndoor();
        }
    }



    @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;
        }
    }

}

我在这里使用片段，所以这里是fragment_main.xml：

<?xml version="1.0" encoding="utf-8" ?>

    <Spinner
        android:id="@+id/spinnerOption"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <CheckBox 
        android:id="@+id/checkbox_traffic"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onTrafficToggled"
        android:text="@string/traffic" />

    <CheckBox
        android:id="@+id/checkbox_location"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onLocationToggled"
        android:text="@string/location" />

    <CheckBox
        android:id="@+id/checkbox_building"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" 
        android:onClick="onBuildingsToggled"
        android:checked="true"
        android:text="@string/building" />

    <CheckBox
        android:id="@+id/checkbox_indoor"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onIndoorToggled"
        android:checked="true"
        android:text="@string/indoor" />
</LinearLayout>
</RelativeLayout>

和我的清单

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.yai.testmap"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="9"
    android:targetSdkVersion="17" />

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

<uses-feature
    android:glEsVersion="0x00020000"
    android:required="true"
/>

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >

    <meta-data
        android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" 
    />

    <activity
        android:name="com.yai.testmap.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <meta-data 
        android:name="com.google.android.maps.v2.API_KEY"
        android:value="SUPER SCREET CODE"
        />
</application>

</manifest>

这里是我的activity_main.xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.yai.testmap.MainActivity"
tools:ignore="MergeRootFrame" />

我很好奇我的Spinner和函数setUpMapIfNeeded（），因为当我评论那部分代码时，我的应用程序正常工作。我想念的是什么？任何的想法？非常感谢。

Answer 1

您的所有View都不属于您的activity_main.xml。它们是您PlaceHolderFragment的一部分。因此，当您拨打findViewById() findViewById(R.id.spinnerOption)时，他们只会返回null，因为它们不属于Activity视图层次结构......

首先将PlaceHolderFragment添加到Activity，然后调用findViewById()，或者将所有View声明及其实现移至PlaceHolderFragment。< / p>

您可以先在视图中添加Fragment添加然后调用findViewById() ...

@Override
protected void onCreate(Bundle savedInstanceState) {    
    /* CREATE OPTION FOR DROPDOWN LIST*/
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_main);

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

Answer 2

试试这个..

将Spinner和所有CheckBox从MainActivity移至PlaceholderFragment，如下所示

public static class PlaceholderFragment extends Fragment implements OnItemSelectedListener{

    public PlaceholderFragment() {
    }

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

    Spinner spinner = (Spinner) rootView.findViewById(R.id.spinnerOption);
    ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(), R.array.spinner_option_string, android.R.layout.simple_spinner_item);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner.setAdapter(adapter);
    spinner.setOnItemSelectedListener(getActivity());

    mTrafficCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_traffic);
    mLocationCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_location);
    mBuildingCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_building);
    mIndoorCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_indoor);

        return rootView;
    }
}

和

@Override
protected void onCreate(Bundle savedInstanceState) {    
    /* CREATE OPTION FOR DROPDOWN LIST*/
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

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

Answer 3

问题是因为Spinner和复选框以及所有其他视图都在

中

fragment_main.xml

文件。您正在尝试在MainActivity中找到该视图的ID，因此每次它都会为您提供Null Poniter异常，因为它位于 PlaceHolderFragment 中所以首先将PlaceHolderFragment添加到您的Activity并查找视图ID。见这个

public static class PlaceholderFragment extends Fragment implements OnItemSelectedListener{

public PlaceholderFragment() {
}

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

Spinner spinner = (Spinner) rootView.findViewById(R.id.spinnerOption);
ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(), R.array.spinner_option_string, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);
spinner.setOnItemSelectedListener(getActivity());

mTrafficCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_traffic);
mLocationCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_location);
mBuildingCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_building);
mIndoorCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_indoor);

    return rootView;
}
}

Android Java.Lang.RuntimeException：无法启动活动组件信息

更新

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3 个答案: