Java 将新项目添加到现有的project android应用程序（如何移动到下一个活动）

我只看过YouTube和互联网上的教程，但我需要一个具体的指导。（还是个学生，只是自学android开发。）

我找到了一个源代码，我想在实验应用程序中作为参考：（归功于所有者）

我想尝试在点击欢迎屏幕上的“开始”按钮后显示它，但按钮的意图对我不起作用。应用程序崩溃了

public class WelcomeScreen extends AppCompatActivity {

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

    Button gsbtn=(Button)findViewById(R.id.getstartedbtn);
    gsbtn.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent i=new Intent(getApplicationContext(),MainActivity.class);
            startActivity(i);
        }
    });

更新 Logcat

java.lang.NullPointerException: Attempt to invoke interface method 'int android.database.Cursor.getCount()' on a null object reference
    at e.alasse.foodiary.views.activities.MainActivity.onLoadFinished(MainActivity.java:382)
    at e.alasse.foodiary.views.activities.MainActivity.onLoadFinished(MainActivity.java:66)
    at android.support.v4.app.LoaderManagerImpl$LoaderObserver.onChanged(LoaderManagerImpl.java:250)
    at android.arch.lifecycle.LiveData.considerNotify(LiveData.java:109)
    at android.arch.lifecycle.LiveData.dispatchingValue(LiveData.java:126)
    at android.arch.lifecycle.LiveData.setValue(LiveData.java:282)
    at android.arch.lifecycle.MutableLiveData.setValue(MutableLiveData.java:33)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.setValue(LoaderManagerImpl.java:189)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManagerImpl.java:174)
    at android.support.v4.content.Loader.deliverResult(Loader.java:132)
    at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:109)
    at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:41)
    at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:258)
    at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:83)
    at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:490)
    at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:507)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="e.alasse.foodiary">
<uses-permission android:name="android.permission.INTERNET" />

<application

    android:name=".MyApplication"
    android:allowBackup="true"
    android:icon="@mipmap/icon2"
    android:roundIcon="@mipmap/icon2"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".WelcomeScreen">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <provider
        android:name="e.alasse.foodiary.providers.RestaurantContentProvider"
        android:authorities="e.alasse.providers.RestaurantContentProvider"
        android:exported="false" />
    <activity android:name=".views.activities.MainActivity"></activity>
    <activity android:name=".ImagesActivity" />
    <activity android:name=".UploadActivity" />
</application>

编辑2：

Android清单

java.lang.NullPointerException: Attempt to invoke interface method 'int android.database.Cursor.getCount()' on a null object reference
    at e.alasse.foodiary.views.activities.MainActivity.onLoadFinished(MainActivity.java:382)
    at e.alasse.foodiary.views.activities.MainActivity.onLoadFinished(MainActivity.java:66)
    at android.support.v4.app.LoaderManagerImpl$LoaderObserver.onChanged(LoaderManagerImpl.java:250)
    at android.arch.lifecycle.LiveData.considerNotify(LiveData.java:109)
    at android.arch.lifecycle.LiveData.dispatchingValue(LiveData.java:126)
    at android.arch.lifecycle.LiveData.setValue(LiveData.java:282)
    at android.arch.lifecycle.MutableLiveData.setValue(MutableLiveData.java:33)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.setValue(LoaderManagerImpl.java:189)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManagerImpl.java:174)
    at android.support.v4.content.Loader.deliverResult(Loader.java:132)
    at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:109)
    at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:41)
    at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:258)
    at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:83)
    at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:490)
    at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:507)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="e.alasse.foodiary">
<uses-permission android:name="android.permission.INTERNET" />

<application

    android:name=".MyApplication"
    android:allowBackup="true"
    android:icon="@mipmap/icon2"
    android:roundIcon="@mipmap/icon2"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".WelcomeScreen">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <provider
        android:name="e.alasse.foodiary.providers.RestaurantContentProvider"
        android:authorities="e.alasse.providers.RestaurantContentProvider"
        android:exported="false" />
    <activity android:name=".views.activities.MainActivity"></activity>
    <activity android:name=".ImagesActivity" />
    <activity android:name=".UploadActivity" />
</application>

更新版本

---

首先，在清单中，您必须使用

名称

属性声明应用程序类。 在您的清单中添加类似的内容：

---

在
onLoadFinished
方法的
MainActivity
类中，您可以看到以下内容：

RestaurantInfoModel model = MyApplication.getGsonInstance().fromJson(cursor.getString(dataIndex), RestaurantInfoModel.class);

它调用MyApplication的方法
getGsonInstance（）
，为了确保此方法成功执行，您应该在
AndroidManifest.xml
中添加此行：

android:name=".MyApplication"

有关自定义应用程序的详细信息是
您能发布错误吗？您好，请发布您的错误，您似乎没有在清单文件中定义MainActivity。请尝试使用Intent i=new Intent（包上下文：WelcomeScreen.this，MainActivity.class）；我编辑了它并添加了日志。请尝试将
getApplicationContext（）
替换为
WelcomScreen。这个
，看看它是否有效？谢谢先生。我在清单上添加了`android:name=“.MyApplication”`。它在按下get Start后工作，加载一段时间后崩溃。我没有得到
getCityId（）
部分