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我在學習Java的早期階段，但希望有人能夠解釋我在這裏做錯了什麼，許多Google搜索都讓我頭腦轉動。getFileDir（）導致NullPointer異常
我一直在關注一個教程，該教程展示瞭如何將一個ArrayList保存到文本文件中，然後獲取詳細信息。
我收到以下錯誤： -
01-10 19:00:37.092 16897-16897/uk.co.c2digital.futurefoto E/AndroidRuntime: FATAL EXCEPTION: main
Process: uk.co.c2digital.futurefoto, PID: 16897
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{uk.co.c2digital.futurefoto/uk.co.c2digital.futurefoto.viewLocations}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2322)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2474)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1359)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:155)
at android.app.ActivityThread.main(ActivityThread.java:5696)
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:1028)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
at android.content.ContextWrapper.getFilesDir(ContextWrapper.java:201)
at uk.co.c2digital.futurefoto.fileControl.<init>(fileControl.java:23)
at uk.co.c2digital.futurefoto.viewLocations.<init>(viewLocations.java:18)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1572)
at android.app.Instrumentation.newActivity(Instrumentation.java:1083)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2312)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2474)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1359)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:155)
at android.app.ActivityThread.main(ActivityThread.java:5696)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
01-10 19:05:37.142 16897-16897/uk.co.c2digital.futurefoto D/Process: killProcess, pid=16897
01-10 19:05:37.162 16897-16897/uk.co.c2digital.futurefoto D/Process: com.android.internal.os.RuntimeInit$UncaughtHandler.uncaughtException:138 java.lang.ThreadGroup.uncaughtException:693 java.lang.ThreadGroup.uncaughtException:690
這是正在當我的文件viewLocations.java運行以下（進口等後）造成
public class viewLocations extends AppCompatActivity {
fileControl filer = new fileControl();
ArrayList imgArray = filer.getSaved();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_locations);
}
FileControl （）;坐落在被稱爲一個單獨的文件「fileControl.java」
public class fileControl extends AppCompatActivity {
// Set filename
String filename = getFilesDir() + "/savedList.txt";
/**
* Following code gets a list of saved image paths from device
* @return
*/
public ArrayList getSaved() {
// Create variable for storing each line as it's read
String line;
// Clear imgArray ready for new entries
ArrayList newList = new ArrayList();
newList.clear(); // ensure new array is clear before continuing
// Try to open file
try {
BufferedReader input = new BufferedReader(new FileReader(filename));
if (!input.ready()) {
// If input file is not ready, show log error
Log.d("getSavedList", "Unable to open save file, input not ready");
} else {
// If input file is ready, read each line one at a time and add to array
while ((line = input.readLine()) != null) {
newList.add(line);
}
input.close();
// Run verification on results to ensure no duplicates or deleted images are recorded
newList = listVerify(newList);
}
} catch (IOException e) {
Log.d("getSavedList", "Read save file failed: " + e);
}
return newList;
}