Android Studio 加载内部存储数据错误
Android Studio Loading Internal Storage Data Error
我刚刚开始学习 Android Studio，并且一直在开发一个基本的笔记应用程序。我在主要片段中添加了 load/save 方法。但是当我第一次启动应用程序时它崩溃了。如果我删除加载方法并启动应用程序，那么应用程序将按我的意愿运行。在我再次添加加载方法并第二次启动应用程序之后，应用程序也可以正常工作。所以我假设我的加载代码有问题，因为它会在我第一次启动时导致应用程序崩溃。您可以找到第一个屏幕的主要片段代码，以及我在下面的 Android Studio 中收到的错误。如果您能帮我解决这个问题，我将不胜感激。
MainProjectList.java片段：
public class MainProjectLister extends ListFragment implements
AdapterView.OnItemClickListener,
AdapterView.OnItemLongClickListener {
Button addProjectButton;
EditText addProjectEditText;
public String projectTitle;
Projekt newProjekt;
ProjektSingleton projektSingleton;
ArrayList<Projekt> projektArrayList;
ArrayAdapter<String> adapter;
ArrayList<Projekt> tempProjektArrayList;
ArrayList<String> ListOfP;
private final String FILENAME="testfile15.txt";
private final String FILENAME2="testfile16.txt";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ListOfP = new ArrayList<String>();
projektSingleton = ProjektSingleton.getInstance();
projektSingleton.setProjektList(getSavedArrayList());
ListOfP = getSavedStringList();
((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(false);
setHasOptionsMenu(true);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View view = inflater.inflate(R.layout.main_project_lister_layout, container, false);
addProjectButton = (Button) view.findViewById(R.id.addProjectButton);
addProjectEditText = (EditText) view.findViewById(R.id.addProjectEditText);
//addProjectButton onClck method
addProjectButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view){
//Checking if nothing is entered in the EditText and gives Error messaga
if(TextUtils.isEmpty(addProjectEditText.getText().toString().trim())){
Toast.makeText(getActivity(), "Please type details", Toast.LENGTH_SHORT).show();
return;
}
projectTitle = addProjectEditText.getText().toString();
newProjekt = new Projekt(projectTitle);
projektArrayList = projektSingleton.getProjectList();
projektArrayList.add(newProjekt);
ListOfP.add(projectTitle);
adapter.notifyDataSetChanged();
saveArrayList(projektArrayList);
saveStringList(ListOfP);
addProjectEditText.setText("");
}
});
getActivity().setTitle("DearBear Notes App");
return view;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, ListOfP);
setListAdapter(adapter);
getListView().setOnItemClickListener(this);
getListView().setOnItemLongClickListener(this);
}
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l){
MainProjectLister frag = new MainProjectLister();
ProjectListFragmentChanger fragInterface = (ProjectListFragmentChanger) getActivity();
fragInterface.projectListChangeFragment(frag);
projektSingleton.setProjectNumber(i);
}
@Override
public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l){
projektArrayList = projektSingleton.getProjectList();
projektArrayList.remove(i);
ListOfP.remove(i);
adapter.notifyDataSetChanged();
saveArrayList(projektArrayList);
saveStringList(ListOfP);
return true;
}
//Interface
public interface ProjectListFragmentChanger{
public void projectListChangeFragment(Fragment fragment);
}
//Save File Method
public void saveArrayList(ArrayList<Projekt> arrayList) {
try {
FileOutputStream fos = getActivity().openFileOutput(FILENAME, Context.MODE_PRIVATE);
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(arrayList);
out.close();
fos.close();
} catch (IOException e) {
e.printStackTrace ();
}
}
public void saveStringList(ArrayList<String> arrayList) {
try {
FileOutputStream fos = getActivity().openFileOutput(FILENAME2, Context.MODE_PRIVATE);
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(arrayList);
out.close();
fos.close();
} catch (IOException e) {
e.printStackTrace ();
}
}
//Load Text File Method
public ArrayList<Projekt> getSavedArrayList() {
ArrayList<Projekt> savedArrayList = null;
try {
FileInputStream fis = getActivity().openFileInput(FILENAME);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<Projekt>) ois.readObject();
ois.close();
fis.close();
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}
public ArrayList<String> getSavedStringList() {
ArrayList<String> savedArrayList = null;
try {
FileInputStream fis = getActivity().openFileInput(FILENAME2);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<String>) ois.readObject();
ois.close();
fis.close();
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}
}
错误：
$ adb shell am start -n "com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER
Client not ready yet..Waiting for process to come online
Connected to process 2394 on device Nexus_5_API_24 [emulator-5554]
I/art: Not late-enabling -Xcheck:jni (already on)
W/art: Unexpected CPU variant for X86 using defaults: x86
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86
I/InstantRun: Instant Run Runtime started. Android package is com.example.lenovo.projectlist, real application class is null.
I/art: Debugger is no longer active
I/art: Starting a blocking GC Instrumentation
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86
W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile15.txt (No such file or directory)
W/System.err: at java.io.FileInputStream.open(Native Method)
W/System.err: at java.io.FileInputStream.<init>(FileInputStream.java:146)
W/System.err: at android.app.ContextImpl.openFileInput(ContextImpl.java:485)
W/System.err: at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.getSavedArrayList(MainProjectLister.java:189)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:53)
W/System.err: at android.app.Fragment.performCreate(Fragment.java:2335)
W/System.err: at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949)
W/System.err: at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857)
W/System.err: at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880)
W/System.err: at android.app.BackStackRecord.run(BackStackRecord.java:727)
W/System.err: at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
W/System.err: at android.app.FragmentController.execPendingActions(FragmentController.java:371)
W/System.err: at android.app.Activity.performStart(Activity.java:6680)
W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
W/System.err: at android.app.ActivityThread.-wrap12(ActivityThread.java)
W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err: at android.os.Looper.loop(Looper.java:154)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:6077)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile16.txt (No such file or directory)
W/System.err: at java.io.FileInputStream.open(Native Method)
W/System.err: at java.io.FileInputStream.<init>(FileInputStream.java:146)
W/System.err: at android.app.ContextImpl.openFileInput(ContextImpl.java:485)
W/System.err: at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.getSavedStringList(MainProjectLister.java:208)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:54)
W/System.err: at android.app.Fragment.performCreate(Fragment.java:2335)
W/System.err: at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949)
W/System.err: at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857)
W/System.err: at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880)
W/System.err: at android.app.BackStackRecord.run(BackStackRecord.java:727)
W/System.err: at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
W/System.err: at android.app.FragmentController.execPendingActions(FragmentController.java:371)
W/System.err: at android.app.Activity.performStart(Activity.java:6680)
W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
W/System.err: at android.app.ActivityThread.-wrap12(ActivityThread.java)
W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err: at android.os.Looper.loop(Looper.java:154)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:6077)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
D/AndroidRuntime: Shutting down VM
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.lenovo.projectlist, PID: 2394
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:344)
at android.widget.ListView.setAdapter(ListView.java:493)
at android.app.ListFragment.setListAdapter(ListFragment.java:240)
at com.example.lenovo.projectlist.MainProjectLister.onActivityCreated(MainProjectLister.java:111)
at android.app.Fragment.performActivityCreated(Fragment.java:2361)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1014)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1171)
at android.app.BackStackRecord.run(BackStackRecord.java:815)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
at android.app.FragmentController.execPendingActions(FragmentController.java:371)
at android.app.Activity.performStart(Activity.java:6680)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Application terminated.
第一个错误发生是因为您在尚未创建的文件上调用 openFileInput()。但是由于您捕获了该错误，getSavedArrayList() returns null 并将其分配给 ListOfP 导致 NullPointerException 当您尝试将 null 分配给 ArrayAdapter 时。
为防止出现这种情况，您应该先检查该文件是否存在，然后将您的列表分配给一个空列表以防止这两种错误；像这样：
public ArrayList<Projekt> getSavedArrayList() {
ArrayList<Projekt> savedArrayList = null;
try {
// get the file and check if it exists before trying to read it
File savedFile = new File(getActivity().getFilesDir(), FILENAME);
if (savedFile.exists()) {
// if the file containing the list exists, read it and return the list
FileInputStream fis = getActivity().openFileInput(FILENAME);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<Projekt>) ois.readObject();
ois.close();
fis.close();
} else {
// if it doesn't exist, return an empty list
savedArrayList = new ArrayList<>();
}
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}
我刚刚开始学习 Android Studio，并且一直在开发一个基本的笔记应用程序。我在主要片段中添加了 load/save 方法。但是当我第一次启动应用程序时它崩溃了。如果我删除加载方法并启动应用程序，那么应用程序将按我的意愿运行。在我再次添加加载方法并第二次启动应用程序之后，应用程序也可以正常工作。所以我假设我的加载代码有问题，因为它会在我第一次启动时导致应用程序崩溃。您可以找到第一个屏幕的主要片段代码，以及我在下面的 Android Studio 中收到的错误。如果您能帮我解决这个问题，我将不胜感激。
MainProjectList.java片段：
public class MainProjectLister extends ListFragment implements
AdapterView.OnItemClickListener,
AdapterView.OnItemLongClickListener {
Button addProjectButton;
EditText addProjectEditText;
public String projectTitle;
Projekt newProjekt;
ProjektSingleton projektSingleton;
ArrayList<Projekt> projektArrayList;
ArrayAdapter<String> adapter;
ArrayList<Projekt> tempProjektArrayList;
ArrayList<String> ListOfP;
private final String FILENAME="testfile15.txt";
private final String FILENAME2="testfile16.txt";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ListOfP = new ArrayList<String>();
projektSingleton = ProjektSingleton.getInstance();
projektSingleton.setProjektList(getSavedArrayList());
ListOfP = getSavedStringList();
((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(false);
setHasOptionsMenu(true);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
View view = inflater.inflate(R.layout.main_project_lister_layout, container, false);
addProjectButton = (Button) view.findViewById(R.id.addProjectButton);
addProjectEditText = (EditText) view.findViewById(R.id.addProjectEditText);
//addProjectButton onClck method
addProjectButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view){
//Checking if nothing is entered in the EditText and gives Error messaga
if(TextUtils.isEmpty(addProjectEditText.getText().toString().trim())){
Toast.makeText(getActivity(), "Please type details", Toast.LENGTH_SHORT).show();
return;
}
projectTitle = addProjectEditText.getText().toString();
newProjekt = new Projekt(projectTitle);
projektArrayList = projektSingleton.getProjectList();
projektArrayList.add(newProjekt);
ListOfP.add(projectTitle);
adapter.notifyDataSetChanged();
saveArrayList(projektArrayList);
saveStringList(ListOfP);
addProjectEditText.setText("");
}
});
getActivity().setTitle("DearBear Notes App");
return view;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, ListOfP);
setListAdapter(adapter);
getListView().setOnItemClickListener(this);
getListView().setOnItemLongClickListener(this);
}
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l){
MainProjectLister frag = new MainProjectLister();
ProjectListFragmentChanger fragInterface = (ProjectListFragmentChanger) getActivity();
fragInterface.projectListChangeFragment(frag);
projektSingleton.setProjectNumber(i);
}
@Override
public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l){
projektArrayList = projektSingleton.getProjectList();
projektArrayList.remove(i);
ListOfP.remove(i);
adapter.notifyDataSetChanged();
saveArrayList(projektArrayList);
saveStringList(ListOfP);
return true;
}
//Interface
public interface ProjectListFragmentChanger{
public void projectListChangeFragment(Fragment fragment);
}
//Save File Method
public void saveArrayList(ArrayList<Projekt> arrayList) {
try {
FileOutputStream fos = getActivity().openFileOutput(FILENAME, Context.MODE_PRIVATE);
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(arrayList);
out.close();
fos.close();
} catch (IOException e) {
e.printStackTrace ();
}
}
public void saveStringList(ArrayList<String> arrayList) {
try {
FileOutputStream fos = getActivity().openFileOutput(FILENAME2, Context.MODE_PRIVATE);
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(arrayList);
out.close();
fos.close();
} catch (IOException e) {
e.printStackTrace ();
}
}
//Load Text File Method
public ArrayList<Projekt> getSavedArrayList() {
ArrayList<Projekt> savedArrayList = null;
try {
FileInputStream fis = getActivity().openFileInput(FILENAME);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<Projekt>) ois.readObject();
ois.close();
fis.close();
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}
public ArrayList<String> getSavedStringList() {
ArrayList<String> savedArrayList = null;
try {
FileInputStream fis = getActivity().openFileInput(FILENAME2);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<String>) ois.readObject();
ois.close();
fis.close();
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}
}
错误：
$ adb shell am start -n "com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER
Client not ready yet..Waiting for process to come online
Connected to process 2394 on device Nexus_5_API_24 [emulator-5554]
I/art: Not late-enabling -Xcheck:jni (already on)
W/art: Unexpected CPU variant for X86 using defaults: x86
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86
I/InstantRun: Instant Run Runtime started. Android package is com.example.lenovo.projectlist, real application class is null.
I/art: Debugger is no longer active
I/art: Starting a blocking GC Instrumentation
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86
W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile15.txt (No such file or directory)
W/System.err: at java.io.FileInputStream.open(Native Method)
W/System.err: at java.io.FileInputStream.<init>(FileInputStream.java:146)
W/System.err: at android.app.ContextImpl.openFileInput(ContextImpl.java:485)
W/System.err: at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.getSavedArrayList(MainProjectLister.java:189)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:53)
W/System.err: at android.app.Fragment.performCreate(Fragment.java:2335)
W/System.err: at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949)
W/System.err: at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857)
W/System.err: at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880)
W/System.err: at android.app.BackStackRecord.run(BackStackRecord.java:727)
W/System.err: at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
W/System.err: at android.app.FragmentController.execPendingActions(FragmentController.java:371)
W/System.err: at android.app.Activity.performStart(Activity.java:6680)
W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
W/System.err: at android.app.ActivityThread.-wrap12(ActivityThread.java)
W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err: at android.os.Looper.loop(Looper.java:154)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:6077)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile16.txt (No such file or directory)
W/System.err: at java.io.FileInputStream.open(Native Method)
W/System.err: at java.io.FileInputStream.<init>(FileInputStream.java:146)
W/System.err: at android.app.ContextImpl.openFileInput(ContextImpl.java:485)
W/System.err: at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.getSavedStringList(MainProjectLister.java:208)
W/System.err: at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:54)
W/System.err: at android.app.Fragment.performCreate(Fragment.java:2335)
W/System.err: at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949)
W/System.err: at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857)
W/System.err: at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880)
W/System.err: at android.app.BackStackRecord.run(BackStackRecord.java:727)
W/System.err: at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
W/System.err: at android.app.FragmentController.execPendingActions(FragmentController.java:371)
W/System.err: at android.app.Activity.performStart(Activity.java:6680)
W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
W/System.err: at android.app.ActivityThread.-wrap12(ActivityThread.java)
W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err: at android.os.Looper.loop(Looper.java:154)
W/System.err: at android.app.ActivityThread.main(ActivityThread.java:6077)
W/System.err: at java.lang.reflect.Method.invoke(Native Method)
W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
D/AndroidRuntime: Shutting down VM
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.lenovo.projectlist, PID: 2394
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:344)
at android.widget.ListView.setAdapter(ListView.java:493)
at android.app.ListFragment.setListAdapter(ListFragment.java:240)
at com.example.lenovo.projectlist.MainProjectLister.onActivityCreated(MainProjectLister.java:111)
at android.app.Fragment.performActivityCreated(Fragment.java:2361)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1014)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1171)
at android.app.BackStackRecord.run(BackStackRecord.java:815)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578)
at android.app.FragmentController.execPendingActions(FragmentController.java:371)
at android.app.Activity.performStart(Activity.java:6680)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
Application terminated.
第一个错误发生是因为您在尚未创建的文件上调用 openFileInput()。但是由于您捕获了该错误，getSavedArrayList() returns null 并将其分配给 ListOfP 导致 NullPointerException 当您尝试将 null 分配给 ArrayAdapter 时。
为防止出现这种情况，您应该先检查该文件是否存在，然后将您的列表分配给一个空列表以防止这两种错误；像这样：
public ArrayList<Projekt> getSavedArrayList() {
ArrayList<Projekt> savedArrayList = null;
try {
// get the file and check if it exists before trying to read it
File savedFile = new File(getActivity().getFilesDir(), FILENAME);
if (savedFile.exists()) {
// if the file containing the list exists, read it and return the list
FileInputStream fis = getActivity().openFileInput(FILENAME);
ObjectInputStream ois = new ObjectInputStream(fis);
savedArrayList = (ArrayList<Projekt>) ois.readObject();
ois.close();
fis.close();
} else {
// if it doesn't exist, return an empty list
savedArrayList = new ArrayList<>();
}
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace ();
}
return savedArrayList;
}