getFileDir（）导致NullPointer异常

Question

我在学习Java的早期阶段，但希望有人能够解释我在这里做错了什么，许多谷歌搜索让我头疼。

我一直在学习如何将ArrayList保存到文本文件然后获取详细信息的教程。

我收到以下错误： -

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 

这是在我的文件viewLocations.java运行以下（导入等之后）时引起的

 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（）; 位于一个名为'fileControl.java'的独立文件中

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;
    }

Answer 1

这里有一堆错误。 首先， Activity类，或者在你的情况下， AppCompatActivity类通常用于呈现带有UI的屏幕，而不是执行谨慎的任务。 你的getSaved()函数可能只是驻留在你的UI实际存在的Activity中。

其次， AppCompatActivity类无法静态执行上下文操作。 换句话说，在使用超类方法之前，需要先实例化Activity，在本例中为getFilesDir() 。

归结为你需要在生命周期函数中初始化filename ，可能是onCreate()