尝试打开文件夹时使用 Android Studio 创建的应用程序崩溃

Question

我在 Android Studio 中收到一条错误消息，指出： java.lang.NullPointerException：尝试获取空数组的长度 代码在这里：

public class Main3Activity extends AppCompatActivity {

Button buttonOpenDialog;
Button buttonUp;
TextView textFolder;

String KEY_TEXTPSS ="TEXTPSS";
static final int CUSTOM_DIALOG_ID = 0;
ListView dialog_ListView;

File root;
File curFolder;

private List<String> fileList = new ArrayList<String>();
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main3);

    buttonOpenDialog = (Button) findViewById(R.id.findMusic);
    buttonOpenDialog.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            showDialog(CUSTOM_DIALOG_ID);
        }
    });

    root = new File(Environment.getExternalStorageDirectory().getAbsolutePath());
    curFolder = root;


}
@Override
protected Dialog onCreateDialog(int id) {

    Dialog dialog = null;

    switch (id) {
        case CUSTOM_DIALOG_ID:
            dialog = new Dialog(Main3Activity.this);
            dialog.setContentView(R.layout.dialoguelayout);
            dialog.setTitle("Custom Dialog");
            dialog.setCancelable(true);
            dialog.setCanceledOnTouchOutside(true);

            textFolder = (TextView) dialog.findViewById(R.id.folder);
            buttonUp = (Button) dialog.findViewById(R.id.up);
            buttonUp.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    ListDir(curFolder.getParentFile());
                }
            });

            dialog_ListView = (ListView) dialog.findViewById(R.id.dialoglist);
            dialog_ListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                    File selected = new File(fileList.get(position));
                    if(selected.isDirectory()) {
                        ListDir(selected);
                    } else {
                        Toast.makeText(Main3Activity.this, selected.toString() + " selected",
                                Toast.LENGTH_LONG).show();
                        dismissDialog(CUSTOM_DIALOG_ID);
                    }
                }
            });

            break;
    }
    return dialog;
}

@Override
protected void onPrepareDialog(int id, Dialog dialog) {
    super.onPrepareDialog(id, dialog);
    switch (id) {
        case CUSTOM_DIALOG_ID:
            ListDir(curFolder);
            break;
    }
}

void ListDir(File f) {
    if(f.equals(root)) {
        buttonUp.setEnabled(false);
    } else {
        buttonUp.setEnabled(true);
    }

    curFolder = f;
    textFolder.setText(f.getPath());

    File[] files = f.listFiles();
    fileList.clear();

    for(File file : files) {
        fileList.add(file.getPath());

    }

    ArrayAdapter<String> directoryList = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, fileList);
    dialog_ListView.setAdapter(directoryList);
}
}

这是错误：

08-16 15:20:05.085 30981-30981/com.example.letwixtor.myapplication2 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                  Process: com.example.letwixtor.myapplication2, PID: 30981
                                                                                  java.lang.NullPointerException: Attempt to get length of null array
                                                                                      at com.example.letwixtor.myapplication2.Main3Activity.ListDir(Main3Activity.java:117)
                                                                                      at com.example.letwixtor.myapplication2.Main3Activity.onPrepareDialog(Main3Activity.java:99)
                                                                                      at android.app.Activity.onPrepareDialog(Activity.java:3380)
                                                                                      at android.app.Activity.showDialog(Activity.java:3444)
                                                                                      at android.app.Activity.showDialog(Activity.java:3394)
                                                                                      at com.example.letwixtor.myapplication2.Main3Activity$1.onClick(Main3Activity.java:43)
                                                                                      at android.view.View.performClick(View.java:5198)
                                                                                      at android.view.View$PerformClick.run(View.java:21147)
                                                                                      at android.os.Handler.handleCallback(Handler.java:739)
                                                                                      at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                      at android.os.Looper.loop(Looper.java:148)
                                                                                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

我认为错误来自“listFiles”函数，因为它无法返回null（即它没有找到目录。）我不太明白为什么在root = new File(Environment.getExternalStorageDirectory().getAbsolutePath());之后找不到目录

运行模拟器和应用程序时，应用程序在尝试打开目录时崩溃。不知道如何解决这个问题。 提前致谢。

Answer 1

确保您在清单中添加了<uses-permission android:name="androi‌​d.permission.READ_EXT‌​ERNAL_STORAGE" />。