無法啟動活動ComponentInfo java.lang.NullPointerException文件瀏覽器

Question

我已經從教程中復制了一個類，該類會打開一個對話框以選擇文件，但我不知道為什么它會引發此異常，有人可以幫助我嗎？

這是課程：

import android.app.Activity;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Environment;
import android.util.Log;

import java.io.File;
import java.io.FilenameFilter;
import java.util.ArrayList;
import java.util.List;

public class FileDialog {
    private static final String PARENT_DIR = "..";
    private final String TAG = getClass().getName();
    private String[] fileList;
    private File currentPath;
    public interface FileSelectedListener {
        void fileSelected(File file);
    }
    public interface DirectorySelectedListener {
        void directorySelected(File directory);
    }
    private ListenerList<FileSelectedListener> fileListenerList = new ListenerList<FileDialog.FileSelectedListener>();
    private ListenerList<DirectorySelectedListener> dirListenerList = new ListenerList<FileDialog.DirectorySelectedListener>();
    private final Activity activity;
    private boolean selectDirectoryOption;
    private String fileEndsWith;

    public FileDialog(Activity activity, File path) {
        this.activity = activity;
        if (!path.exists()) path = Environment.getExternalStorageDirectory();
        loadFileList(path);
    }

    public Dialog createFileDialog() {
        Dialog dialog = null;
        AlertDialog.Builder builder = new AlertDialog.Builder(activity);

        builder.setTitle(currentPath.getPath());
        if (selectDirectoryOption) {
            builder.setPositiveButton("Select directory", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                    Log.d(TAG, currentPath.getPath());
                    fireDirectorySelectedEvent(currentPath);
                }
            });
        }

        builder.setItems(fileList, new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
                String fileChosen = fileList[which];
                File chosenFile = getChosenFile(fileChosen);
                if (chosenFile.isDirectory()) {
                    loadFileList(chosenFile);
                    dialog.cancel();
                    dialog.dismiss();
                    showDialog();
                } else fireFileSelectedEvent(chosenFile);
            }
        });

        dialog = builder.show();
        return dialog;
    }


    public void addFileListener(FileSelectedListener listener) {
        fileListenerList.add(listener);
    }

    public void removeFileListener(FileSelectedListener listener) {
        fileListenerList.remove(listener);
    }

    public void setSelectDirectoryOption(boolean selectDirectoryOption) {
        this.selectDirectoryOption = selectDirectoryOption;
    }

    public void addDirectoryListener(DirectorySelectedListener listener) {
        dirListenerList.add(listener);
    }

    public void removeDirectoryListener(DirectorySelectedListener listener) {
        dirListenerList.remove(listener);
    }

    /**
     * Show file dialog
     */
    public void showDialog() {
        createFileDialog().show();
    }

    private void fireFileSelectedEvent(final File file) {
        fileListenerList.fireEvent(new ListenerList.FireHandler<FileSelectedListener>() {
            public void fireEvent(FileSelectedListener listener) {
                listener.fileSelected(file);
            }
        });
    }

    private void fireDirectorySelectedEvent(final File directory) {
        dirListenerList.fireEvent(new ListenerList.FireHandler<DirectorySelectedListener>() {
            public void fireEvent(DirectorySelectedListener listener) {
                listener.directorySelected(directory);
            }
        });
    }

    private void loadFileList(File path) {
        this.currentPath = path;
        List<String> r = new ArrayList<String>();
        if (path.exists()) {
            if (path.getParentFile() != null) r.add(PARENT_DIR);
            FilenameFilter filter = new FilenameFilter() {
                public boolean accept(File dir, String filename) {
                    File sel = new File(dir, filename);
                    if (!sel.canRead()) return false;
                    if (selectDirectoryOption) return sel.isDirectory();
                    else {
                        boolean endsWith = fileEndsWith != null ? filename.toLowerCase().endsWith(fileEndsWith) : true;
                        return endsWith || sel.isDirectory();
                    }
                }
            };
            String[] fileList1 = path.list(filter);
            for (String file : fileList1) {
                r.add(file);
            }
        }
        fileList = (String[]) r.toArray(new String[]{});
    }

    private File getChosenFile(String fileChosen) {
        if (fileChosen.equals(PARENT_DIR)) return currentPath.getParentFile();
        else return new File(currentPath, fileChosen);
    }

    public void setFileEndsWith(String fileEndsWith) {
        this.fileEndsWith = fileEndsWith != null ? fileEndsWith.toLowerCase() : fileEndsWith;
    }
}

class ListenerList<L> {
    private List<L> listenerList = new ArrayList<L>();

    public interface FireHandler<L> {
        void fireEvent(L listener);
    }

    public void add(L listener) {
        listenerList.add(listener);
    }

    public void fireEvent(FireHandler<L> fireHandler) {
        List<L> copy = new ArrayList<L>(listenerList);
        for (L l : copy) {
            fireHandler.fireEvent(l);
        }
    }

    public void remove(L listener) {
        listenerList.remove(listener);
    }

    public List<L> getListenerList() {
        return listenerList;
    }
}

這是我在活動中稱呼它的地方：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    File mPath = new File(Environment.getExternalStorageDirectory()+ "//DIR//");
    fileDialog = new FileDialog(this, mPath);
    fileDialog.setFileEndsWith(".txt");
    fileDialog.addFileListener(new FileDialog.FileSelectedListener() {
        public void fileSelected(File file) {
            Log.d(getClass().getName(), "selected file " + file.toString());
        }
    });
    fileDialog.showDialog();
}

最后是我的堆棧跟蹤：

java.lang.RuntimeException: Unable to start activity ComponentInfo{MainActivity}                                                                                                             java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2200)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2250)
        at android.app.ActivityThread.access$800(ActivityThread.java:139)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1200)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5105)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:608)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at rpstudios.com.FileDialog.loadFileList(FileDialog.java:130)
        at rpstudios.com.FileDialog.<init>(FileDialog.java:35)
        at rpstudios.com.MainActivity.onCreate(MainActivity.java:22)

任何建議都歡迎

編輯：NullPointerException在行130： for(String file : fileList1)在loadFileList()方法中。

Answer 1

我認為在公共對話框createFileDialog（）中，您缺少create（）

dialog = builder.show();

應該

dialog = builder.create().show();

我們試試吧 ;）