Java 编写arrayList<；类别>；归档

我是android应用程序开发新手，我曾尝试将arrayList写入文件，但在代码中尝试这样做时遇到了问题：

public class AddActivity extends Activity implements Serializable {

/**
 * 
 */
private static final long serialVersionUID = 1L;
public transient Context ctx;
private final String filename = "Accounts";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add);

    final EditText name = (EditText) findViewById(R.id.name);
    final EditText username = (EditText) findViewById(R.id.username);
    final EditText password = (EditText) findViewById(R.id.password);
    final EditText type = (EditText) findViewById(R.id.type);
    Button add = (Button) findViewById(R.id.add);

    add.setOnClickListener(new View.OnClickListener() {
        @SuppressWarnings("unchecked")
        public void onClick(View arg0) {
            String n = name.getText().toString();
            String u = username.getText().toString();
            String p = password.getText().toString();
            String t = type.getText().toString();
            Account newAccount = new Account (n, u , p, t);
            ArrayList<Account> accounts;
            String filePath = getFilesDir().getPath().toString() + "/" + filename;
            File fl = new File(filePath);
            try {
                if (fl.exists()){
                    FileInputStream fis = ctx.openFileInput(filename);
                    ObjectInputStream oi = new ObjectInputStream(fis);
                    accounts = (ArrayList<Account>) oi.readObject();        
                    accounts.add(newAccount);
                    oi.close();
                    fis.close();
                    fl.delete();
                }else{
                    accounts = new ArrayList<Account>();
                    accounts.add(newAccount);
                }
                fl.createNewFile();
                FileOutputStream fos = (openFileOutput(filename, Context.MODE_PRIVATE));
                ObjectOutputStream oos = new ObjectOutputStream(fos);
                oos.writeObject(accounts);
                oos.flush();
                oos.close();
                fos.close();

            } catch (IOException e) {
                Log.e("InternalStorage", e.getMessage()); 
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }

        }


    });

}

}

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每次单击“添加”按钮，我都会收到一个错误，上面写着“很遗憾，（应用程序名称）已停止”。请帮忙。谢谢。

我想您也可以尝试以下代码将列表写入文件：

File myfile = getFileStreamPath(FileName);
    try {
        if (myfile.exists() || myfile.createNewFile()) {
            FileOutputStream fos = openFileOutput(FileName, MODE_PRIVATE);
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(masterlistrev);
            fos.close();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

---

logcat中对此异常的描述是什么？请发布堆栈跟踪如果应用程序崩溃，这意味着您有一个堆栈跟踪。请用你的问题贴出来，不要让我们猜测错误是什么。第56行的NPE是哪一个？

File myfile = getFileStreamPath(FileName);
    try {
        if (myfile.exists() || myfile.createNewFile()) {
            FileOutputStream fos = openFileOutput(FileName, MODE_PRIVATE);
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(masterlistrev);
            fos.close();
        }
    } catch (Exception e) {
        e.printStackTrace();
    }