java - 应用程序在启动时崩溃，尝试查看登录屏幕是否存在文件

Question

我正在创建一个类，用于检查文件是否已创建(有用户名和密码)。如果已创建，它会创建一个 Intent 去另一个类读取数据并通过服务器检查它FTP。由于某种原因，我无法让它工作，我已经尝试了一切并阅读了我能阅读的每个网页，但没有运气。

我的代码:

public class LogIn extends Activity implements OnClickListener {
Button send;
EditText user;
EditText pass;
CheckBox staySignedIn;
FileOutputStream Fos;
String a;
String b;
String string = a;
String string2 = b;

String FILENAME = "userandpass";

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.login);
    send = (Button) findViewById(R.id.bLogIn);
    user = (EditText) findViewById(R.id.eTuser);
    pass = (EditText) findViewById(R.id.eTpassword);
    staySignedIn = (CheckBox) findViewById(R.id.Cbstay);
    send.setOnClickListener(this);

    if (staySignedIn.isChecked()) {

        String a = user.getText().toString();
        String b = pass.getText().toString();
        File f = new File(FILENAME);
        try {
            Fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
            Fos.write(string.getBytes());
            Fos.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        File file = getBaseContext().getFileStreamPath(FILENAME);
        if(file.exists());
        Intent i = new Intent(LogIn.this, ChatService.class);
    }

}

public void onClick(View v) {
    // TODO Auto-generated method stub
    switch (v.getId()) {
    case R.id.bLogIn:
        if (pass.length() == 0)
            Toast.makeText(this,
                    "Try to type in your username and password again!",
                    Toast.LENGTH_LONG).show();
        else if (user.length() == 0)
            Toast.makeText(this,
                    "Try to type in your username and password again!",
                    Toast.LENGTH_LONG).show();
        else {

            String u = user.getText().toString();
            String p = pass.getText().toString();
            Bundle send = new Bundle();
            send.putString("key", u);
            send.putString("key1", p);
            Intent a = new Intent(LogIn.this, logincheck.class);
            a.putExtra("key", u);
            a.putExtra("key1", p);
            startActivity(a);
            Toast.makeText(this, "Were signing you in!", Toast.LENGTH_LONG)
                    .show();
            break;
        }
    }

}
}

日志猫:

 01-19 11:37:17.601: W/dalvikvm(4411): threadid=1: thread exiting with uncaught  exception (group=0x4001d800)
    01-19 11:37:17.621: E/AndroidRuntime(4411): FATAL EXCEPTION: main
    01-19 11:37:17.621: E/AndroidRuntime(4411): java.lang.RuntimeException: Unable to start  activity ComponentInfo{com.gta5news.bananaphone/com.gta5news.bananaphone.LogIn}:  java.lang.NullPointerException
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.os.Handler.dispatchMessage(Handler.java:99)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.os.Looper.loop(Looper.java:123)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread.main(ActivityThread.java:4627)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at java.lang.reflect.Method.invokeNative(Native Method)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at java.lang.reflect.Method.invoke(Method.java:521)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
   01-19 11:37:17.621: E/AndroidRuntime(4411):    
    at dalvik.system.NativeStart.main(Native  Method)
    01-19 11:37:17.621: E/AndroidRuntime(4411): Caused by: java.lang.NullPointerException
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at  com.gta5news.bananaphone.LogIn.onCreate(LogIn.java:55)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    01-19 11:37:17.621: E/AndroidRuntime(4411):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)

Answer 1

如果第 55 行是

Fos.write(string.getBytes());

那么 Fos 或 string 未初始化。鉴于 string 被初始化为 a ，而它本身尚未初始化，这就解释了这一点。您需要为string分配一个正确的值。