java - 当我在 Activity 中调用构造函数时应用程序崩溃

Question

我是 android studio 的新手，我正在尝试构建一个记事本应用程序。

 button.setOnClickListener(new View.OnClickListener() 
 {
   @Override
     public void onClick(View v) {
     String title = findViewById(R.id.textView3).toString();
     String note_content = findViewById(R.id.textView).toString();

     FileOutputStream outputStream;

     try 
       {
           outputStream = openFileOutput(title, Context.MODE_PRIVATE);
           outputStream.write(note_content.getBytes());
       } 
      catch (FileNotFoundException e) 
       {
           e.printStackTrace();
       } 
      catch (IOException e) 
       {
           e.printStackTrace();
       }
          MainActivity mainActivity = new MainActivity(title);
    }
    });

这是用户单击以保存注释的按钮。代码保存注释后，它应该将标题发送到 MainActivity，以便可以将其发送到 Recycleview 适配器 - 这将在 View 保持器中将其显示为文本。

目前，当我运行代码时，它会崩溃 - 但是，当我删除构造函数时，应用程序运行正常。 (MainActivity mainActivity = new MainActivity(title);)

Error:
     10-02 02:39:13.822 27279-27279/? D/AndroidRuntime: Shutting down VM
10-02 02:39:13.824 27279-27279/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.quicknote, PID: 27279
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.quicknote/com.example.quicknote.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.quicknote.MainActivity> has no zero argument constructor
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2337)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5443)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
     Caused by: java.lang.InstantiationException: java.lang.Class<com.example.quicknote.MainActivity> has no zero argument constructor
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1090)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2327)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
        at android.app.ActivityThread.-wrap11(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:148) 
        at android.app.ActivityThread.main(ActivityThread.java:5443) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
10-02 02:39:20.081 27279-27279/com.example.quicknote I/Process: Sending signal. PID: 27279 SIG: 9

Answer 1

根据错误消息:

java.lang.InstantiationException: java.lang.Class has no zero argument constructor

Android 系统实例化 Activity 需要零参数构造函数。您永远不应该自己手动调用 Activity 构造函数，因为只有系统才能正确创建 Activity。

Parcelables and Bundles documentation详细介绍了使用额外的 Bundle 将信息发送到 Activity 的正确方法。