在片段中需要帮助（Android）

我刚开始学习android。所以当我决定自己创建一个片段时，我正在做主题片段，以下是我编写的代码。但是，在运行代码时，仿真器显示“ fragment2已停止”。请查看我的代码，并告诉我我要去哪里错了。提前致谢。

1.碎片

package com.example.adi.fragment2;

import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import org.w3c.dom.Text;

public class MainActivity extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.activity_main,container,false);
        final EditText editText = (EditText)view.findViewById(R.id.editText);
        final TextView textView = (TextView)view.findViewById(R.id.textView);
        Button button = (Button) view.findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                textView.setText(editText.getText().toString());
            }
        });
        return view;
    }
}

2.主启动器活动

package com.example.adi.fragment2;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;



public class Launcher extends FragmentActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FragmentManager fm = getSupportFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.box);
        if(fragment==null) {
            fragment = new MainActivity();
            fm.beginTransaction().add(R.id.box, fragment).commit();
        }
    }
}

编辑：这是日志状态

01-07 06:05:26.423 2183-2183/? I/art: Late-enabling -Xcheck:jni
01-07 06:05:26.423 2183-2183/? W/art: Unexpected CPU variant for X86 using defaults: x86
01-07 06:05:26.550 2183-2183/com.example.adi.fragment2 W/System: ClassLoader referenced unknown path: /data/app/com.example.adi.fragment2-2/lib/x86
01-07 06:05:26.573 2183-2183/com.example.adi.fragment2 I/InstantRun: Instant Run Runtime started. Android package is com.example.adi.fragment2, real application class is null.
01-07 06:05:26.944 2183-2183/com.example.adi.fragment2 W/System: ClassLoader referenced unknown path: /data/app/com.example.adi.fragment2-2/lib/x86
01-07 06:05:26.985 2183-2183/com.example.adi.fragment2 D/AndroidRuntime: Shutting down VM
01-07 06:05:26.986 2183-2183/com.example.adi.fragment2 E/AndroidRuntime: FATAL EXCEPTION: main
             Process: com.example.adi.fragment2, PID: 2183
             java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.adi.fragment2/com.example.adi.fragment2.MainActivity}: java.lang.ClassCastException: com.example.adi.fragment2.MainActivity cannot be cast to android.app.Activity
                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                 at android.app.ActivityThread.-wrap12(ActivityThread.java)
                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                 at android.os.Handler.dispatchMessage(Handler.java:102)
                 at android.os.Looper.loop(Looper.java:154)
                 at android.app.ActivityThread.main(ActivityThread.java:6077)
                 at java.lang.reflect.Method.invoke(Native Method)
                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
              Caused by: java.lang.ClassCastException: com.example.adi.fragment2.MainActivity cannot be cast to android.app.Activity
                 at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                 at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                 at android.os.Looper.loop(Looper.java:154) 
                 at android.app.ActivityThread.main(ActivityThread.java:6077) 
                 at java.lang.reflect.Method.invoke(Native Method) 
                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)