Fragment中Button的android:onClick 无法监听相应

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.jjc.demo.MainActivity$PlaceholderFragment" >

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello_world"
        android:onClick="button" />

</RelativeLayout>

package com.jjc.demo;

import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

public class MainActivity extends ActionBarActivity {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }
    }
    
    public void button(View v){
        Toast.makeText(this, "Hello", Toast.LENGTH_SHORT).show();
    }
    
    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
        
        @Override
        public void onActivityCreated(Bundle savedInstanceState) {
            super.onActivityCreated(savedInstanceState);
            initView();
        }

        private void initView() {
            
        }
    }
}

    public void button(View v){
        Toast.makeText(this, "Hello", Toast.LENGTH_SHORT).show();
    }