变更到Android4.4的问题

更新到Android 4.4，写了个小程序。发现运行不起来了。抛空指针异常。debug模式下，发现在onCreate方法中获取Button是null。

Android 4.4把layout进行了重组，全部放到了Fragment下，在onCreate方法执行时，Fragment还没有被调用，所以在这个方法中获取Fragment中的Button是空值。新类中多了一个Fragment的内部类，当xml的Fragment被装载时，会调用内部类的onCreateView方法，所以要将所有之前在layout中而现在移到fragment中的操作全部移到此方法中进行。

package com.bignerdranch.android.geoquiz;

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

public class TestActivity extends ActionBarActivity {

	private static Button mFirstButton;
	private static Button mSecondButton;

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

		if (savedInstanceState == null) {
			getSupportFragmentManager().beginTransaction()
					.add(R.id.container, new PlaceholderFragment()).commit();
		}
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {

		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.test, menu);
		return true;
	}

	@Override
	public boolean onOptionsItemSelected(MenuItem item) {
		// Handle action bar item clicks here. The action bar will
		// automatically handle clicks on the Home/Up button, so long
		// as you specify a parent activity in AndroidManifest.xml.
		int id = item.getItemId();
		if (id == R.id.action_settings) {
			return true;
		}
		return super.onOptionsItemSelected(item);
	}

	/**
	 * 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) {
			final View rootView = inflater.inflate(R.layout.fragment_test, container,
					false);
			
			mFirstButton = (Button) rootView.findViewById(R.id.first_button);
			mFirstButton.setOnClickListener(new View.OnClickListener() {

				@Override
				public void onClick(View v) {
					Toast.makeText(rootView.getContext(), R.string.first_toast,
							Toast.LENGTH_SHORT).show();
				}

			});
			
			mSecondButton = (Button) rootView.findViewById(R.id.second_button);
			mSecondButton.setOnClickListener(new View.OnClickListener() {

				@Override
				public void onClick(View v) {
					Toast.makeText(rootView.getContext(), R.string.second_toast,
							Toast.LENGTH_SHORT).show();
				}

			});
			
			return rootView;
		}
	}

}