變更到Android4.4的問題

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更新到Android 4.4，寫了個小程式。發現運行不起來了。拋null 指標異常。debug模式下，發現在onCreate方法中擷取Button是null。

Android 4.4把layout進行了重組，全部放到了Fragment下，在onCreate方法執行時，Fragment還沒有被調用，所以在這個方法中擷取Fragment中的Button是空值。新類中多了一個Fragment的內部類，當xml的Fragment被裝載時，會調用內部類的onCreateView方法，所以要將所有之前在layout中而現在移到fragment中的操作全部移到此方法中進行。

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;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_test);if (savedInstanceState == null) {getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();}}@Overridepublic 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;}@Overridepublic 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() {}@Overridepublic 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() {@Overridepublic 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() {@Overridepublic void onClick(View v) {Toast.makeText(rootView.getContext(), R.string.second_toast,Toast.LENGTH_SHORT).show();}});return rootView;}}}