我正在android中开发一个新闻应用程序。我有以下内容
News Item Fragment是应用程序中的第一页。它有一个列表视图，显示新闻项目。当用户点击列表视图中的项目时，它会打开第二页（News Itemdetails Fragment）。我希望在第二页处于活动状态时在工具栏上显示“向上”按钮图标，以便用户可以再次返回到第一页。为了显示工具栏中的UP按钮，我在'News Itemdetails Fragment'（第二页）中尝试了'setDisplayHomeAsUpEnabled'，但是当在'News Item Fragment'（第一页）中点击一个项目时，它会抛出错误。
我已提供以下详细信息供您参考。
新闻项目片段
package com.uss.navigationdrawer3;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;
public class HomeFragment extends Fragment {
// Listview Adapter
ArrayAdapter<String> adapter;
// Search EditText
EditText inputSearch;
public HomeFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view=inflater.inflate(R.layout.fragment_home, container, false);
String[] planets = new String[] { "News Item 1", "News Item 2", "News Item 3","News Item 4","News Item 5","News Item 6"
};
final ListView listView=(ListView) view.findViewById(R.id.list_view_fragment_home);
//adapter = new ArrayAdapter<String>(this, R.layout.list_item, R.id.product_name, planets);
adapter=new ArrayAdapter<String>(getActivity(),R.layout.list_item,R.id.product_name,planets);
//ArrayAdapter adapter = new ArrayAdapter(this,
// R.layout.list_item, R.id.product_name, planets);
// Assign adapter to ListView
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> arg0, View arg1,
int position, long arg3) {
// TODO Auto-generated method stub
int itemPosition = position;
// ListView Clicked item value
String itemValue = (String) listView.getItemAtPosition(position);
// Show Alert
Toast.makeText(getActivity(),
"Position :"+itemPosition+" ListItem : " +itemValue , Toast.LENGTH_LONG)
.show();
FragmentTransaction fragmentTransaction=getFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.main_container,new MessageFragment());
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();
//getSupportActionBar().setTitle("Message Fragment");
}
});
inputSearch = (EditText) view.findViewById(R.id.inputSearch);
inputSearch.addTextChangedListener(new TextWatcher() {
@Override
public void onTextChanged(CharSequence cs, int arg1, int arg2, int arg3) {
// When user changed the Text
HomeFragment.this.adapter.getFilter().filter(cs);
}
@Override
public void beforeTextChanged(CharSequence arg0, int arg1, int arg2,
int arg3) {
// TODO Auto-generated method stub
}
@Override
public void afterTextChanged(Editable arg0) {
// TODO Auto-generated method stub
}
}
);
// Inflate the layout for this fragment
return view;
}
}
新闻Itemdetails Fragment
package com.uss.navigationdrawer3;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
/**
* A simple {@link Fragment} subclass.
*/
public class MessageFragment extends Fragment{
Toolbar toolbar;
public MessageFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
//return inflater.inflate(R.layout.fragment_message, container, false);
View view=inflater.inflate(R.layout.fragment_message, container, false);
toolbar=(Toolbar)view.findViewById(R.id.toolbar);
AppCompatActivity activity = (AppCompatActivity) getActivity();
activity.setSupportActionBar(toolbar);
if (activity != null) {
ActionBar ab = activity.getSupportActionBar();
if (ab != null) {
ab.setDisplayHomeAsUpEnabled(true);
}
}
toolbar.setTitle("Title");
return view;
}
}
错误
12-14 12:06:37.675 2005-2005/com.uss.navigationdrawer3 D/AndroidRuntime﹕ Shutting down VM
--------- beginning of crash
12-14 12:06:37.675 2005-2005/com.uss.navigationdrawer3 E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.uss.navigationdrawer3, PID: 2005
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence android.support.v7.widget.Toolbar.getTitle()' on a null object reference
at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:94)
at android.support.v7.internal.widget.ToolbarWidgetWrapper.<init>(ToolbarWidgetWrapper.java:87)
at android.support.v7.internal.app.ToolbarActionBar.<init>(ToolbarActionBar.java:77)
at android.support.v7.app.AppCompatDelegateImplV7.setSupportActionBar(AppCompatDelegateImplV7.java:198)
at android.support.v7.app.AppCompatActivity.setSupportActionBar(AppCompatActivity.java:96)
at com.uss.navigationdrawer3.MessageFragment.onCreateView(MessageFragment.java:36)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:458)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
12-14 12:06:37.867 2005-2017/com.uss.navigationdrawer3 I/art﹕ Background sticky concurrent mark sweep GC freed 3409(281KB) AllocSpace objects, 0(0B) LOS objects, 18% free, 1831KB/2MB, paused 77.322ms total 183.824ms
12-14 12:11:37.853 2005-2005/com.uss.navigationdrawer3 I/Process﹕ Sending signal. PID: 2005 SIG: 9
请在工具栏中按下向上按钮时帮助如何从第二页进入第一页。
答案 0 :(得分：0)
似乎您已在活动xml中定义了工具栏，并尝试在片段中访问它，返回工具栏值