我的主要包含带有一些片段的菜单抽屉。然后在主页面之前显示主要内容。我对Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
我尝试更改getSupportActionBar().setDisplayHomeAsUpEnabled(true);
但public class MainActivity extends Activity {或public class MainActivity extends AppcompactActivity {
我的主要是：
package thyroid.com.thyroid;
import thyroid.com.thyroid.adapter.NavDrawerListAdapter;
import thyroid.com.thyroid.model.NavDrawerItem;
import java.util.ArrayList;
import android.support.v7.app.ActionBarActivity;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.content.res.Configuration;
import android.content.res.TypedArray;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBar;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.common.api.GoogleApiClient;
public class MainActivity extends Activity {
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private ActionBarDrawerToggle mDrawerToggle;
// nav drawer title
private CharSequence mDrawerTitle;
// used to store app title
private CharSequence mTitle;
// slide menu items
private String[] navMenuTitles;
private TypedArray navMenuIcons;
private ArrayList<NavDrawerItem> navDrawerItems;
private NavDrawerListAdapter adapter;
/**
* ATTENTION: This was auto-generated to implement the App Indexing API.
* See https://g.co/AppIndexing/AndroidStudio for more information.
*/
private GoogleApiClient client;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTitle = mDrawerTitle = getTitle();
// load slide menu items
navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items);
// nav drawer icons from resources
navMenuIcons = getResources()
.obtainTypedArray(R.array.nav_drawer_icons);
mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mDrawerList = (ListView) findViewById(R.id.list_slidermenu);
navDrawerItems = new ArrayList<NavDrawerItem>();
// adding nav drawer items to array
// Home
navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));
// Find People
navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
// Photos
navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));
// Communities, Will add a counter here
navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1), true, "22"));
// Pages
navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1)));
// What's hot, We will add a counter here
navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1), true, "50+"));
// Recycle the typed array
navMenuIcons.recycle();
mDrawerList.setOnItemClickListener(new SlideMenuClickListener());
// setting the nav drawer list adapter
adapter = new NavDrawerListAdapter(getApplicationContext(),
navDrawerItems);
mDrawerList.setAdapter(adapter);
// enabling action bar app icon and behaving it as toggle button
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
getSupportActionBar().setHomeButtonEnabled(true);
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
R.drawable.ic_drawer, //nav menu toggle icon
R.string.app_name, // nav drawer open - description for accessibility
R.string.app_name // nav drawer close - description for accessibility
) {
public void onDrawerClosed(View view) {
getSupportActionBar().setTitle(mTitle);
// calling onPrepareOptionsMenu() to show action bar icons
invalidateOptionsMenu();
}
public void onDrawerOpened(View drawerView) {
getSupportActionBar().setTitle(mDrawerTitle);
// calling onPrepareOptionsMenu() to hide action bar icons
invalidateOptionsMenu();
}
};
mDrawerLayout.setDrawerListener(mDrawerToggle);
if (savedInstanceState == null) {
// on first time display view for first nav item
displayView(0);
}
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
}
private ActionBar getSupportActionBar() {
return null;
}
@Override
public void onStart() {
super.onStart();
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
client.connect();
Action viewAction = Action.newAction(
Action.TYPE_VIEW, // TODO: choose an action type.
"Main Page", // TODO: Define a title for the content shown.
// TODO: If you have web page content that matches this app activity's content,
// make sure this auto-generated web page URL is correct.
// Otherwise, set the URL to null.
Uri.parse("http://host/path"),
// TODO: Make sure this auto-generated app URL is correct.
Uri.parse("android-app://thyroid.com.thyroid/http/host/path")
);
AppIndex.AppIndexApi.start(client, viewAction);
}
@Override
public void onStop() {
super.onStop();
// ATTENTION: This was auto-generated to implement the App Indexing API.
// See https://g.co/AppIndexing/AndroidStudio for more information.
Action viewAction = Action.newAction(
Action.TYPE_VIEW, // TODO: choose an action type.
"Main Page", // TODO: Define a title for the content shown.
// TODO: If you have web page content that matches this app activity's content,
// make sure this auto-generated web page URL is correct.
// Otherwise, set the URL to null.
Uri.parse("http://host/path"),
// TODO: Make sure this auto-generated app URL is correct.
Uri.parse("android-app://thyroid.com.thyroid/http/host/path")
);
AppIndex.AppIndexApi.end(client, viewAction);
client.disconnect();
}
/**
* Slide menu item click listener
*/
private class SlideMenuClickListener implements
ListView.OnItemClickListener {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id) {
// display view for selected nav drawer item
displayView(position);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// toggle nav drawer on selecting action bar app icon/title
if (mDrawerToggle.onOptionsItemSelected(item)) {
return true;
}
// Handle action bar actions click
switch (item.getItemId()) {
case R.id.action_settings:
return true;
default:
return super.onOptionsItemSelected(item);
}
}
/* *
* Called when invalidateOptionsMenu() is triggered
*/
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
// if nav drawer is opened, hide the action items
boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
return super.onPrepareOptionsMenu(menu);
}
/**
* Diplaying fragment view for selected nav drawer list item
*/
private void displayView(int position) {
// update the main content by replacing fragments
Fragment fragment = null;
switch (position) {
case 0:
fragment = new LoginFragment();
break;
case 1:
fragment = new RegisterFragment();
break;
case 2:
fragment = new LoginFragment();
break;
case 3:
fragment = new LoginFragment();
break;
case 4:
fragment = new LoginFragment();
break;
case 5:
fragment = new LoginFragment();
break;
default:
break;
}
if (fragment != null) {
FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
.replace(R.id.frame_container, fragment).commit();
// update selected item and title, then close the drawer
mDrawerList.setItemChecked(position, true);
mDrawerList.setSelection(position);
setTitle(navMenuTitles[position]);
mDrawerLayout.closeDrawer(mDrawerList);
} else {
// error in creating fragment
Log.e("MainActivity", "Error in creating fragment");
}
}
@Override
public void setTitle(CharSequence title) {
mTitle = title;
getSupportActionBar().setTitle(mTitle);
}
/**
* When using the ActionBarDrawerToggle, you must call it during
* onPostCreate() and onConfigurationChanged()...
*/
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
mDrawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
// Pass any configuration change to the drawer toggls
mDrawerToggle.onConfigurationChanged(newConfig);
}
}
这里有一个片段：
package thyroid.com.thyroid;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.design.widget.Snackbar;
import android.support.v7.widget.AppCompatButton;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
import thyroid.com.thyroid.model.ServerRequest;
import thyroid.com.thyroid.model.ServerResponse;
import thyroid.com.thyroid.model.User;
public class LoginFragment extends Fragment implements View.OnClickListener{
private AppCompatButton btn_login;
private EditText et_email,et_password;
private TextView tv_register,tv_reset_password;
private ProgressBar progress;
private SharedPreferences pref;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_login,container,false);
initViews(view);
return view;
}
private void initViews(View view){
pref = getActivity().getPreferences(0);
btn_login = (AppCompatButton)view.findViewById(R.id.btn_login);
tv_register = (TextView)view.findViewById(R.id.tv_register);
tv_reset_password = (TextView)view.findViewById(R.id.tv_reset_password);
et_email = (EditText)view.findViewById(R.id.et_email);
et_password = (EditText)view.findViewById(R.id.et_password);
progress = (ProgressBar)view.findViewById(R.id.progress);
btn_login.setOnClickListener(this);
tv_register.setOnClickListener(this);
tv_reset_password.setOnClickListener(this);
}
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.tv_register:
goToRegister();
break;
case R.id.btn_login:
String email = et_email.getText().toString();
String password = et_password.getText().toString();
if(!email.isEmpty() && !password.isEmpty()) {
progress.setVisibility(View.VISIBLE);
loginProcess(email,password);
} else {
Snackbar.make(getView(), "Fields are empty !", Snackbar.LENGTH_LONG).show();
}
break;
case R.id.tv_reset_password:
goToResetPassword();
break;
}
}
private void loginProcess(String email,String password){
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(Constants.BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build();
RequestInterface requestInterface = retrofit.create(RequestInterface.class);
User user = new User();
user.setEmail(email);
user.setPassword(password);
ServerRequest request = new ServerRequest();
request.setOperation(Constants.LOGIN_OPERATION);
request.setUser(user);
Call<ServerResponse> response = requestInterface.operation(request);
response.enqueue(new Callback<ServerResponse>() {
@Override
public void onResponse(Call<ServerResponse> call, retrofit2.Response<ServerResponse> response) {
ServerResponse resp = response.body();
Snackbar.make(getView(), resp.getMessage(), Snackbar.LENGTH_LONG).show();
if(resp.getResult().equals(Constants.SUCCESS)){
SharedPreferences.Editor editor = pref.edit();
editor.putBoolean(Constants.IS_LOGGED_IN,true);
editor.putString(Constants.EMAIL,resp.getUser().getEmail());
editor.putString(Constants.NAME,resp.getUser().getName());
editor.putString(Constants.UNIQUE_ID,resp.getUser().getUnique_id());
editor.apply();
goToProfile();
}
progress.setVisibility(View.INVISIBLE);
}
@Override
public void onFailure(Call<ServerResponse> call, Throwable t) {
progress.setVisibility(View.INVISIBLE);
Log.d(Constants.TAG,"failed");
Snackbar.make(getView(), t.getLocalizedMessage(), Snackbar.LENGTH_LONG).show();
}
});
}
private void goToResetPassword(){
Fragment reset = new Fragment();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.fragment_frame,reset);
ft.commit();
}
private void goToRegister(){
Fragment register = new RegisterFragment();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.fragment_frame,register);
ft.commit();
}
private void goToProfile(){
Fragment profile = new ProfileFragment();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.fragment_frame,profile);
ft.commit();
}
}
现在是.class，它是main之前的“主页”：
package thyroid.com.thyroid;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import static thyroid.com.thyroid.R.id.button;
public class SplashScreen extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.splashscreen);
Button button = (Button) findViewById(R.id.button);
assert button != null;
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(v.getContext(),MainActivity.class);
startActivityForResult(intent,0);
}
});
Thread timerThread = new Thread() {
public void run() {
try {
sleep(6000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
Intent intent = new Intent(SplashScreen.this, MainActivity.class);
startActivity(intent);
}
}
};
timerThread.start();
}
@Override
protected void onPause() {
// TODO Auto-generated method stub
super.onPause();
finish();
}
}
这是我的错误：
06-20 11:23:33.603 29939-29939/thyroid.com.thyroid E/AndroidRuntime: FATAL EXCEPTION: main
Process: thyroid.com.thyroid, PID: 29939
java.lang.RuntimeException: Unable to start activity ComponentInfo{thyroid.com.thyroid/thyroid.com.thyroid.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
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:898)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at thyroid.com.thyroid.MainActivity.onCreate(MainActivity.java:101)
at android.app.Activity.performCreate(Activity.java:5976)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
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:898)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
答案 0 :(得分：0)
在您的代码中
private ActionBar getSupportActionBar() {
return null;
}
因此，getSupportActionBar().setDisplayHomeAsUpEnabled(true)抛出了NPE。你需要实际实现那个方法......