我正在尝试根据用户是否已注册在应用启动时更改menuitem的标题,但出现此错误:
Process: com.cloud.services, PID: 25186
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cloud.services/com.cloud.services.MainActivity}:
java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.Menu.findItem(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2345)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2405)
at android.app.ActivityThread.access$800(ActivityThread.java:149)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1324)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:211)
at android.app.ActivityThread.main(ActivityThread.java:5315)
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:941)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:736)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.Menu.findItem(int)' on a null object reference
at com.cloud.services.MainActivity.onCreate(MainActivity.java:35)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2405)
at android.app.ActivityThread.access$800(ActivityThread.java:149)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1324)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:211)
at android.app.ActivityThread.main(ActivityThread.java:5315)
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:941)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:736)
我的MainActivity代码:
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity";
private SectionsPageAdapter mSectionsPageAdapter;
private FirebaseAuth auth;
private Menu menu;
private ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
auth = FirebaseAuth.getInstance();
if (auth.getCurrentUser() != null) {
MenuItem item= menu.findItem(R.id.action_settings);
item.setTitle("LogOut");
}
setContentView(R.layout.activity_main);
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.loginmenu, menu);
return true;
}
//and this to handle actions
@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) {
if (item.getTitle().equals("Login\SignUp**strong text**")){
Intent intent4 = new Intent(MainActivity.this, LoginActivity.class);
startActivity(intent4);}
else{
auth.signOut();
}
return true;
}
return super.onOptionsItemSelected(item);
}
}
根据Android生命周期,在菜单膨胀之前首先调用onCreate()
。因此,在onCreate()
方法中引用菜单项将导致应用崩溃,因为菜单尚未膨胀。这就是为什么你得到这个:
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'android.view.MenuItem android.view.Menu.findItem(int)' on a null object reference
[要解决此问题,只需覆盖打开选项菜单时调用的onPrepareOptionsMenu(Menu menu)
,然后将登录逻辑检查移至该方法。像这样:
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
MenuItem item= menu.findItem(R.id.action_settings);
if (auth.getCurrentUser() != null) {
item.setTitle("LogOut");
}
return super.onPrepareOptionsMenu(menu);
}
Edit:另外,由于您未为其分配任何内容,因此menu
将为null。因此,对其调用findItem(id)
将使您的应用程序崩溃。从代码中删除全局menu
,因为在这种情况下添加onPrepareOptionsMenu(menu)
将使其变得多余。
如果您需要对此进行进一步的澄清,请在评论部分告知我。