我的应用程序具有主要活动，注册活动，登录活动和主要活动2，我想在其中使用底部导航活动
它在我创建新内容时起作用，然后在通知片段中进行了一些修改之后，所以知道当我oggin用户应重定向2 mainactivity2但未重定向
时，会发生此运行时错误Mainacyivity2 xml代码
package com.example.myapplication;
import android.os.Bundle;
import com.google.android.material.bottomnavigation.BottomNavigationView;
import androidx.appcompat.app.AppCompatActivity;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
public class Main2Activity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
BottomNavigationView navView = findViewById(R.id.nav_view);
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications)
.build();
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
NavigationUI.setupWithNavController(navView, navController);
}
}
Mainactivity2 java code
package com.example.myapplication;
import android.os.Bundle;
import com.google.android.material.bottomnavigation.BottomNavigationView;
import androidx.appcompat.app.AppCompatActivity;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
public class Main2Activity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
BottomNavigationView navView = findViewById(R.id.nav_view);
// Passing each menu ID as a set of Ids because each
// menu should be considered as top level destinations.
AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications)
.build();
NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
NavigationUI.setupWithNavController(navView, navController);
}
}
这是我的LoginActivity
package com.example.myapplication;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import com.example.myapplication.Model.Users;
import com.example.myapplication.Prevalent.Prevalent;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;
import com.rey.material.widget.CheckBox;
import io.paperdb.Paper;
public class Login extends AppCompatActivity
{
private EditText inputphone, inputpassword;
private Button LoginButton;
private ProgressBar pb;
private String ParentDatabaseName= "Users";
private CheckBox chkBowRememberme;
private TextView Admin,NotAdmin;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
LoginButton= (Button) findViewById(R.id.loginbutton);
inputphone = (EditText) findViewById(R.id.phno);
inputpassword = (EditText) findViewById(R.id.password2);
LoginButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
LoginUser();
}
});
Admin = (TextView) findViewById(R.id.admin);
NotAdmin = (TextView) findViewById(R.id.Notadmin);
chkBowRememberme = (CheckBox) findViewById(R.id.checkbox);
Paper.init(this);
Admin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
LoginButton.setText("Login Admin");
Admin.setVisibility(View.INVISIBLE);
NotAdmin.setVisibility(View.VISIBLE);
ParentDatabaseName = "Admin";
}
});
NotAdmin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
LoginButton.setText("Login");
NotAdmin.setVisibility(View.INVISIBLE);
Admin.setVisibility(View.VISIBLE);
ParentDatabaseName = "Users";
}
});
}
private void LoginUser()
{
String l_phno= inputphone.getText().toString();
String l_password= inputpassword.getText().toString();
if (TextUtils.isEmpty(l_phno))
{
Toast.makeText(this, "Write Phone No", Toast.LENGTH_SHORT).show();
}
else if (TextUtils.isEmpty(l_password))
{
Toast.makeText(this, "Write Your Password", Toast.LENGTH_SHORT).show();
}
else
{
Allow_User(l_phno,l_password);
}
}
private void Allow_User(final String l_phno, final String l_password)
{
if (chkBowRememberme.isChecked())
{
Paper.book().write(Prevalent.UserPhoneKey, l_phno);
Paper.book().write(Prevalent.UserPasswordKey, l_password);
}
final DatabaseReference RootRef;
RootRef = FirebaseDatabase.getInstance().getReference();
RootRef.addListenerForSingleValueEvent(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot)
{
if (dataSnapshot.child(ParentDatabaseName).child(l_phno).exists())
{
Users userData = dataSnapshot.child(ParentDatabaseName).child(l_phno).getValue(Users.class);
if (userData.getPhone().equals(l_phno))
{
if (userData.getPassword().equals(l_password))
{
if (ParentDatabaseName.equals("Admin"))
{
Toast.makeText(Login.this, "Logged in ", Toast.LENGTH_SHORT).show();
Intent intent = new Intent ( Login.this, Main2Activity.class);
startActivity(intent);
}
else if (ParentDatabaseName.equals("Users"))
{
Toast.makeText(Login.this, "Logged in ", Toast.LENGTH_SHORT).show();
Intent intent = new Intent ( Login.this, Main2Activity.class);
startActivity(intent);
}
}
else
{
Toast.makeText(Login.this, "Password is incorect ", Toast.LENGTH_SHORT).show();
}
}
}
else
{
Toast.makeText(Login.this, "Create Account With This" + l_phno + "Number", Toast.LENGTH_SHORT).show();
}
}
@Override
public void onCancelled(@NonNull DatabaseError databaseError)
{
}
});
}
}
Eror
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 23908
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.Main2Activity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:201)
at android.app.ActivityThread.main(ActivityThread.java:6810)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
at androidx.navigation.ui.ActionBarOnDestinationChangedListener.setTitle(ActionBarOnDestinationChangedListener.java:48)
at androidx.navigation.ui.AbstractAppBarOnDestinationChangedListener.onDestinationChanged(AbstractAppBarOnDestinationChangedListener.java:100)
at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:218)
at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:220)
at com.example.myapplication.Main2Activity.onCreate(Main2Activity.java:26)
at android.app.Activity.performCreate(Activity.java:7224)
at android.app.Activity.performCreate(Activity.java:7213)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:201)
at android.app.ActivityThread.main(ActivityThread.java:6810)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)