android-studio - 我的应用程序在显示启动画面后无法启动另一个活动，并且显示此错误
问题描述
Activity我的应用程序在显示启动画面后无法启动另一个。这是我在运行我的应用程序时遇到的错误：
例外：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ref, PID: 7683
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ref/com.example.ref.LoginActivity}: 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:3449)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
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 com.example.ref.LoginActivity.onCreate(LoginActivity.java:46)
at android.app.Activity.performCreate(Activity.java:8000)
at android.app.Activity.performCreate(Activity.java:7984)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
I/Process: Sending signal. PID: 7683 SIG: 9
Disconnected from the target VM, address: 'localhost:49932', transport: 'socket'
SpalshScreen：
package com.example.ref;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.content.Intent;
import android.os.Handler;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
public class SpalshScreen extends AppCompatActivity {
FirebaseUser currentUser;
private FirebaseAuth mAuth;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_spalsh_screen2);
mAuth = FirebaseAuth.getInstance();
if (mAuth != null) {
currentUser = mAuth.getCurrentUser(); }
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
FirebaseUser user = mAuth.getCurrentUser();
if (user == null) {
Intent intent = new Intent(SpalshScreen.this, LoginActivity.class);
startActivity(intent);
finish();
} else {
Intent mainIntent = new Intent(SpalshScreen.this, DashboardActivity.class);
mainIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(mainIntent);
finish();
}
}
}, 1000);
}
}
LoginActivity:
package com.example.ref;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.text.InputType;
import android.util.Patterns;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;
import androidx.annotation.NonNull;
import androidx.appcompat.app.ActionBar;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import java.util.HashMap;
import java.util.Objects;
public class LoginActivity extends AppCompatActivity {
private EditText email, password, name;
private Button mlogin;
private TextView newdnewaccount, reocverpass;
FirebaseUser currentUser;
private ProgressDialog loadingBar;
private FirebaseAuth mAuth;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
ActionBar actionBar = getSupportActionBar();
actionBar.setTitle("Create Account");
actionBar.setDisplayShowHomeEnabled(true);
actionBar.setDisplayHomeAsUpEnabled(true);
// initialising the layout items
email = findViewById(R.id.login_email);
password = findViewById(R.id.login_password);
newdnewaccount = findViewById(R.id.needs_new_account);
reocverpass = findViewById(R.id.forgetp);
mAuth = FirebaseAuth.getInstance();
mlogin = findViewById(R.id.login_button);
loadingBar = new ProgressDialog(this);
mAuth = FirebaseAuth.getInstance();
// checking if user is null or not
if (mAuth != null) {
currentUser = mAuth.getCurrentUser();
}
mlogin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String emaill = email.getText().toString().trim();
String pass = password.getText().toString().trim();
// if format of email doesn't matches return null
if (!Patterns.EMAIL_ADDRESS.matcher(emaill).matches()) {
email.setError("Invalid Email");
email.setFocusable(true);
} else {
loginUser(emaill, pass);
}
}
});
// If new account then move to Registration Activity
newdnewaccount.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(LoginActivity.this, RegistrationActivity.class));
}
});
// Recover Your Password using email
reocverpass.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
showRecoverPasswordDialog();
}
});
}
private void showRecoverPasswordDialog() {
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setTitle("Recover Password");
LinearLayout linearLayout = new LinearLayout(this);
final EditText emailet = new EditText(this);//write your registered email
emailet.setText("Email");
emailet.setMinEms(16);
emailet.setInputType(InputType.TYPE_TEXT_VARIATION_EMAIL_ADDRESS);
linearLayout.addView(emailet);
linearLayout.setPadding(10, 10, 10, 10);
builder.setView(linearLayout);
builder.setPositiveButton("Recover", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
String emaill = emailet.getText().toString().trim();
beginRecovery(emaill);//send a mail on the mail to recover password
}
});
builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
}
});
builder.create().show();
}
private void beginRecovery(String emaill) {
loadingBar.setMessage("Sending Email....");
loadingBar.setCanceledOnTouchOutside(false);
loadingBar.show();
// send reset password email
mAuth.sendPasswordResetEmail(emaill).addOnCompleteListener(new OnCompleteListener<Void>() {
@Override
public void onComplete(@NonNull Task<Void> task) {
loadingBar.dismiss();
if (task.isSuccessful()) {
Toast.makeText(LoginActivity.this, "Done sent", Toast.LENGTH_LONG).show();
} else {
Toast.makeText(LoginActivity.this, "Error Occured", Toast.LENGTH_LONG).show();
}
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
loadingBar.dismiss();
Toast.makeText(LoginActivity.this, "Error Failed", Toast.LENGTH_LONG).show();
}
});
}
private void loginUser(String emaill, String pass) {
loadingBar.setMessage("Logging In....");
loadingBar.show();
// sign in with email and password after authenticating
mAuth.signInWithEmailAndPassword(emaill, pass).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
if (task.isSuccessful()) {
loadingBar.dismiss();
FirebaseUser user = mAuth.getCurrentUser();
if (task.getResult().getAdditionalUserInfo().isNewUser()) {
String email = user.getEmail();
String uid = user.getUid();
HashMap<Object, String> hashMap = new HashMap<>();
hashMap.put("email", email);
hashMap.put("uid", uid);
hashMap.put("name", "");
hashMap.put("onlineStatus", "online");
hashMap.put("typingTo", "noOne");
hashMap.put("phone", "");
hashMap.put("image", "");
hashMap.put("cover", "");
FirebaseDatabase database = FirebaseDatabase.getInstance();
// store the value in Database in "Users" Node
DatabaseReference reference = database.getReference("Users");
// storing the value in Firebase
reference.child(uid).setValue(hashMap);
}
Toast.makeText(LoginActivity.this, "Registered User " + user.getEmail(), Toast.LENGTH_LONG).show();
Intent mainIntent = new Intent(LoginActivity.this, DashboardActivity.class);
mainIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(mainIntent);
finish();
} else {
loadingBar.dismiss();
Toast.makeText(LoginActivity.this, "Login Failed", Toast.LENGTH_LONG).show();
}
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
loadingBar.dismiss();
Toast.makeText(LoginActivity.this, "Error Occured", Toast.LENGTH_LONG).show();
}
});
}
@Override
public boolean onSupportNavigateUp() {
onBackPressed();
return super.onSupportNavigateUp();
}
}
<resources xmlns:tools="http://schemas.android.com/tools">
<!-- Base application theme. -->
<style name="Theme.Ref" parent="Theme.MaterialComponents.DayNight.NoActionBar">
<!-- Primary brand color. -->
<item name="colorPrimary">@color/purple_500</item>
<item name="colorPrimaryVariant">@color/purple_700</item>
<item name="colorOnPrimary">@color/white</item>
<!-- Secondary brand color. -->
<item name="colorSecondary">@color/teal_200</item>
<item name="colorSecondaryVariant">@color/teal_700</item>
<item name="colorOnSecondary">@color/black</item>
<!-- Status bar color. -->
<item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
<!-- Customize your theme here. -->
</style>
</resources>
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".LoginActivity">
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<TextView
android:id="@+id/welcom"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginStart="120dp"
android:layout_marginTop="50dp"
android:text="Welcome!"
android:textColor="@color/colorBlack"
android:textSize="30sp"
android:textStyle="italic|bold" />
<TextView
android:id="@+id/email"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@id/welcom"
android:layout_marginStart="35dp"
android:layout_marginTop="120dp"
android:text="Email"
android:textColor="@color/colorBlack" />
<EditText
android:id="@+id/login_email"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/email"
android:layout_alignParentStart="true"
android:layout_marginLeft="30dp"
android:layout_marginTop="5dp"
android:layout_marginRight="30dp"
android:background="@drawable/edit"
android:drawableStart="@drawable/ic_email"
android:hint="Email..."
android:inputType="textEmailAddress"
android:padding="8dp" />
<TextView
android:id="@+id/password"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@id/login_email"
android:layout_marginStart="32dp"
android:layout_marginTop="20dp"
android:text="Password"
android:textColor="@color/colorBlack" />
<EditText
android:id="@+id/login_password"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/password"
android:layout_alignParentStart="true"
android:layout_marginLeft="30dp"
android:layout_marginTop="5dp"
android:layout_marginRight="30dp"
android:background="@drawable/edit"
android:drawableStart="@drawable/password"
android:hint="Password..."
android:inputType="textPassword"
android:padding="8dp" />
<TextView
android:id="@+id/forgetp"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/login_password"
android:layout_marginStart="210dp"
android:layout_marginTop="15dp"
android:layout_marginEnd="23dp"
android:text="@string/forget_password"
android:textAlignment="center"
android:textColor="@color/colorPrimaryDark"
android:textSize="17sp"
android:textStyle="bold" />
<Button
android:id="@+id/login_button"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/forgetp"
android:layout_marginLeft="30dp"
android:layout_marginTop="15dp"
android:layout_marginRight="30dp"
android:background="@drawable/buttonss"
android:padding="4dp"
android:text="Login"
android:textAllCaps="false"
android:textColor="@android:color/background_light"
android:textSize="24sp" />
<TextView
android:id="@+id/needs_new_account"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@+id/login_button"
android:layout_centerHorizontal="true"
android:layout_marginStart="15dp"
android:layout_marginTop="20dp"
android:layout_marginEnd="23dp"
android:text="@string/need_new_account_sign_up_here"
android:textAlignment="center"
android:textColor="@color/colorPrimary"
android:textSize="17sp"
android:textStyle="bold" />
</RelativeLayout>
</ScrollView>
</LinearLayout>
解决方案
当编译器onCreate()在您的第二个活动中启动时，他找不到对您尝试的对象的引用setTitle()，因此基本上您的Toolbar或ActionBar没有对 View(xml) 中的对象的引用。
前任：
Toolbar toolbar = findViewById(R.id.name_of_the_object);
以这种方式toolbar获取对视图中对象的引用（activity.xml）；
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