在Android中的另一个活动中启动活动

Question

按下注册按钮后，显示错误：不幸的是，应用程序已停止

LogCat错误：java.lang.RuntimeException：无法启动活动ComponentInfo｛com.example.lolacpuckcakes.com.example.lolacpucks.HomeActivity｝：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法"void android.widget.Button.setOnClickListener(android.view.view$OnClickListen(">

注册活动：Java代码

public class SignUpActivity extends AppCompatActivity {
EditText emailId, password;
Button btnSignUp;
TextView tvSignIn;
FirebaseAuth mFirebaseAuth;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_signup);
mFirebaseAuth = FirebaseAuth.getInstance();
emailId = findViewById(R.id.emailIDSignUp);
password = findViewById(R.id.pwdIDSignUp);
btnSignUp = findViewById(R.id.btnIDSignUp);
tvSignIn = findViewById(R.id.textView);
btnSignUp.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String email = emailId.getText().toString();
String pwd = password.getText().toString();
if(email.isEmpty())
{
emailId.setError("Please Enter Email Id");
emailId.requestFocus();
}
else if (pwd.isEmpty())
{
password.setError("Please Enter Email Id");
password.requestFocus();
}
else if (email.isEmpty() && pwd.isEmpty())
{
Toast.makeText(SignUpActivity.this, "Fields are Empty",Toast.LENGTH_SHORT).show();
}
else if (!(email.isEmpty() && pwd.isEmpty()))
{
mFirebaseAuth.createUserWithEmailAndPassword(email,pwd).addOnCompleteListener(SignUpActivity.this, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
if (!task.isSuccessful())
{
Toast.makeText(SignUpActivity.this, "SignUp Unsuccessful, Please Try Again",Toast.LENGTH_SHORT).show();
}
else
{
startActivity(new Intent(SignUpActivity.this,HomeActivity.class));
}
}
});
}
else
{
Toast.makeText(SignUpActivity.this, "Error Occurred!",Toast.LENGTH_SHORT).show();
}
}
});
tvSignIn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(SignUpActivity.this, LoginActivity.class);
startActivity(i);
}
});

}
}

SignUpActivity XML:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".SignUpActivity">

<Button
android:id="@+id/btnIDSignUp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="132dp"
android:text="Sign Up" />
<ImageView
android:id="@+id/imageView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="62dp"
android:src="@drawable/logo_lola" />
<EditText
android:id="@+id/pwdIDSignUp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="206dp"
android:ems="10"
android:hint="Password"
android:inputType="textPassword" />
<EditText
android:id="@+id/emailIDSignUp"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="269dp"
android:ems="10"
android:hint="Email"
android:inputType="textEmailAddress" />
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_marginBottom="87dp"
android:text="Already have an account? Sign in here." />

</RelativeLayout>

HomeActivity Java代码：

public class HomeActivity extends AppCompatActivity {
Button btnlogout;
FirebaseAuth mFirebaseAuth;
private FirebaseAuth.AuthStateListener mAuthStateListener;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home);
btnlogout.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
FirebaseAuth.getInstance().signOut();
Intent intoMain  = new Intent(HomeActivity.this, SignUpActivity.class);
startActivity(intoMain);
}
});
}
}

HomeActivityXML:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".HomeActivity">

<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:text="Welcome"
android:textSize="50dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="1.0" />
<Button
android:id="@+id/btnLogout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Logout"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
tools:layout_editor_absoluteY="424dp" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

您错过了初始化btnlogout。在设置View.OnClickListener之前，在HomeActivityonCreate中添加此行

btnlogout = findViewById(R.id.btnLogout);

Answer 2

在为按钮设置listener之前，您需要创建btnSignUp对象btnSignUp=findViewById(R.id.btnLogout(；

Answer 3

您需要在setonclicklistener之前创建btnSignUp对象。