I want to send email verification to another account in firebase that is not used for sign up of account

I have 3 fields in my registration activity.

Username Email and Password.

So the account will be created with the Username which will show up on the authentication page but I want to send the email to the Email field but how do I send it to the email specified in the editTextEmail. Here is the code I used.

package net.simplifiedlearning.firebaseauth;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.util.Patterns;
import android.view.View;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.Toast;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseAuthUserCollisionException;
import com.google.firebase.auth.FirebaseUser;
import java.util.AbstractCollection;
public class ChildSignUpActivity extends AppCompatActivity implements View.OnClickListener{
EditText editTextEmail,editTextUsername, editTextPassword;
ProgressBar progressBar;
private FirebaseAuth mAuth;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_sign_up);
editTextUsername = (EditText) findViewById(R.id.editTextUsername);
editTextEmail = (EditText) findViewById(R.id.editTextEmail);
editTextPassword = (EditText) findViewById(R.id.editTextPassword);
progressBar = (ProgressBar) findViewById(R.id.progressbar);
mAuth = FirebaseAuth.getInstance();
findViewById(R.id.buttonSignUp).setOnClickListener(this);
findViewById(R.id.textViewLogin).setOnClickListener(this);
}
private void registerUser (){
String email = editTextEmail.getText().toString().trim();
String username = editTextUsername.getText().toString().trim();
String password = editTextPassword.getText().toString().trim();
if (email.isEmpty()) {
editTextEmail.setError("Email is required");
editTextEmail.requestFocus();
return;
}
if (!Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
editTextEmail.setError(("Please enter a valid email"));
editTextEmail.requestFocus();
return;
}
if (password.isEmpty()) {
editTextPassword.setError("Password is required");
editTextPassword.requestFocus();
return;
}
if (password.length() < 6) {
editTextPassword.setError("Your password must be at least 6 characters");
return;
}
progressBar.setVisibility(View.VISIBLE);
mAuth.createUserWithEmailAndPassword(username, password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull final Task<AuthResult> task) {
if (task.isSuccessful()) {
// send user verification link using firebase
FirebaseUser user = mAuth.getCurrentUser();
user.sendEmailVerification().addOnSuccessListener(new OnSuccessListener<Void>() {
@Override
public void onSuccess(Void aVoid) {
Toast.makeText(ChildSignUpActivity.this, "Verification Email Has Been Sent", Toast.LENGTH_SHORT).show();
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
Toast.makeText(getApplicationContext(), task.getException().getMessage(), Toast.LENGTH_SHORT).show();;
}
});
Toast.makeText(getApplicationContext(), "User Register Succesfull", Toast.LENGTH_SHORT).show();
} else {
if(task.getException() instanceof FirebaseAuthUserCollisionException) {
Toast.makeText(getApplicationContext(), "User already exists", Toast.LENGTH_SHORT).show();
}else{
Toast.makeText(getApplicationContext(), task.getException().getMessage(), Toast.LENGTH_SHORT);
}
}
}
});
}
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.buttonSignUp:
registerUser();
break;
case R.id.textViewLogin:
startActivity(new Intent(this, MainActivity.class));
break;
}
}
}

So I want the field called email to get the verification email could anyone offer a solution?

Create a document in firestore with document name as username and field email.

>profiles
> - fatalcoder524 //username
> 1. email: test@test.com //email to validate
> 2. dob: 12/12/12 //other details

First check if the username is present in firestore.

• If exists, use the email in firestore and password by user to validate.
• If not, ask user to signup.

Sample Code: (Signup)

String email; //user input
String username; //user input
String password; //user input

Map<String, Object> user = new HashMap<>();
user.put("email", email);   // data to add to firestore

db.collection("profiles").document(username) //username is set as firestore document name, profiles is the collection name
        .set(user)  //Store the email in above document.
        .addOnSuccessListener(new OnSuccessListener<Void>() {
            @Override
            public void onSuccess(Void aVoid) // If data is added successfully
            {
                mAuth.createUserWithEmailAndPassword(email, password) // create account
        .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    // Sign in success, update UI with the signed-in user's information
                    Log.d(TAG, "createUserWithEmail:success");
                } else {
                    // If sign in fails, display a message to the user.
                    Log.w(TAG, "createUserWithEmail:failure", task.getException());
                }
            }
        });

            }
        });

Sample Code: (Signin)

String username;  //get username from user
String password; //get password from user
DocumentReference docRef = db.collection("profiles").document(username); //get doc reference
docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
        if (task.isSuccessful()) {
            DocumentSnapshot document = task.getResult();
            if (document.exists()) // if username is present
            { 
                String email=document.getData().email; //get email from document
                mAuth.signInWithEmailAndPassword(email, password) //signin with email
        .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) // if password and email id matches
                {
                    // Sign in success, update UI with the signed-in user's information
                    Log.d(TAG, "signInWithEmail:success");
                } else {
                    // If sign in fails, display a message to the user.
                    Log.w(TAG, "signInWithEmail:failure", task.getException());
                }
            }
        });
            } else {
                Log.d(TAG, "No such document");
                //add code (user not found/registered) 
            }
        } else {
            Log.d(TAG, "get failed with ", task.getException());
        }
    }
});

Modification of your code:

FirebaseFirestore db = FirebaseFirestore.getInstance();
private void registerUser (){
String email = editTextEmail.getText().toString().trim();
String username = editTextUsername.getText().toString().trim();
String password = editTextPassword.getText().toString().trim();
if (email.isEmpty()) {
editTextEmail.setError("Email is required");
editTextEmail.requestFocus();
return;
}
if (!Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
editTextEmail.setError(("Please enter a valid email"));
editTextEmail.requestFocus();
return;
}
if (password.isEmpty()) {
editTextPassword.setError("Password is required");
editTextPassword.requestFocus();
return;
}
if (password.length() < 6) {
editTextPassword.setError("Your password must be at least 6 characters");
return;
}
progressBar.setVisibility(View.VISIBLE);
Map<String, Object> user = new HashMap<>();
user.put("email", email);
db.collection("profiles").document(username) //username is set as firestore document name, profiles is the collection name
.set(user)  //Store the email in above document.
.addOnSuccessListener(new OnSuccessListener<Void>() {
@Override
public void onSuccess(Void aVoid) // If data is added successfully
{
mAuth.createUserWithEmailAndPassword(email, password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull final Task<AuthResult> task) {
if (task.isSuccessful()) {
// send user verification link using firebase
FirebaseUser user = mAuth.getCurrentUser();
user.sendEmailVerification().addOnSuccessListener(new OnSuccessListener<Void>() {
@Override
public void onSuccess(Void aVoid) {
Toast.makeText(ChildSignUpActivity.this, "Verification Email Has Been Sent", Toast.LENGTH_SHORT).show();
}
}).addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
Toast.makeText(getApplicationContext(), task.getException().getMessage(), Toast.LENGTH_SHORT).show();;
}
});
Toast.makeText(getApplicationContext(), "User Register Succesfull", Toast.LENGTH_SHORT).show();
} else {
if(task.getException() instanceof FirebaseAuthUserCollisionException) {
Toast.makeText(getApplicationContext(), "User already exists", Toast.LENGTH_SHORT).show();
}else{
Toast.makeText(getApplicationContext(), task.getException().getMessage(), Toast.LENGTH_SHORT);
}
}
}
});
}
});
}

"So the account will be created with the Username which will show up on the authentication page." Do you want the username to appear on the authentication page in your firebase console. I don't think that can be done or why it should be done. The right way would be to create user with email and password and then update the user's profile with the username he wants like this-

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
UserProfileChangeRequest profileUpdates = new UserProfileChangeRequest.Builder()
.setDisplayName("Jane Q. User")
.build();
user.updateProfile(profileUpdates)
.addOnCompleteListener(new OnCompleteListener<Void>() {
@Override
public void onComplete(@NonNull Task<Void> task) {
if (task.isSuccessful()) {
Log.d(TAG, "User profile updated.");
}
}
});

Later, if you need the username, you can access it using-

FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
if (user != null) {
for (UserInfo profile : user.getProviderData()) {
String name = profile.getDisplayName();
}
}

The best way to fix my problem is the first use the firestore to save the user data for username then use the tricks in the fatal coders post then that should fix it and let you sign in.