Android:Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Question

I am new to Android and I am trying to make a registration portal. But I keep getting this error

Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Register.java:

package com.example.trackandtrigger;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
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.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class register extends AppCompatActivity {
    final String valid_email = "^[\w!#$%&'*+/=?`{|}~^-]+(?:\.[\w!#$%&'*+/=?`{|}~^-]+)*@(?:[a- 
zA-Z0-9-]+\.)+[a-zA-Z]{2,6}$";
    final String valid_password = "^(?=.*[A-Za-z])(?=.*\d)(?=.*[@$!%*#?&])[A-Za-z\d@$!%*#?&] 
{8,}$";//Minimum eight characters, at least one letter, one number and one special character
    final String valid_phone = "^\d{10}$";
    final Pattern mail = Pattern.compile(valid_email);
    final Pattern pass = Pattern.compile(valid_password);
    final Pattern phno = Pattern.compile(valid_phone);
    EditText mFullName,mEmail,mPassword,mPhone;
    Button mRegisterBtn;
    TextView mLoginBtn;
    FirebaseAuth fAuth;
    ProgressBar progressBar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);

        mFullName   =   findViewById(R.id.FullName);
        mEmail      =   findViewById(R.id.Email);
        mPassword   =   findViewById(R.id.Password);
    mPhone      =   findViewById(R.id.Phone);
    mRegisterBtn=   findViewById(R.id.Regbtn);
    mLoginBtn   =   findViewById(R.id.Login);

    fAuth=FirebaseAuth.getInstance();
    progressBar=findViewById(R.id.progressBar);

    mRegisterBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String email = mEmail.getText().toString().trim();
            String password = mPassword.getText().toString();
            String phone=mPhone.getText().toString();

            Matcher mat1=mail.matcher(email);
            Matcher mat2=pass.matcher(password);
            Matcher mat3=phno.matcher(phone);

            if(!mat1.matches()) {
                mEmail.setError("Enter valid Email");
                return;
            }

            else if(!mat2.matches()) {
                mPassword.setError("Minimum 8 characters, at least 1 letter, 1 number and 1 special character");
                return;
            }

            else if(!mat3.matches()) {
                mPhone.setError("Enter 10 digit phone no.");
                return;
            }
            else {

                progressBar.setVisibility(View.VISIBLE);
            }
        }
    });
}
}

activity_register.xml:

The problem seems to be with Register Button(id:Regbtn). Please help me out.

user_14245643 · Answer

I will suggest to start debugging and check if mRegisterBtn is being returned as null or is actively finding the related button on the layout.

mRegisterBtn=   findViewById(R.id.Regbtn);

user_2216110 · Answer

It's a side effect due to the fact that you named your class register instead of Register. You need to have the same case between your filename and your class

Register.java -> public class Register / xml: tools:context=".Register"
register.java -> public class register / xml: tools:context=".register"

However it's still (mostly) recommended for code clarity/coding standards to keep java class uppercased

Android:Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

2 Answers