Unable to start activity : Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener' on a null object reference

Others 2022-04-20 21:21:22 views: 0
RAVI KACHHADIYA :

It is a simple signup and login app for my learning purpose. I wrote the code without any errors but when I try to click on the Sign UP or Already Account? then the app closes automatically.

Error

V/FA: onActivityCreated
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.signup, PID: 10899
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.signup/com.example.signup.Home}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.signup.Home.onCreate(Home.java:25)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 
Disconnected from the target VM, address: 'localhost:8600', transport: 'socket'

AndroidMainfest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.signup">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".Home"></activity>
        <activity android:name=".login" />
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

activity_main.xml

<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=".MainActivity">

    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="85dp"
        android:layout_marginTop="50dp"
        android:layout_marginEnd="85dp"
        android:hint="Email"
        android:ems="10"
        android:inputType="textEmailAddress"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.4"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="85dp"
        android:layout_marginTop="39dp"
        android:layout_marginEnd="85dp"
        android:layout_marginBottom="7dp"
        android:ems="10"
        android:hint="password"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="120dp"
        android:layout_marginTop="68dp"
        android:layout_marginEnd="120dp"
        android:layout_marginBottom="121dp"
        android:text="Sign up"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="54dp"
        android:layout_marginEnd="150dp"
        android:text="Already Account?"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

package com.example.signup;

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.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;

public class MainActivity extends AppCompatActivity {
    EditText emailId, password;
    Button btnSignUp;
    TextView tvSignIn;
    FirebaseAuth mFirebaseAuth;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mFirebaseAuth = FirebaseAuth.getInstance();
        emailId = findViewById(R.id.editText);
        password = findViewById(R.id.editText2);
        btnSignUp = findViewById(R.id.button);
        tvSignIn = findViewById(R.id.textView2);

        btnSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String email = emailId.getText().toString();
                String pwd = password.getText().toString();

                if(email.isEmpty() && pwd.isEmpty()){
                    Toast.makeText(MainActivity.this,"Fields are empty", Toast.LENGTH_SHORT);
                }
                else if(email.isEmpty()){
                    emailId.setError("Please enter email Id");
                    emailId.requestFocus();
                }
                else if(pwd.isEmpty()){
                    password.setError("Please enter Password ");
                    password.requestFocus();
                }
                else if(!email.isEmpty() && !pwd.isEmpty()){
                    mFirebaseAuth.createUserWithEmailAndPassword(email, pwd).addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            if(!task.isSuccessful()){
                                Toast.makeText(MainActivity.this,"Sign Up Unsuccessful, try again", Toast.LENGTH_SHORT);
                            }
                            else {
                                startActivity(new Intent(MainActivity.this,Home.class));
                            }
                        }
                    });
                }
                else{
                    Toast.makeText(MainActivity.this,"Error Occured! ", Toast.LENGTH_SHORT);
                }
            }
        });
        tvSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(MainActivity.this, login.class);
                startActivity(i);
            }
        });
    }
}

activity_login.xml

<?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=".login">

    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="85dp"
        android:layout_marginTop="205dp"
        android:layout_marginEnd="85dp"
        android:hint="Email"
        android:ems="10"
        android:inputType="textEmailAddress"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.4"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="85dp"
        android:layout_marginTop="39dp"
        android:layout_marginEnd="85dp"
        android:layout_marginBottom="50dp"
        android:ems="10"
        android:hint="password"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="120dp"
        android:layout_marginTop="68dp"
        android:layout_marginEnd="120dp"
        android:layout_marginBottom="20dp"
        android:text="Sign in"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/editText2" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="54dp"
        android:layout_marginEnd="150dp"
        android:layout_marginBottom="50dp"
        android:text="Not an Account?"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button" />
</androidx.constraintlayout.widget.ConstraintLayout>

login.java

package com.example.signup;

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.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 com.google.firebase.auth.FirebaseUser;

public class login extends AppCompatActivity {

    EditText emailId, password;
    Button btnSignIn;
    TextView tvSignUp;
    FirebaseAuth mFirebaseAuth;
    private FirebaseAuth.AuthStateListener mAuthStateListener;

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

        mFirebaseAuth = FirebaseAuth.getInstance();
        emailId = findViewById(R.id.editText);
        password = findViewById(R.id.editText2);
        btnSignIn = findViewById(R.id.button);
        tvSignUp = findViewById(R.id.textView2);

        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
            FirebaseUser mFirebaseUser = mFirebaseAuth.getCurrentUser();
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser mFirebaseUser = mFirebaseAuth.getCurrentUser();
                if (mFirebaseUser != null) {
                    Toast.makeText(login.this, "Login Successfull", Toast.LENGTH_SHORT);
                    Intent i = new Intent(login.this, Home.class);
                    startActivity(i);
                }
                else{
                    Toast.makeText(login.this,"Please login",Toast.LENGTH_SHORT);
                }
            }
        };

        btnSignIn.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 Password ");
                    password.requestFocus();
                }
                else if(email.isEmpty() && pwd.isEmpty()){
                    Toast.makeText(login.this,"Fields are empty", Toast.LENGTH_SHORT);
                }
                else if(!email.isEmpty() && !pwd.isEmpty()){
                        mFirebaseAuth.signInWithEmailAndPassword(email,pwd).addOnCompleteListener(login.this, new OnCompleteListener<AuthResult>() {
                            @Override
                            public void onComplete(@NonNull Task<AuthResult> task) {
                                if(!task.isSuccessful()){
                                    Toast.makeText(login.this,"Login Error, try again", Toast.LENGTH_SHORT);
                                }
                                else {
                                    startActivity(new Intent(login.this,Home.class));
                                }
                            }
                        });
                }
                else{
                    Toast.makeText(login.this,"Error Occured! ", Toast.LENGTH_SHORT);
                }



            }
        });
        tvSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(login.this, MainActivity.class);
                startActivity(i);
            }
        });
    }

    @Override
    protected void onStart() {
        super.onStart();
        mFirebaseAuth.addAuthStateListener(mAuthStateListener);
    }
}

activity_home.xml

<?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"
    android:orientation="vertical"
    tools:context=".Home">


    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Welcome"
        android:textSize="48sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="52dp"
        android:layout_marginBottom="233dp"
        android:text="Logout"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView2" />

</androidx.constraintlayout.widget.ConstraintLayout>

Home.java

package com.example.signup;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

public class Home 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 = findViewById(R.id.button);

        btnLogout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FirebaseAuth.getInstance().signOut();
                Intent i = new Intent(Home.this, login.class);
                startActivity(i);
            }
        });

    }
}

build.gradle (Signup)

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        google()
        jcenter()

    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.6.1'
        classpath 'com.google.gms:google-services:4.3.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()

    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

build.gradle(:app)

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "com.example.signup"
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'com.google.firebase:firebase-database:16.0.4'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
    // Add the Firebase SDK for Google Analytics
    implementation 'com.google.firebase:firebase-analytics:17.2.2'

    // Add the SDKs for any other Firebase products you want to use in your app
    // For example, to use Firebase Authentication and Cloud Firestore
    implementation 'com.google.firebase:firebase-auth:19.2.0'
    //implementation 'com.google.firebase:firebase-firestore:21.4.0'

}
thanhlv :

Because in activity_home.xml you define Button with id is button3 and in onCreate of Home you init button with id is button

update code in Home class like this:

btnLogout = findViewById(R.id.button3);

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