How to fix Null Pointer Exception error in Java? - android-studio

Question

My app keeps crashing when I try to log in (after LoginActivity file). Here's the code. I"d really appreciate some help with this.
MainActivity (opens the register/login screen)
package com.jovanovic.stefan.sqlitetutorial;
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.Toast;
public class MainActivity extends AppCompatActivity {
EditText username, password, repassword;
Button signup, signin;
DBHelper DB;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
username = (EditText) findViewById(R.id.username);
password = (EditText) findViewById(R.id.password);
repassword = (EditText) findViewById(R.id.repassword);
signup = (Button) findViewById(R.id.btnsignup);
signin = (Button) findViewById(R.id.btnsignin);
DB = new DBHelper(this);
signup.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
String user = username.getText().toString();
String pass = password.getText().toString();
String repass = repassword.getText().toString();
if(user.equals("")||pass.equals("")||repass.equals(""))
Toast.makeText(MainActivity.this, "Please enter all the fields", Toast.LENGTH_SHORT).show();
else{
if(pass.equals(repass)){
Boolean checkuser = DB.checkusername(user);
if(checkuser==false){
Boolean insert = DB.insertData(user, pass);
if(insert==true){
Toast.makeText(MainActivity.this, "Registered successfully", Toast.LENGTH_SHORT).show();
Intent intent = new Intent(getApplicationContext(),HomeActivity.class);
startActivity(intent);
}else{
Toast.makeText(MainActivity.this, "Registration failed", Toast.LENGTH_SHORT).show();
}
}
else{
Toast.makeText(MainActivity.this, "User already exists! please sign in", Toast.LENGTH_SHORT).show();
}
}else{
Toast.makeText(MainActivity.this, "Passwords not matching", Toast.LENGTH_SHORT).show();
}
} }
});
signin.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent intent = new Intent(getApplicationContext(), LoginActivity.class);
startActivity(intent);
}
});
}
}
LoginActivity (once registered then proceed to login)
package com.jovanovic.stefan.sqlitetutorial;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class LoginActivity extends AppCompatActivity {
EditText username, password;
Button btnlogin;
DBHelper DB;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
username = (EditText) findViewById(R.id.username1);
password = (EditText) findViewById(R.id.password1);
btnlogin = (Button) findViewById(R.id.btnsignin1);
DB = new DBHelper(this);
btnlogin.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
String user = username.getText().toString();
String pass = password.getText().toString();
if(user.equals("")||pass.equals(""))
Toast.makeText(LoginActivity.this, "Please enter all the fields", Toast.LENGTH_SHORT).show();
else{
Boolean checkuserpass = DB.checkusernamepassword(user, pass);
if(checkuserpass==true){
Log.d("mytag","1");
Toast.makeText(LoginActivity.this, "Sign in successfull", Toast.LENGTH_SHORT).show();
Log.d("mytag","2");
Intent intent = new Intent(getApplicationContext(), HomeActivity.class);
Log.d("mytag","3");
startActivity(intent);
}else{
Toast.makeText(LoginActivity.this, "Invalid Credentials", Toast.LENGTH_SHORT).show();
}
}
}
});
}
}
HomeActivity (this should open the main screen of the app. The app has a add button which adds items using recyclerview)
package com.jovanovic.stefan.sqlitetutorial;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.app.Activity;
import android.content.DialogInterface;
import android.content.Intent;
import android.database.Cursor;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import java.util.ArrayList;
public class HomeActivity extends Activity {
RecyclerView recyclerView;
FloatingActionButton add_button;
ImageView empty_imageview;
TextView no_data;
MyDatabaseHelper myDB;
ArrayList<String> book_id, book_title, book_author, book_pages;
CustomAdapter customAdapter;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
recyclerView = findViewById(R.id.recyclerView);
add_button = findViewById(R.id.add_button);
empty_imageview = findViewById(R.id.empty_imageview);
no_data = findViewById(R.id.no_data);
add_button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent intent = new Intent(HomeActivity.this, AddActivity.class);
startActivity(intent);
}
});
myDB = new MyDatabaseHelper(HomeActivity.this);
book_id = new ArrayList<>();
book_title = new ArrayList<>();
book_author = new ArrayList<>();
book_pages = new ArrayList<>();
storeDataInArrays();
customAdapter = new CustomAdapter(HomeActivity.this,this, book_id, book_title, book_author,
book_pages);
recyclerView.setAdapter(customAdapter);
recyclerView.setLayoutManager(new LinearLayoutManager(HomeActivity.this));
}
#Override
protected void onActivityResult(int requestCode, int resultCode, #Nullable Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if(requestCode == 1){
recreate();
}
}
void storeDataInArrays(){
Cursor cursor = myDB.readAllData();
if(cursor.getCount() == 0){
empty_imageview.setVisibility(View.VISIBLE);
no_data.setVisibility(View.VISIBLE);
}else{
while (cursor.moveToNext()){
book_id.add(cursor.getString(0));
book_title.add(cursor.getString(1));
book_author.add(cursor.getString(2));
book_pages.add(cursor.getString(3));
}
empty_imageview.setVisibility(View.GONE);
no_data.setVisibility(View.GONE);
}
}
#Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.my_menu, menu);
return super.onCreateOptionsMenu(menu);
}
#Override
public boolean onOptionsItemSelected(MenuItem item) {
if(item.getItemId() == R.id.delete_all){
confirmDialog();
}
return super.onOptionsItemSelected(item);
}
void confirmDialog(){
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setTitle("Delete All?");
builder.setMessage("Are you sure you want to delete all Data?");
builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialogInterface, int i) {
MyDatabaseHelper myDB = new MyDatabaseHelper(HomeActivity.this);
myDB.deleteAllData();
//Refresh Activity
Intent intent = new Intent(HomeActivity.this, HomeActivity.class);
startActivity(intent);
finish();
}
});
builder.setNegativeButton("No", new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialogInterface, int i) {
}
});
builder.create().show();
}
}
Manifest file
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.jovanovic.stefan.sqlitetutorial">
<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=".UpdateActivity"
android:parentActivityName=".HomeActivity"/>
<activity android:name=".HomeActivity"
android:label="GDSC Items App"/>
<activity android:name=".LoginActivity"
android:label="GDSC Items App"/>
<activity
android:name=".AddActivity"
android:label="Add Item"
android:parentActivityName=".HomeActivity" />
<activity
android:name=".MainActivity"
android:label="GDSC Items App">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<action android:name="android.intent.action.VIEW" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
Error log
2022-01-27 20:31:01.505 3354-3354/com.jovanovic.stefan.sqlitetutorial E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.jovanovic.stefan.sqlitetutorial, PID: 3354
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jovanovic.stefan.sqlitetutorial/com.jovanovic.stefan.sqlitetutorial.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.floatingactionbutton.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:201)
at android.app.ActivityThread.main(ActivityThread.java:6810)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.floatingactionbutton.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.jovanovic.stefan.sqlitetutorial.HomeActivity.onCreate(HomeActivity.java:45)
at android.app.Activity.performCreate(Activity.java:7224)
at android.app.Activity.performCreate(Activity.java:7213)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)

Answer 1

Make sure you setting the correct layout inside the setContentView() method
For example:
setContentView(R.layout.activity_home);