Develop Reference

java.lang.NullPointerException when setting value of attributes within a loop

Trying to make it so if the user types "end", in the second input which is "Enter the first name of student", the loop automatically assigns each object in the array the attributes of "null" for id and name, and 0 for age and id, as well as breaking the outerloop. However, I get the error java.lang.NullPointerException.
Any help would be appreciated.
import java.util.*;
class Main {
public static void main(String[] args) {
Scanner myObj = new Scanner(System.in);
System.out.println("Enter number of students");
int numberof = myObj.nextInt();
System.out.println("Number of students is " + numberof);
Student Studentz[] = new Student[numberof];
outerloop:
for (int i = 0; i < numberof; ++i) {
Studentz[i] = new Student();
System.out.println("Enter first name of student " + (i + 1));
Scanner myObj1 = new Scanner(System.in);
String firstname = myObj1.nextLine();
System.out.println("Firstname is: " + firstname);
if (firstname.equals("end")) {
for (int g = i; g < numberof; ++g) {
Studentz[g].Setfirst("null");
Studentz[g].Setlast("null");
Studentz[g].Setage(0);
Studentz[g].Setid(0);
}
break outerloop;
} else {
Studentz[i].Setfirst(firstname);
System.out.println("Enter last name of student " + (i + 1));
Scanner myObj2 = new Scanner(System.in);
String lastname = myObj2.nextLine();
System.out.println("Last name is: " + lastname);
Studentz[i].Setlast(lastname);;
System.out.println("Enter age of student " + (i + 1));
Scanner myObj3 = new Scanner(System.in);
int nazca = myObj3.nextInt();
System.out.println("Age is: " + nazca);
Studentz[i].Setage(nazca);
System.out.println("Enter ID of student " + (i + 1));
Scanner myObj4 = new Scanner(System.in);
int nazca1 = myObj4.nextInt();
System.out.println("ID is: " + nazca1);
Studentz[i].Setid(nazca1);
}
for (int c = 0; c < numberof; ++c) {
System.out.println(Studentz[c].Snake());
}
}
}
}
public class Student {
private String first;
private String last;
private int age;
private int id;
public int getid() {
return
this.id;
}
public void Studentss(String f, String l, int a, int i) {
first = f;
last = l;
age = a;
id = i;
}
public void Setfirst(String z) {
this.first = z;
}
public void Setlast(String za) {
this.last = za;
}
public void Setage(int zb) {
this.age = zb;
}
public void Setid(int zc) {
this.id = zc;
}
public String Snake() {
String snek = "Name is " + this.first + " " + this.last + " , Age is " + this.age + " ,ID is " + this.id;
return snek;
}
}

you fail here because you try to print students data before you initialized all elements of Studentz array:
for (int c = 0; c < numberof; ++c) {
System.out.println(Studentz[c].Snake());
}
also your code fails here by the same reason:
for (int g = i; g < numberof; ++g) {
Studentz[g].Setfirst("null"); // NPE - no Student created yet in the array

Some Possibly Helpful Tips:
Follow Java naming rules for your variables and method names, Studentz should be studentz, even for your Arrays. I know...your tired of reading that but as you progress, you'll see how beneficial it really is.
If you can, don't use multiple Scanner objects, there is no need for that in your use-case. Declare one Scanner object and stick with it. You're probably doing this because you are using the nextLine() method after the nextInt() method and you're finding that it skips the first name prompt and provides a Null String (""). This happens because the nextInt() method does not consume the newline character when the Enter key is hit. To solve this problem you would either, place the code line myObj.nextLine(); directly under the int numberof = myObj.nextInt(); code line:
int numberof = myObj.nextInt();
myObj.nextLine(); // Consume ENTER key hit
or don't use the nextInt() method at all. Instead just stick with the nextLine() method and not worry about consumption:
Scanner myObj = new Scanner(System.in);
int numberof = 0;
String val = "";
while (val.equals("")) {
System.out.println("Enter number of students");
val = myObj.nextLine();
if (!val.matches("\\d+")) {
System.err.println("Invalid number supplied!");
val = "";
continue;
}
numberof = Integer.parseInt(val);
}
System.out.println("Number of students is " + numberof);
Student[] studentz = new Student[numberof];
It's always a good idea to validate any input from the User (as shown above), even if they're in a for loop. Give the User an opportunity to make a correct entry, after all, typo's do happen.
Get rid of the outerLoop: label. As a matter of fact, try avoid ever using them if you can....and you can. It would only be on a relatively rare occasion where you might ever need one.
Give your variables meaningful names, at least to some extent. This can benefit you later on down the road when you want to read your old code in the future.
In your Student class you made yourself a this real nice method named Studentss(). Well, I think it should be a constructor instead and save yourself a lot of code entry for calls to Setter methods. After all, that's mostly what the constructor is for. Your Student class constructor can look like this:
public Student(String firstName, String lastName, int age, int id) {
this.first = firstName;
this.last = lastName;
this.age = age;
this.id = id;
}
And be used like this. You will notice that upon each iteration of the outer for loop, all the prompt answers are placed within variables then at the end of all those prompts the constructor is used instantiate a student object, for example:
studentz[i] = new Student(firstname, lastname, age, id);
Doing it this way mean that there is no need to make calls to Setter methods. There is nothing wrong with making setter calls, it's just easier using the constructor. This is demonstrated below:
Scanner myObj = new Scanner(System.in);
int numberof = 0;
String val = "";
while (val.equals("")) {
System.out.println("Enter number of students");
val = myObj.nextLine();
if (!val.matches("\\d+")) {
System.err.println("Invalid number supplied!");
val = "";
continue;
}
numberof = Integer.parseInt(val);
}
System.out.println("Number of students is " + numberof);
Student[] studentz = new Student[numberof];
for (int i = 0; i < numberof; ++i) {
String firstname = "null";
String lastname = "null";
int age = 0;
int id = 0;
boolean exitOuterForLoop = false;
// Student First Name Prompt:
// (with option to End and default remaining to null's and 0's)
while (firstname.equals("null")) {
System.out.println("Enter first name of student " + (i + 1) + " (End to stop):");
firstname = myObj.nextLine();
if (firstname.equalsIgnoreCase("end")) {
firstname = "null";
//Make all remaining Student instances null and 0
for (int g = i; g < numberof; ++g) {
studentz[g] = new Student(firstname, lastname, age, id); // Use Student class constructor
}
exitOuterForLoop = true;
break; // Exit this 'while' loop
}
// Validate first name (no numbers or crazy characters)
if (!firstname.matches("(?i)[a-z']+")) {
System.err.println("Invalid First Name! (" + firstname + ") Try Again...");
firstname = "null";
}
}
if (exitOuterForLoop) {
break; // Exit this outer 'for' loop.
}
System.out.println("Firstname is: " + firstname);
// Student Last Name Prompt
while (lastname.equals("null")) {
System.out.println("Enter last name of student " + (i + 1));
lastname = myObj.nextLine();
// Validate last name (no numbers or crazy characters)
if (!lastname.matches("(?i)[a-z']+")) {
System.err.println("Invalid Last Name! (" + lastname + ") Try Again...");
lastname = "null";
}
}
System.out.println("Last name is: " + lastname);
// Student Age Prompt
val = "";
while (val.equals("")) {
System.out.println("Enter age of student " + (i + 1));
val = myObj.nextLine();
// Validate age (digits 0 to 9 only)
if (!val.matches("\\d+")) {
System.err.println("Invalid Age Supplied! (" + val + ") Try Again...");
val = "";
}
}
age = Integer.parseInt(val);
System.out.println("Student age is: " + age);
// Student ID Prompt
val = "";
while (val.equals("")) {
System.out.println("Enter ID of student " + (i + 1));
val = myObj.nextLine();
// Validate age (digits 0 to 9 only)
if (!val.matches("\\d+")) {
System.err.println("Invalid ID Supplied! (" + val + ") Try Again...");
val = "";
}
}
id = Integer.parseInt(val);
System.out.println("Student ID is: " + id);
studentz[i] = new Student(firstname, lastname, age, id); // Use Student class constructor
}
// Display the instances of Student contained within the 'studentz[]' array.
for (int c = 0; c < numberof; ++c) {
System.out.println(studentz[c].toString());
}
The snake() method is actually just another toString() method and there is nothing wrong with that however you might want to consider using StringBuilder class to create the returned string rather than doing concatenations, for example:
public String snake() {
return new StringBuilder("Name is ").append(this.first).append(" ").append(this.last)
.append(" , Age is ").append(this.age).append(" , ID is ")
.append(this.id).toString();
}
Not overly important here but it can save you memory if you do a lot concatenating with many large strings.

An application for a temporary management with Java

Hello the StackOverflow community! I am recent Java learner with 1 year of experience only. I was asked to design a software that mimics a school DB, storing all names, class, roll, and other details. When asked to, it would display appropriate messages, like calculating performance, deleting records, etc. It is yet an incomplete work but the first part (accepting and storing details) are done. I have spent a lot of time behind this and the only thing I get is a nullPointerError. Sorry, but I have been asked to stick to the basics, so no glitzy code. I have used inheritance. The superclass is "Student".
public class Student {
int roll,age = 0; // Roll to be auto-updated
String cl,name;
// Marks variables now
int m_eng, m_math, m_physics, m_chem, m_bio = 0;
public Student(){
}
public Student(int a, String cla){
age = a;
cl = cla; // Assign values
}
void setMarks(int eng, int math, int phy, int chem, int bio){
m_eng = eng; m_math = math; m_physics = phy; m_chem = chem; m_bio = bio;
}
}
Here's the error:
java.lang.NullPointerException
at Application.accept_data(Application.java:35)
at Application.execute(Application.java:23)
at Application.input(Application.java:16)
at Application.main(Application.java:101)
Here is the code, though:
import java.util.Scanner;
public class Application extends Student {
static int n; static Scanner sc = new Scanner(System.in);
static Student s[];
void input(){
System.out.println("Enter the number of students: ");
n = sc.nextInt();
s = new Student[n]; // Create array for n students
System.out.println("Enter your choice: ");
System.out.println("1. Accept student's details ");
System.out.println("2. Display all records ");
System.out.println("3. Display data student-wise ");
System.out.println("4. Delete record");
System.out.println("5. Display performance status");
System.out.println("6. Exit");
execute();
}
static void execute(){
boolean correct = false;
while (!correct){
int op = sc.nextInt();
switch(op){
case 1: accept_data(); correct = true;
case 2: disp_data();correct = true;
case 3: disp_studentwise();correct = true;
case 4: del_record();correct = true;
case 5: performance();correct = true;
case 6: System.exit(0); correct = true;//Terminate
default: System.out.println("You must enter a choice. Kindly re-enter: ");correct = false;
}
}
}
static void accept_data(){
for (int i = 0; i<s.length; i++){
s[i].roll = i+1; //Autoupdate roll
System.out.println("Enter name: "); s[i].name = sc.nextLine();
System.out.println("Enter age: "); s[i].age = sc.nextInt(); // Refer to object prope.
System.out.println("Enter class: "); s[i].cl = sc.nextLine();
System.out.println("We're heading for marks entry!");
System.out.println("Enter marks in the following order: ENGLISH, MATH, PHYSICS, CHEMISTRY, BIOLOGY");
s[i].setMarks(sc.nextInt(),sc.nextInt(),sc.nextInt(),sc.nextInt(),sc.nextInt());
}
System.out.println("Thanks. Main menu, please enter your choice now: ");
execute();
}
static void disp_data(){
System.out.println("The system will display all stored information of students available.");
for (int i = 0; i<s.length; i++){
if (s[i].roll != -1){
continue; // In case record is deleted, it won't display
}
else {
printrec(i);
}
}
System.out.println("Main menu, please enter your choice: ");
execute();
}
static void disp_studentwise(){
System.out.println("Enter the roll number");
int r = sc.nextInt();
boolean ok = (r>s.length||r<0)?false:true;
while (!ok){
System.out.println("Incorrect roll. Please re-enter: ");
r = sc.nextInt();
if (r>s.length) ok = false;
else ok = true;
}
printrec(r-1);
System.out.println("Main menu, please enter your choice: ");
execute();
}
static void printrec(int n){
int i = n;
System.out.println("For roll number " + s[i].roll + ", details: ");
System.out.println("Name: " + s[i].name); System.out.println("Age: " + s[i].age);
System.out.println("Class: " + s[i].cl);
System.out.println("Subject \t Marks");
System.out.println("English: \t " + s[i].m_eng); // Display record with marks
System.out.println("Maths: \t " + s[i].m_math);
System.out.println("Physics: \t " + s[i].m_physics);
System.out.println("Chemistry: \t " + s[i].m_chem);
System.out.println("Biology: \t " + s[i].m_bio);
}
static void del_record(){
System.out.println("Enter the roll number you want to delete: ");
int rll = sc.nextInt();
for (int i = 0; i<s.length; i++){
if (rll == s[i].roll){
s[i].roll = -1; // Assign non-positive value to refer deleted items
}
}
}
static void performance(){
}
public static void main(String[] args){
Application ob = new Application();
ob.input(); // Start program
}
}
Can anyone point out what's going wrong? Why there's a problem with accepting details of students after pressing for the 1st option? It shows nullPointer on s[i].roll. Keep in mind that roll is autoupdated, and user doesn't intervene there. It serves as a primary key. An explanation would be beneficial, if possible of course, I am eager to learn. Thanks.

this :
s = new Student[n]; // Create array for n students
You are just creating an array of 'n' Student objects here ... that doesn't mean that your 'n' Students are initialized ... your array contains only 'null' values ...
you may want in your accept_data method do a :
for (int i = 0; i<s.length; i++){
s[i] = new Student();
s[i].roll = i+1; //Autoupdate roll
....

You are getting an NPE because you create an array of Students in your input method, but you never populate it with Student objects, so in accept_data, you're trying to access the roll field on a non-existent object.
You will need to fill in the array with new Student objects in your input method before you call accept_data.

How to fix my Array class

I am trying to figure out how to get my array to run correctly, I know I have to change the array value to an input but I cannot get the program to compile if any one can help that be great.
I am trying to have the program take input for grades and names of students and in the end output their name and grade.
Edit sorry this is my first it posting i have an error
Student.java:60: error: class, interface, or enum expected I am in java 101 so this is why it is such low level java, we only know the basics
import java.util.Scanner;
public class students
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("How many students?: ");
int numofstudents = keyboard.nextInt();
Student s = new Student();
s.setMultipleStudents();
s.toString();
System.out.println("Enter the Grade for the student: ");
int gradeofstudnets = keyboard.nextInt();
}
}
and my second class is
import java.util.Scanner;
public class Student
{
Scanner scan = new Scanner(System.in);
private String name;
private int grade;
private int[] multiplegradeinputs = new int[10];
private String[] multipleStudent = new String[10];
public Student()
{
}
public Student(String n, int g)
{
name = n;
grade = g;
}
public String setMultipleStudents()
{
String n = "";
for(int i = 1; i < multipleStudent.length; i++)
{
System.out.println("Enter student #" + i +" name: " );
n = scan.nextLine();
multipleStudent[i] = n;
}
return null;
}
public String multiplegradeinputs()
{
for(int i = 1; i <multiplegradeinputs.length; i++)
{
System.out.println("Enter the Grade of the student#" + i +" : ");
grade = scan.nextInt();
multiplegradeinputs[i] = grade;
}
} <--- error here
public String toString()
{
String temp = "";
for(int i = 1; i < multipleStudent.length; i++)
{
temp += multipleStudent[i] + " ";
}
return temp;
}
}

Add return statement in your multiplegradeinputs() method:
public String multiplegradeinputs()
{
for(int i = 1; i <multiplegradeinputs.length; i++)
{
System.out.println("Enter the Grade of the student#" + i +" : ");
grade = scan.nextInt();
multiplegradeinputs[i] = grade;
}
return null; //Add this line
}
Or change your methods to void return type if they dont return anything.

Class names have to be capitalized in java, so instead of
public class students
you should write
public class Students
Also instead of writing
keyboard.nextInt();
You should write
Integer.parseInt(keyboard.nextLine());
This is mainly because java is full of bugs and technical specifications that you won't find easily. Let me know if this fixes it for you, since you didn't post the exact error message you got.
As for the error that you pointed out, it's because your function expects a String as a return value no matter what, so either change that to void if you can or return a null string. To do that just add the following line at the very end of the method.
return null;

You should create a Student object which holds the properties of the student, e.g. Name and Grades. You should then store all the student objects in some kind of data structure such as an array list in the students class.

Adding to the answer provided by #hitz
You have a bug in the for loops:
for(int i = 1; i <multiplegradeinputs.length; i++)
for(int i = 1; i < multipleStudent.length; i++)
You will never populated multiplegradeinputs[0] and multipleStudent[0] because you start the loop at index == 1 and thus you will have only 9 student names stored instead of 10.
Change to:
for(int i = 0; i <multiplegradeinputs.length; i++)
for(int i = 0; i < multipleStudent.length; i++)
Remember even though the length in 10, the indices always start with 0 in Java and in your case will end with 9.

import java.util.Scanner;
public class Student
{
Scanner scan = new Scanner(System.in);
private String name;
private int grade;
private int[] multiplegradeinputs = new int[10];
private String[] multipleStudent = new String[10];
public Student()
{
}
public Student(String n, int g)
{
name = n;
grade = g;
}
public String setMultipleStudents()
{
String n = "";
for(int i = 1; i < multipleStudent.length; i++)
{
System.out.println("Enter student #" + i +" name: " );
n = scan.nextLine();
multipleStudent[i] = n;
}
return null;
}
public void multiplegradeinputs()
{
for(int i = 1; i <multiplegradeinputs.length; i++)
{
System.out.println("Enter the Grade of the student#" + i +" : ");
grade = scan.nextInt();
multiplegradeinputs[i] = grade;
}
}
public String toString()
{
String temp = "";
for(int i = 1; i < multipleStudent.length; i++)
{
temp += multipleStudent[i] + " ";
}
return temp;
}
}
this is the 2nd class
import java.util.Scanner;
public class students
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner(System.in);
System.out.println("How many students?: ");
int numofstudents = keyboard.nextInt();
Student s = new Student();
s.setMultipleStudents();
s.toString();
System.out.println("Enter the Grade for the student: ");
int gradeofstudnets = keyboard.nextInt();
}
}

You are missing a return value in the multiplegradeinputs() method.

Having trouble with array

I am currently trying to program a array based program. We have to make a employee class then a tester main class that holds a array of five user input employee names, salaries, and performance rating. The performance rating is used to determine a supplied raise amount. I have it basically done, but when i run the program, nothing happens even though java virtual machine is running. I have looked up and down for the error, anyone can point out what i am doing wrong?
Employee Class
public class Employee
{
private String employeeName;
private int salary;
private int performanceRating;
public Employee()
{
employeeName = "";
salary = 0;
performanceRating = 0;
}
public String getEmployeeName()
{
return employeeName;
}
public int getSalary()
{
return salary;
}
public int getPerformanceRating()
{
return performanceRating;
}
}
And this is the tester main class where the error comes in somewhere
import java.util.Scanner;
public class Tester
{
public static void main(String[] args)
{
Employee[] work = new Employee[5];
Scanner scanString = new Scanner(System.in);
Scanner scanInt = new Scanner(System.in);
String employeeName = "";
double salary = 0;
double performanceRating = 0;
String choice = scanString.nextLine();
while(choice.equals("yes"))
{
for(int i = 0; i < 5 ;i++)
{
System.out.println("What is the employee's name?");
employeeName = scanString.nextLine();
System.out.println("Enter Employee's salary");
salary = scanInt.nextInt();
System.out.println("Performance? 1 = excellent, 2 = good, 3 = poor");
performanceRating = scanInt.nextInt();
work[i] = new Employee();
}
for(int j = 0; j < 5; j ++)
if(work[j].getPerformanceRating() == 1)
{
salary = salary * 0.06;
System.out.println("Employee " + employeeName + " salary raised to " + salary);
}
else if(performanceRating == 2)
{
salary = salary * 0.04;
System.out.println("Employee " + employeeName + " salary raised to " + salary);
}
else if(performanceRating == 3)
{
salary = salary * 0.015;
System.out.println("Employee " + employeeName + " salary raised to " + salary);
}
else if(performanceRating < 5)
{
salary = salary;
System.out.println("Rating is off scale. No raise for emplyee " + employeeName);
}
System.out.println("Enter more employees? type 'yes' if you want to go again");
choice = scanString.nextLine();
}
System.out.println("Done");
}
}

The program reads from System.in. If you don't enter anything, nothing will happen.
Not sure why you have 2 Scanners instead of just one.

You have while(choice.equals("yes")) except you never prompt the user to make a choice. The program does do stuff (some of which may not all work properly), but you have to give the program input. What you can do is ask the user a question before the line String choice = scanString.nextLine();.
As a side note, you could use a switch in place of the if and else if's and it might be a little easier to read and understand.

Combining array indexes

I have created two arrays. One to store names and one to store sales. I am trying to link the indexes of each together so that index 1 from then name array will bring up the value of index 1 of the sales array. I am trying to get it to sort out the sales array and return the max sales and the person that brought in those sales. All the values are inputted by the user including the size of the arrays.
import java.util.Scanner;
import java.util.Arrays;
public class EmpArray {
public static int employee(){
Scanner input = new Scanner(System.in);
int numemp;
System.out.println("Enter how many employees to be compared: \n");
numemp = input.nextInt();
String name[] = new String[numemp];
int annsales[] = new int[numemp];
int maxannsales;
for (int i = 0; i < name.length; i++) {
System.out.println("Enter your name: \n");
String employeename = input.next();
name[i] = employeename;
System.out.println("\n");
}
for (int j = 0; j < annsales.length; j++) {
System.out.println("Enter in your annual sales: \n");
int employeesales = input.nextInt();
annsales[j] = employeesales;
System.out.println("\n");
}
System.out.println("Employee Name: " + Arrays.toString(name));
System.out.println("Total Sales: " + Arrays.toString(annsales));
Arrays.sort(annsales);
//look at page 456 of starting out with java and page 460 of the same book, p.464
System.out.println("Top Salary is : $"+annsales[annsales.length-1]);
maxannsales = annsales[annsales.length-1];
return maxannsales;
}
}
What am I doing wrong I have been at this for two weeks now.

You should make a class to store the date rather than the use two separate arrays.
public class Employee {
private int sales;
private String name;
public Employee(String name, int sales){
this.name = name;
this.sales = sales;
}
public String getName(){
return this.name;
}
public int getSales(){
return this.sales;
}
}
Now store the name and sales as local variables when you read in from your Scanner and pass them to the Employee constructor. You can then create an Employee array[] or ArrayList<Employee> and sort this array/arraylist on Employee.getSales()
Like so:
import java.util.Scanner;
import java.util.Arrays;
public class EmpArray {
public static void main(String[] args){
employee();
}
public static int employee(){
Scanner input = new Scanner(System.in);
int numEmp;
System.out.println("Enter how many employees to be compared: ");
numEmp = input.nextInt();
input.nextLine();
Employee[] employees = new Employee[numEmp];
for (int i = 0; i < numEmp; i++) {
System.out.print("Enter your name: ");
String employeeName = input.nextLine();
System.out.println();
System.out.print("Enter in your annual sales:");
int employeeSales = input.nextInt();
input.nextLine();
System.out.println();
//creates an Employee object based on the constructor defined in Employee
Employee employee = new Employee(employeeName, employeeSales);
employees[i] = employee;
}
//initialize maxSeller to first employee
Employee maxSeller = employees[0];
for(int i = 0; i < employees.length; i++){
//using the getters we created in Employee
System.out.println("Employee Name: " + employees[i].getName());
System.out.println("Total Sales: " + employees[i].getSales());
System.out.println();
//checks if employees[i] sold more than maxSeller
if(maxSeller.getSales() < employees[i].getSales()){
maxSeller = employees[i];
}
}
System.out.println();
System.out.println("Top Seller: " + maxSeller.getName());
System.out.println("Top Sales: " + maxSeller.getSales());
return maxSeller.getSales();
}
}

If you sort the array with the numbers, then that gets sorted. You can find the top or bottom sales amount, but you cannot get the name of the salesperson. The two arrays are not linked in any way, so you cannot expect sorting one to cause the sorting of the other.
You need to take an approach that is fundamentally different., using different data structures.
For instance, one approach would be to have a single Array, but not to make it of String(s) or int(s), but to store both the name and the sales together, as a pair. You could pout the pair into an object as shown by sunrize920, or you could put them both in a map.
Having paired them together into some type of object, you can then have an Array of such objects.
Then, you could sort that array. To do so, you will need to write a custom comparator.
Alternatively, keep your two arrays and don;t do any sorting. Just loop through on and find the largest number. Remember the position of the largest sales number. Then, look up the name in the other array, using the same position.