How do I enter a String for a persons name followed by a number for a student id into a for loop of an array for objects belong to a class?

Question

import java.util.Scanner;

class Student {
    public int id;
    public String name;
    Student(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

The first for loop is where I am having trouble, I am trying to enter a name followed by an id for 3 students but it keeps throwing a null pointer exception

The error is displayed as follows:

javac -classpath.:/run_dir/junit-4.12.jar:/run_dir/hamcrest-core-1.3.jar:/run_dir/json-simple-1.1.1.jar -d. Main.java

java -classpath.:/run_dir/junit-4.12.jar:/run_dir/hamcrest-core-1.3.jar:/run_dir/json-simple-1.1.1.jar

Main Enter the name for student 1: John Exception in thread "main" java.lang.NullPointerException at Main.main(Main.java:28) exit status 1

public class Main {
public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);

    Student children[] = new Student[3];
    
    for (int i = 0; i < children.length; i++) {
        System.out.print("Enter the name for student " + (i + 1) + ": ");
        children[i].name = keyboard.nextLine();
        System.out.print("Enter the id for student " + (i + 1) + ": ");
        children[i].id = keyboard.nextInt();
    }

    // Display the name and the id for the 3 students.
    for (int i = 0; i < children.length; i++) {
        System.out.println("The first student is " + children[i].name + 
        " and their student id is " + children[i].id);
    }
    keyboard.close();
}

Answer 1

You're getting a NullPointerException error because your Student array is empty. Student children[] = new Student[3]; Just creates a new array of size three which will hold Student objects. In the loop before you try assigning values to items in the array, you need to add a new Student.

children[i] = new Student(params);

You could also create variables to hold the input values and then create the Student instance and add it at the end of each iteration.

Answer 2

import java.util.Scanner;

class Student {
  public int id;
  public String name;
  Student(int id, String name) {
    this.id = id;
    this.name = name;
  }
}

public class Main {
  public static void main(String[] args) {
    Scanner keyboard = new Scanner(System.in);

    Student children[] = new Student[3];

    for (int i = 0; i < children.length; i++) {
      children[i] = new Student(0, ""); // We have to initialize each student
      System.out.print("Enter the name for student " + (i + 1) + ": ");
      children[i].name = keyboard.nextLine();
      System.out.print("Enter the id for student " + (i + 1) + ": ");
      children[i].id = keyboard.nextInt();
      keyboard.nextLine(); // We have to read the new line (\n)
    }

    // Display the name and the id for the 3 students.
    for (int i = 0; i < children.length; i++) {
      System.out.println("The student number " + (i+1) + " is " + children[i].name + " and his student id is " + children[i].id);
    }
    keyboard.close();
  }
}