Displaying list from an array, input data numerically

String [] lastNames = new String [num];
String [] firstNames = new String [num];
int [] grades = new int [num];

for (int i = 0; i < num; i++)
{
    System.out.print ("Enter Student " + (i + 1) + " last name:");
    lastNames [i] = in.nextLine ();
    System.out.print ("Enter Student " + (i + 1) + " first name:");
    firstNames [i] = in.nextLine ();
    System.out.print ("Enter Student " + (i + 1) + " grade:");
    gradess [i] = in.nextInt ();
}

In my opinion, it isn't a good practice to handle associations between arrays, anyway it's up to you to decide your design. If you want to do that, then @Mikhail Vladimirov 's suggestion is the way to go.

On the other hand, just design a class for your needs, and store objects of the class in an array or list.

public class StudentScore{
    String firstName;
    String lastName;
    int grade;

    pulbic StudnetScore(String firstName, String lastName, int grade){
        this.firstName = firstName;
        this.lastName = lastName;
        this.grade = grade;
    }

    //getters(), setters()
}

In the main class:

StudentScore[] studentScores = new StudentScore[num];
for (int i = 0; i < studentScores.length; i++){
    System.out.print ("Enter Student " + (i + 1) + " last name:");
    String lastName = in.nextLine ();
    System.out.print ("Enter Student " + (i + 1) + " first name:");
    String firstName = in.nextLine ();
    System.out.print ("Enter Student " + (i + 1) + " grade:");
    int grade = in.nextInt ();
    studentScores[i] = new StudentScore(firstName,lastName,grade);
}

I recommend you to use an arrayList to store the Student objects. Consider the below example for better understanding:

First, you can create a model class to store the student details with getters() & setters(). It must look something like this:

package com.stack.overflow.works.model;

public class Student {

    private String firstName;
    private String lastName;
    private int score;

    public Student() {}

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public int getScore() {
        return score;
    }

    public void setScore(int score) {
        this.score = score;
    }
}

Next, you may create your StudentScoresApp as shown below to read the input from user:

package com.stack.overflow.works.main;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

import com.stack.overflow.works.model.Student;

public class StudentScoresApp {

    public static List<Student> getStudentScores() {
        List<Student> students = new ArrayList<Student>();
        Student student = null;
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter number of students to enter: ");
        int numberOfStudents = scanner.nextInt();

        for (int i = 0; i < numberOfStudents; i++) {
            student = new Student();
            System.out.print("Enter Student " + (i + 1) + " First Name:");
            String firstName = scanner.next();
            student.setFirstName(firstName);
            System.out.print("Enter Student " + (i + 1) + " Last Name:");
            String lastName = scanner.next();
            student.setLastName(lastName);
            System.out.print("Enter Student " + (i + 1) + " Score:");
            int score = scanner.nextInt();
            student.setScore(score);
            students.add(student);
        }
        scanner.close();

        return students;
    }

    public static void displayStudentScores(List<Student> students) {
        int i = 1;
        for (Student student: students) {
            System.out.println("Student " + (i) + " First Name:" + student.getFirstName());
            System.out.println("Student " + (i) + " Last Name:" + student.getLastName());
            System.out.println("Student " + (i) + " Score:" + student.getScore());
            i++;
        }
    }

    public static void main(String[] args) {
        System.out.println("Welcome to the Student Scores Application");
        System.out.println("*****************************************");
        List<Student> students = StudentScoresApp.getStudentScores();
        System.out.println();
        System.out.println("Displaying Student Scores:");
        System.out.println("*************************");
        StudentScoresApp.displayStudentScores(students);
    }

}

Now, you can run the StudentScoresApp. Sample test result is shown below:

Welcome to the Student Scores Application
*****************************************
Enter number of students to enter: 3
Enter Student 1 First Name:Sandeep
Enter Student 1 Last Name:Thulaseedharan
Enter Student 1 Score:100
Enter Student 2 First Name:Sathya
Enter Student 2 Last Name:Narayanan
Enter Student 2 Score:100
Enter Student 3 First Name:Jayakrishnan
Enter Student 3 Last Name:Lal
Enter Student 3 Score:100

Displaying Student Scores:
*************************
Student 1 First Name:Sandeep
Student 1 Last Name:Thulaseedharan
Student 1 Score:100
Student 2 First Name:Sathya
Student 2 Last Name:Narayanan
Student 2 Score:100
Student 3 First Name:Jayakrishnan
Student 3 Last Name:Lal
Student 3 Score:100

Hope this helps..

Thank you..Happy coding...