Problem filling an array of objects. The result is always null

Question

I created a three-class program. In the third class, I should fill the array with the values "name", "surname", "code", and "age". What happens though is that my array is always "null", and I can't fill it. Thank you all for your help.

First class:

import java.util.Scanner;

public class Main {
public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    String name, surname;
    int code, age, i=0, min=150;        
    
    System.out.println("How many teachers do you want to enter information?");
    i=in.nextInt();     

            
    for(int j=0; j<i; j++) 
    {
        System.out.println("Enter the teacher's name");
        name=in.next();
        System.out.println("Enter the teacher's surname");
        surname=in.next();
        System.out.println("Enter the teacher's code");
        code=in.nextInt();
        System.out.println("Enter the teacher's age");
        age=in.nextInt();
        
        Teacher d = new Teacher(name, surname, code, age);
        System.out.println(d.getCode());
        System.out.println(d.getSurname());
        System.out.println(d.getAge());
        
        University c = new University(name, surname, code, age);
        min=c.MinimumAge(age, min);

    }
    
    System.out.println(min);

}
}

Second class:

public class Teacher
{
    protected String name;
    protected String surname;
    protected int code;
    protected int age;
    
    
    public Teacher(String name, String surname, int code, int age)
    {
        this.name=name;
        this.surname=surname;
        this.code=code;
        this.age=age;
    }
    

    public String getName()
    {
        return name;
    }
    
    public int getCode()
    {
        return code;
    }
    
    public String getSurname()
    {
        return surname;
    }
    
    public int getAge()
    {
        return age;
    }
}

Third class (I put "0" as a location for filling the array just to do tests.):

public class University
{
    private Teacher[] array=new Teacher[4];;
    public University(String name, String surname, int code, int age)
    {
        array[0] = new Teacher(name, surname, code, age);
    }
    
    public int MinimumAge(int age, int min)
    {
        if (age<min)
            min=age;
        return min;
    }
}

None of the variables in the constructor of Teacher exist e.g. this.nome. — Arvind Kumar Avinash
– Arvind Kumar Avinash, Commented Dec 28, 2020 at 17:06
@LiveandLetLive Sorry, I translated the program from Italian. Now I've solved it. Thanks for notifying me. — user14584436
– user14584436, Commented Dec 28, 2020 at 17:09
There is an array, created with size 4, and first element is assigned a value, so the other 3 will be null. What's surprising about that? Besides, how would you know, since you never use the array for anything? — Andreas
– Andreas, Commented Dec 28, 2020 at 17:13

Arvind Kumar Avinash · Accepted Answer · 2020-12-28 17:50:45Z

The class University has not been implemented correctly. You can implement it as:

class University {
    private int counter = 0;
    private final int MAX = 4;
    private Teacher[] array = new Teacher[MAX];

    public void addTeacher(Teacher teacher) {
        if (counter < MAX) {
            array[counter++] = teacher;
        }
    }

    public int minimumAge() {
        if (counter == 0) {
            return 0;
        }
        int min = array[0].getAge();
        for (int i = 1; i < counter; i++) {
            if (array[i].getAge() < min) {
                min = array[i].getAge();
            }
        }
        return min;
    }
}

Accordingly, you need to change your code in main as shown below:

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String name, surname;
        int code, age, i = 0, min = 150;

        University university = new University();

        System.out.print("How many teachers do you want to enter information?: ");
        i = in.nextInt();

        for (int j = 0; j < i; j++) {
            System.out.print("Enter the teacher's name: ");
            name = in.next();
            System.out.print("Enter the teacher's surname: ");
            surname = in.next();
            System.out.print("Enter the teacher's code: ");
            code = in.nextInt();
            System.out.print("Enter the teacher's age: ");
            age = in.nextInt();

            university.addTeacher(new Teacher(name, surname, code, age));
        }

        System.out.println(university.minimumAge());
    }
}

A sample run:

How many teachers do you want to enter information?: 2
Enter the teacher's name: a
Enter the teacher's surname: b
Enter the teacher's code: 1
Enter the teacher's age: 23
Enter the teacher's name: x
Enter the teacher's surname: y
Enter the teacher's code: 2
Enter the teacher's age: 15
15

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