Java three classes hierarchy

Question

I've been reading book algorithms on Java by Robert Lafore. And there is an problem - Object sorting. So there are three classes which make hierarchy. In book it is fine working but in Eclipse I have problem "Implicit super constructor Person() is undefined. Must explicitly invoke another constructor". What shall I do with extentions classes, or what shall I change to make this code run? Please help. Thank you.

class Person {
    private String lastName;
    private String firstName;
    private int age;

    public Person(String last, String first, int a) {
        lastName = last;
        firstName = first;
        age = a;
    } 

    public void displayPerson() {
        System.out.print("Last name: " + lastName);
        System.out.print(". First name: " + firstName);
        System.out.println(". Age: " + age);
    }

    public String getLast() {
        return lastName;
    }
}

Next class extends person.

public class ArrayInObj extends Person  {

    private Person a[];
    private int nElems;

    public ArrayInObj(int max) {        // Here is the problem
        a = new Person[max];
        nElems = 0;
    }

    public void insert(String last, String first, int age) {
        a[nElems] = new Person(last, first, age);
    }

    public void display() {
        for (int i = 0; i < nElems; i++) {
            a[i].displayPerson();
        }
        System.out.println(" ");
    }

    public void insertionSort( ) {
        int in, out;

        for (out = 1; out < nElems; out++) {
            Person temp = a[out];
            in = out;

            while (in > 0 && a[in-1].getLast().compareTo(temp.getLast()) > 0) {
                a[in] = a[in -1];
                --in;
            }
            a[in] = temp;
        }
    }
}

And the main class with main function.

public class ObjectSort extends ArrayInObj {

    public ObjectSort(int max) {
        super(max);
    }

    public static void main(String[] args) {

        int maxSize = 10;
        ArrayInObj arr = new ArrayInObj(maxSize);

        arr.insert("Evans", "Patty", 24);
        arr.insert("Smith", "Lorainne", 37);
        arr.insert("Yee", "Tom", 43);
        arr.insert("Adams", "Henry", 63);
        arr.insert("Hashimoto", "Sato", 21);

        System.out.println("Before sorting: ");
        arr.display();

        arr.insertionSort();

        System.out.println("After sorting: ");
        arr.display();

    }

}

define no-arg constructor in Person call like `public Person(){}` I would question is ArrayObj really a person? — SMA, Jul 11 '16 at 13:37
You have a class ``ArrayInObj`` that is used as a container for multiple instances of ``Person``. As such, it should not extend ``Person``. Remove the inheritance (by removing ``extends Person``) and you dont have to worry about that error anymore. — f1sh, Jul 11 '16 at 13:38
@f1sh, +1. The question is a typical sample of inheritance misuse. Probably it's enough to use `List` here. If author needs a tree of persons (each person needs to have list of related persons), it should be done in a different manner by implementing type agnostic generic data structure like `MyTree`, then a type specific tree of persons could be created: `new MyTree()`. — Vladimir Vagaytsev, Jul 11 '16 at 13:43
Is this an example in a book (I highly doubt that)? ArrayInObj is certainly not a Person. Why does it inherit from Person? — Nuri Tasdemir, Jul 11 '16 at 13:43
You have transcribed the code incorrectly. There is no inheritance among these three classes in the Lafore book. Recommend closing as a typo. — Erick G. Hagstrom, Jul 11 '16 at 14:29
Yes in book there are no inheritance. Sorry that I made this thread. Thank you all guys! — Turtality, Jul 11 '16 at 15:59

Laurenz Albe · Accepted Answer · 2016-07-11T13:57:14.017

The technical reason why you are getting this error is that the only constructor of ArrayInObj implicitly calls the constructor of its superclass as follows:

public ArrayInObj(int max) {
    /* this gets implicitly added and fails to compile */
    super();
    a = new Person[max];
    nElems = 0;
}

To fix the compile error, you can either explicitly call the superclass's existing constructor or define a zero-argument constructor for Person.

Reading your code I get the impression that the correct solution would be to remove the extends Person from ArrayInObj's definition. Ask yourself: Is a ArrayInObj a Person? No, it seems to be a container for an array of Person.

score 1 · Answer 2 · edited Jul 11 '16 at 13:52

1

ArrayInObj is extending Person, and in the constructor for ArrayInObj, it is implicitly calling the Person constructor super() (by not explicitly calling a different super()). But, the Person class does not have a zero-argument constructor. Either explicitly call a Person constructor that is defined, super(a,b,c), or do not have ArrayInObj extends Person.

edited Jul 11 '16 at 13:52

Vladimir Vagaytsev

2,871
9
33
36

answered Jul 11 '16 at 13:43

Isaac Hanson

1,088
9
14

score 0 · Answer 3 · answered Jul 11 '16 at 13:40

There are three possibilities:

you add an constructor Person() to your Person class
you call the existing constructor with super(last, first, a) as first call in your ArrayInObj classes constructor
Is your ArrayInObj-class realy a Person? Does not look like it is. Remove extends Person

score 0 · Answer 4 · edited May 23 '17 at 10:33

0

As others have said, you need to provide a default constructor in your Person class, so that the ArrayInObj class may use it.

You will also benefit from reading about constructors in the documentation https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html, or regarding default constructors this site Java default constructor.

edited May 23 '17 at 10:33

Community

1
1

answered Jul 11 '16 at 13:44

Sumiya

317
4
5

Java three classes hierarchy

4 Answers4