CODEWITHSUNDEEP
javaconstructorsubclass
user1200325
user1200325

Reputation: 31

Subclass constructor

I created a superclass (Person) & a subclass (Student)

public class Person{
private String name;
private Date birthdate;
//0-arg constructor
public Person() {
    birthdate = new Date("January", 1, 1000);
    name = "unknown name";
}
//2-arg constructor
public Person(String newName, Date newBirthdate){
    this.name = newName;
    this.birthdate = newBirthdate;
}

//Subclass
public class Student extends Person{

    public Student(){
        super(name, birthdate)
}

I get the error: cannor reference name & birthdate before supertype cosntructor has been called. I tried:

public Student(){
    super()
}

but my course tester says I should use super(name, birthdate);

Upvotes: 3

Views: 250

Answers (4)

Brad Mace
Brad Mace

Reputation: 27926

Looks like there are a few misconceptions here:

When you create Student, there isn't a separate Person object--there's just a Student which has all the properties of a Person.

The constructor is what builds the Student, so inside the constructor there is no other Student/Person whose fields you could reference. Once you call super you've initialized the Person portion of the object, and the fields from the Person are accessible, but since it's a new object they can't have been set to anything unless you do it in the constructor.

Your options are either to :

1) use the defaults as set up in Person:

public Student() {
   super(); // this line can be omitted as it's done by default
}

2) Take values as parameters and pass them to the Person constructor:

public Student(String newName, Date newBirthdate) {
    super(newName, newBirthdate);
}

3) Provide new defaults:

public Student() {
    super("Bob", new Date("January", 1, 1990));
}

Upvotes: 1

Ted Hopp
Ted Hopp

Reputation: 234867

If your default constructor for Student needs to use the two-argument constructor of Person, you'll have to define your subclass like this:

public class Student extends Person{

    public Student() {
        super("unknown name", "new Date("January", 1, 1000));
    }

    public Student(String name, Date birthdate) {
        super(name, birthdate);
    }
}

Note also that Person.name and Person.birthdate are not visible in subclasses because they are declared private.

Upvotes: 4

MByD
MByD

Reputation: 137442

You need to get the name and birthdate parameters for student somehow. How about:

public Student(String name, Date birthdate){
    super(name, birthdate)
}

you can also do:

public Student(){
    super("unknown name", new Date("January", 1, 1000));
}

Upvotes: 0

jahroy
jahroy

Reputation: 22692

You'll need to create a Student constructor that takes the name and birthday as parameters.

The example you've provided won't work unless the Student is already instantiated.

Upvotes: 1

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