Using subclass method as parameter in super call to superclass constructor

Question

In my code below, I'm trying to call the RandomStringChooser constructor in the RandomLetterChooser subclass constructor.

In order to pass an array as the parameter, I call the getSingleLetters method in super(). However, I'm getting the following error:

"cannot reference this before superclass constructor has been called"

with an arrow pointing at the "g" in getSingleLetters (3rd line of subclass).

Why is this and how should I fix?

FYI: This is my attempted solution to the 1.b. of the AP Computer Science 2016 FRQ (https://secure-media.collegeboard.org/digitalServices/pdf/ap/ap16_frq_computer_science_a.pdf). Their solution (https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap16_compsci_a_q1.pdf) involves an ArrayList (or List, I'm not completely sure about the difference between these two), which I realize is much cleaner, but still calls the getSingleLetters method in the same way as I have done below (and thus should have the same error when run).

class RandomStringChooser {
    private int length;
    private int count = 0;
    private String word;
    private String newArray[];

    RandomStringChooser(String[] array) {
        length = array.length;
        newArray = new String[length];
        for(int i = 0; i < length; i++) {
            newArray[i] = array[i];
        }
    }

    String getNext() {
        if(count == length) return "NONE";
        word = "";
        while(word.equals("")) {
            int index = (int) (Math.random() * length);
            word = newArray[index];
            newArray[index] = "";
        }
        count++;
        return word;
    }
}

class RandomLetterChooser extends RandomStringChooser{
    RandomLetterChooser(String str) {
        super(getSingleLetters(str)); //here's the problem
    }

    String[] getSingleLetters(String str) {
        String[] letterArray = new String[str.length()];
        for(int i = 0; i < str.length(); i++) {
            letterArray[i] = str.substring(i, i+1);
        }
        return letterArray;
    }
}

And here's where I run the program, in case it helps:

public class AP2 {
    public static void main(String[] args) {
        String ball = "basketball";
        RandomLetterChooser s = new RandomLetterChooser(ball);
        for(int i = 0; i < 12; i++) {
            System.out.print(s.getNext() + " ");
        }
    }
}

The `getSingleLetters` method in the assignment is `static`. — Jorn Vernee, Jul 16 '17 at 18:44

davidxxx · Accepted Answer · 2017-07-17T06:51:45.220

"cannot reference this before superclass constructor has been called"

Invoking any instance method during (as you are doing)

RandomLetterChooser(String str) {
    super(getSingleLetters(str)); //here's the problem
}

or before invoking the super class constructor results to a compilation error.

JLS Chapter 8. Classes :

An explicit constructor invocation statement in a constructor body may not refer to any instance variables or instance methods or inner classes declared in this class or any superclass, or use this or super in any expression; otherwise, a compile-time error occurs.

Or said in an other manner, an object cannot be used (instance fields and methods) while this hierarchy (parent class) has not be fully constructed.

By changing the modifier of the getSingleLetters() method to static you could use this code as a static method is not associated to an instance of the class :

 RandomLetterChooser(String str) {
    super(RandomLetterChooser.getSingleLetters(str)); //here's the problem
}

Thanks for the response. Just for additional clarification, why does static make this work? Is it because static methods don't require an instance of a class to run and thus don't require the super constructor to have completed? — kid, Jul 17 '17 at 00:17
Can you achieve this without making the method private? Many of the times it's useful to provide an instance method since the result of the method depends on the object itself. — Haonan Chen, May 12 '20 at 12:33

score 1 · Answer 2 · answered Jul 16 '17 at 19:10

As the other answers explain - this is what should happen.

You see the "construction" of a new object is only finished after the constructor of that class is completely done - and that includes the super class constructors. Only then you can assume for sure that your object is fully initialized.

In other words: you absolutely don't want to do things like your example code: avoid calling "real" methods on your class while it is still in its initialization phase!

score 0 · Answer 3 · answered Jul 16 '17 at 18:52

This is because

getSingleLetters method

Cannot be used before you done with superclass constructor. Actually I mean same as exception text. So You can put this method to your superclass constructor or take a look here:

Is it possible to do computation before super() in the constructor?

Using subclass method as parameter in super call to superclass constructor

3 Answers3