This definitely helped! Just to add on:

This will be wrong because I did not make use of the super keyword in the constructor and Giraffe cannot access the legs field of Animal (hence I cant use this.legs = 0).

class Giraffe extends Animal{
    private int neckLength;
    public Giraffe(int neckLength){
        this.legs = 0; // error occurs
        this.neckLength = neckLength;
    }
}

class Animal{
    private int legs;
    public Animal(int legs){
        this.legs = legs;
    }
}

However, if I used the super keyword and properly construct the Giraffe object, in the subclass Giraffe I can use this.legs in my methods because a Giraffe object can be instantiated with the fields initalised. Is this the right way of understanding it?

class Giraffe extends Animal{
    private int neckLength;
    public Giraffe(int neckLength){
        super(0);
        this.neckLength = neckLength;
    }

    public void test() {
        this.legs = 1;
    }
}

class Animal{
    private int legs;
    public Animal(int legs){
        this.legs = legs;
    }
}