assigning super class a reference java

Question

I have a class Vector with a constructor

Vector(int dimension) // creates a vector of size dimension

I have a class Neuron that extends the Vector class

public class Neuron extends Vector {

    public Neuron(int dimension, ... other parameters in here ...) { 
         super(dimension);
         // other assignments below here ...
     }    
}

What I want to be able to do is assign the Vector in the Neuron class a reference to another Vector. Something along the lines of

    public Neuron(Vector v, ... other parameters in here ...) { 
         super = v;
         // other assignments below here ...
     }

Of course, I can't do this. Is there some work around? Even if I was not able to do this in the constructor of the Neuron class, that would probably be OK.

Wanting to do this is often a good indication that you should be using [composition instead of inheritance](http://stackoverflow.com/a/2399554/228171) — Mark Peters, Jul 26 '12 at 15:39
I wouldn't use Vector unless you have to. I would use ArrayList instead and I wouldn't sub-class it. It is better to use delegation as required. — Peter Lawrey, Jul 26 '12 at 15:45
Vector is my own class that mimics the behavior of mathematical vectors. I should have made it clear that I wasn't talking about the java.util Vector class here. However, it's more of a general question on assigning a reference to the super class. — COM, Aug 02 '12 at 04:13

score 11 · Accepted Answer · edited May 23 '17 at 12:03

11

You need to create a copy constructor in the Vector class:

public Vector(Vector toCopy) {
    this.dimension = toCopy.dimension;

    // ... copy other attributes
}

and then in Neuron you do

public Neuron(Vector v, ... other parameters in here ...) { 
     super(v);
     // other assignments below here ...
}

You may also consider using on composition instead of inheritance. In fact, that is one of the recommendations in Effective Java. In such case you would do

class Neuron {
    Vector data;

    public Neuron(Vector v, ... other parameters in here ...) {
        data = v;
        // other assignments below here ...
    }
}

Related questions:

edited May 23 '17 at 12:03

Community

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answered Jul 26 '12 at 15:29

aioobe

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I implemented something like that and it didn't give me exactly what I was looking for. The dimension specifies the size of the array in the Vector class. The array is the only member of the class so this is what I have: 'public Vector(Vector toCopy) { this.array = toCopy.array }' However, when I use the copy constructor to do 'Vector v2 = new Vector(v1); System.out.println(v1); System.out.println(v2);' it prints out two different things which is different than v1 = v2. Though when I make a change to one vector it carries through to the other. – COM Jul 26 '12 at 15:50
As long as you do `this.array = toCopy.array` changes to one array should affect the other too. Just avoid things such as `System.arraycopy` and you should be fine. – aioobe Jul 26 '12 at 15:51
Interesting that you posted those links about composition vs inheritance because I was using composition before and switched to inheritance as it mad the code a bit cleaner, but maybe i should switch back. – COM Jul 26 '12 at 16:20
It usually looks a bit cleaner code-wise when changing to inheritance, but it's not as flexible and easy to maintain. The rule of thumb is to use inheritance, if and only if there is a true *is-a* relationship between the classes. So, ask yourself, *is* a Neuron a Vector? – aioobe Jul 26 '12 at 16:46

assigning super class a reference java

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