has-a and is-a (inheritance)

[Ryan Tracy]

Can someone explain in detail what has-a and is-a in inheritance is?
I am trying add to a class some functionality from another class and a co-worker said to me,

There is a difference between a class that HAS and one that IS.

[Greg Charles]

"is a" and "has a" distinguish inheritance from delegation. For example, let's say you want a Vector that only allows objects of type Animal to be added to it. You have a choice of extendig java.util.Vector and overriding the methods to make that restriction. That's "is a". On the other hand, you could make a class that contained a Vector as a member variable. Your methods could mimic the interface of Vector, and delegate the work to the Vector member. That's "has a". Each approach has pros and cons. The GOF Patterns book is a good reference for this issue.

[Ryan Tracy]

Anyone have another example or explanation?

[Marilyn de Queiroz]

A Car "is-a" Vehicle.
A Vehicle "has-a" Motor.
A Car "is-a" Vehicle.
A Car "has-a" Motor.
A Bicycle "is-a" Vehicle.
A Bicycle does not have a Motor.

[Matthew Phillips]

Here is the code that someone showed me to explain it.

public class Vehicle
{
    protected Motor motor ;
}

public class Car extends Vehicle
{
    private Tire tire ;
    private Trunk trunk ;
}
public class Motor
{
}

public class Tire
{
}

public class Trunk
{
    
}

All vehicles have a motor so the vehicle has a member variable of type motor. A Car is a specialized type of vehicle because it inherits from vehicle. Since the Motor in Vehicle is protected Car can access this. A Car also has a Trunk and a Tire.

[Greg Charles]

Ouch!

[Matthew Phillips]

Originally posted by Greg Charles:
Ouch!

???