let me give you example to understand the the dimnation of this type of classes
public interface transport{
public double getSpeed(){return(0/*the speed*/);}
public void setSpeed(){return(0/*the speed*/);}
}
class car extend transport{
//implementation of transport interface
public double getSpeed(){return(speed/*the speed*/);}
public void setSpeed(){speed=250;}
}
class train extend transport{
//implementation of transport interface
....
}
class plane extend transport{
//implementation of transport interface
....
}
so the interface class is the general concept .
