Static variable declaration is superclass - Java

Question

I have the following program:

class Vehicle{  
  static int speed=50;  
  Vehicle(){
      this.speed=speed;
  }
}  


class Bike4 extends Vehicle{  
  int speed=100;  

  void display(){  
   System.out.println(Vehicle.speed);//will print speed of Vehicle now  
  }  
  public static void main(String args[]){  
   Bike4 b=new Bike4();  
   b.display();  
   }  
}

Supposing I don't want to use the keyword "super" and I simply call the function "Vehicle.speed" why do I have to change the type of the variable speed in the superclass to static? What would happen if I run the program without using the static keyword? (again, supposing it compiles)

What do you mean by "simply call the function Vehicle.speed"? That's a field, not a function. And static isn't a type, it's a modifier... — Jon Skeet
– Jon Skeet, Commented Feb 19, 2015 at 19:39
I think you should review the class notes about class (static) vs. instance (non-static) fields. Vehicle.speed as you wrote it is not the speed of a given vehicle. It's the speed of all vehicles, which makes no sense. — RealSkeptic
– RealSkeptic, Commented Feb 19, 2015 at 19:40

spudone · Accepted Answer · 2015-02-19 20:14:59Z

1

Because you're defining a different speed for Bike4 as opposed to the parent Vehicle, it looks like you want to change the derived value. A static variable won't work because it belongs to the class, not the instance. I think you want something like this instead:

public class Vehicle{  
    protected int speed;  
    Vehicle(int speed) {
        this.speed=speed;
    }
}  


public class Bike4 extends Vehicle {
    public Bike4(int speed) {
        super(speed);
    }

    void display() {  
        System.out.println(speed);
    }
    public static void main(String args[]) {  
        Bike4 b=new Bike4(100);  
        b.display();  
    }  
}

answered Feb 19, 2015 at 20:14

spudone

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