object and instance variable says that they can change at runtime.

[Tomendra Singh]

hi all,
i am studying head first java,in 2nd chapter there is line which says"i can change at runtime--object, instance variable"

firstly i am confused that "object and instance variable can change something" or it is " they can be changed at run time"

whichever exists,

then please help me to understand that.if possible then please with an example.

thank you very much.

[Jackie Gleason]

I will venture a guess....

I believe it is saying an Object's Instance Variables can be changed at run-time. This means that when an object is created a new copy of the variable is created....

Think of it like this Instance variables are different for each instance of the class(object) while static variable values are shared throughout all instances of the object.

The only type of variables that cannot be "changed" at runtime are final variables.

Does that help?

[Campbell Ritchie]

public class Car
{
   private int speed;
 
   public Car( . . . )
   {
       . . .
       speed = 0; //Redundant, but <a href="http://java.sun.com/docs/books/tutorial/java/nutsandbolts/datatypes.html" target="_blank" rel="nofollow">this page</a> says it's bad style otherwise.
       . . .
   }
 
   public void stop()
   {
      speed = 0;
   }
 
   public void goFaster(int mph)
   {
      speed = Math.max(0, speed + mph);
   }
 
   public void goSlower(int mph)
   {
      goFaster(-mph);
   }
}

public class CarDemo
{
   public static void main(String[] args)
   {
      Car ford = new Car("Ford", "red", "NU58 CRC");
 . . .
      ford.goFaster(50);
 . . .
      ford.goSlower(20);
 . . .
      ford.stop();
   }
}

Like that.