Thread: Difference with static and non static variables

Hello, I am very new to Java and was going through the Oracle tutorial. I got to the variables page and have got stuck on understanding the difference between static and non-static variables.

I am using the famous Bicycle object as an example. The class defines non static variables and from what the Java definition is of a non static variable, I was under the impression that this means no matter how many objects you create from this class, they will all have their own values even when the variable is manipulated in various objects.

Here is Java's definition:
Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in "non-static fields", that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.

And the example code:

package Bicycle;
 
public class Bicycle {
    int cadence = 0;
    int speed = 0;
    int gear = 1;
 
    void changeCadence(int newValue){
        cadence = newValue;
    }
 
    void changeGear(int newValue){
        gear = newValue;
    }
 
    void speedUp(int increment){
        speed = speed + increment;
    }
 
    void applyBrakes(int decrement){
        speed = speed - decrement;
    }
 
    void printStates(){
        System.out.println("Cadence: "+cadence+" Speed: "+speed+" gear: "+gear);
    }
 
 
}

 
package Bicycle;
 
public class BicycleDemo {
    public static void main(String[] args){
        Bicycle bike1 = new Bicycle();
        Bicycle bike2 = new Bicycle();
 
        bike1.changeCadence(50);
        bike1.speedUp(10);
        bike1.changeGear(2);
        bike1.printStates();
 
        bike2.changeCadence(50);
        bike2.speedUp(2);
        bike2.changeGear(2);
        bike2.changeCadence(40);
        bike2.speedUp(10);
        bike2.changeGear(3);
        bike2.printStates();
    }
}

From what Java states about static variables, I thought the static modifier acted the same how the '&' in PHP in that it would create a reference. Java states that when declared it tells java that there is exactly 1 copy of the variable in existence. I thought this means if you change a value in one object, it would set the same value in the other object.

So, I added the static keyword to all variables when declared but I get the exact same result. Am I completely getting the wrong idea?

Please help!

Regards,

SS

Zaphod thank you so much for breaking that down into simple terms for me. I understand the difference a lot better than before. Obviously I'm sure there is more to learn but I just needed to grasp the basic concept first.

It's the same as a reference variable in PHP then in that if you change the value to reference, it changes the value of the original.

Thank you all for your help.

Kind regards,

SS

if you change the value to reference, it changes the value of the original.

Not sure that is the right concept. Not sure what you mean by reference and by original.
With a static variable, there is only one variable for the class, not one variable for each instance of the class.

Let's say we have a php variable. You create a reference like so:

$var = 0;
$new = &$var;

Here I set $new to point to the same memory as $var so if I make any changes to $new, it also changes the value in the original $var variable whereas if I leave off the ampersand before $var, it creates it's own separate copy of the value, so changing $new will not affect $var, which is similar to instance variables?

I've only just really been able to get on today and I've just looked again at the docs. Can I think of static variables as pointing to the same memory address in all instances of the variable, be it manipulated in the same object or a second object from the same class? For the example above, we create 2 objects from the same class and when declared with static, they both have the same value when printed.

It might be the wrong concept to think of static variables as references but I think they work in a very similar way in comparison to my PHP example.

Am I right in saying this or totally off the mark?

I know it's been answered but I want to really understand it before moving on because I'll just get to a stage where I won't know what I'm declaring and why.

Kind regards,

SS.

static variables are variables that exist for the class and not just in instances of the class.
I don't see where references are related to that.