Static final fields and final fields

[Kevin Young]

Static final fields and final fields. What is the difference between the two?

 public class FinalData {
private final int v1 = 1;
private static final int V2 = 2;

public static void main(String[] args) {
FinalData f1 = new FinalData();
System.out.println(f1.v1);
// not in a static way, but still compiles..
System.out.println(f1.V2);
// OK. Another possibility to read out the variable. Is that all?
System.out.println(FinalData.V2);
	
}
}   

[Sebastian Janisch]

Uhm, that the one is a static variable and the other one an instance variable ?

[Daniel Matthews]

static members belong to the class whereas non-static members belong to the object instance.

[Campbell Ritchie]

Welcome to JavaRanch

"Static" is one of those things often not well explained. I wrote here about "Campbell's classification" and suggested when method might be made static. Anything marked static belongs to the class, not a particular object, and you can see examples if you explore the Math class. Code in a static method is prohibited from accessing instance methods or instance fields or using the keywords this and super. That is because those things all belong to the object, and anything static belongs to the class. "Class cannot access object."

Fields marked static belong to the class

There is one copy of the field

All members of the class have access to that static field.

All instances of the class have access to it ("object can access class").

There is only one copy of that static field, (still only one copy ) so access from objects is on a shared basis.

If one object can change a static field, then all other objects of that class "see" the change.

Fields marked final can be set up once and once only. Fields which are set up immediately can be used as constants, and should be static because there is no point in having multiple copies of the same information which won't change.

public class Car
{
    private static int nextSerial = 0;
/*
 * A final instance field: different for each instance. Declared and initialised on the same line.
 * Once given a value, that cannot change
 */
    private final int serialNumber = ++nextSerial;
/*
 * A final instance field. May be different for different instances. Declared here: can only
 * be given a value in the constructor. Must have a value given before the constructor
 * completes.
 */
 . . .
    private final String make;

    public Car(String make, etc . . . )
    {
        this.make = make;
        . . .
    }
 . . .
}
 . . .
Car car1 = new Car("Ford");
Car car2 = new Car("BMW");

public class Water
{
/*
 * A static final field used as a constant: boiling point will be the same for all
 * Water instances. Note that constant fields may be given public access.
 * Declared and initialised here: a compile time constant.
 */
    public static final int BOILING_POINT = 100;
/*
 * Another constant, but in this case iniatialised in a static initializer block
 */
    public static final int FREEZING_POINT;
    static
    {
        FREEZING_POINT = 0;
    }

/*
 * Final fields, declared as if they were constants, but not marked static.
 * You will have multiple copies of the same value which cannot change:
 * not efficient programming
 */
/**
 * Density of water: the weight of 1l water in kg.
 */
    public final double DENSITY;
/**
 * Specific heat of water: the number of KJoules to heat 1kg water by 1°C
 */
    public final double SPECIFIC_HEAT = 4.16; 

    public Water ( . . . )
    {
        DENSITY = 1.0;
/*
 * Why would you want to initialise density here? All objects will have the same value 1.0
 * which cannot change. Why didn't you declare this as a static field?
 */
 . . .
    }
 . . .
}

[Abhijit Kangale]

v1 is instance variable & v2 is class variable. class variable is one which is member variable of class but is static.

[Campbell Ritchie]

I forgot to say you usually call static members by ClassName.memberName, not objectName.memberName.

[Pratik D mehta]

Thankyou Mr Campbell . This was extremely helpful.

[Campbell Ritchie]

You're welcome