CODEWITHSUNDEEP
javaarchitecturegame-engine
DraxCode
DraxCode

Reputation: 43

Assign unique ID for every new Class (not instance!)

I was wondering, whether it would be possible to assign a unique ID for every new class (not instance!).

Example of what I mean:

public class ChildObjectOne extends SuperClass {}
public class ChildObjectTwo extends SuperClass {}

public SuperClass {
   private final int ID;

   public SuperClass() {
      this.ID = newID();
   }
}

final ChildObjectOne childObjectOne = new ChildObjectOne();
final ChildObjectTwo childObjectTwo = new ChildObjectTwo();

System.out.println(childObjectOne.getID()); //prints 1
System.out.println(childObjectOne.getID()); //prints 2

childObjectOne = new ChildObjectOne();
childObjectTwo = new ChildObjectTwo();

System.out.println(childObjectOne.getID()); //prints 3
System.out.println(childObjectOne.getID()); //prints 4

What I want it to do instead is print 1 and 2 again. It should generate a new ID for every new class, but if I create a new instance of that class, I want the ID to stay the same.

I tried to achieve this using generics:

public SuperClass<T> {
   private static int ID;

   public SuperClass() {
      this.ID = newID();
   }
}

public class ChildObjectOne extends SuperClass<ChildObjectOne> {}
public class ChildObjectTwo extends SuperClass<ChildObjectTwo> {}

I was hoping, it would count passing a different T as a new class, but that didn't work. Instead the ID is the last one that was set.

How can I achieve this kind of ID system?

Upvotes: 1

Views: 1518

Answers (2)

HomeIsWhereThePcIs
HomeIsWhereThePcIs

Reputation: 1464

To expand upon my comment, the class name will give you an unique String ID. If you want that ID to be a number you could do something like this:

class IdGenerator{
    private static int counter = 0;
    private static HashMap<String,Integer> classIdMap = new HashMap<>();

    public static synchronized int getId(Class clazz){
        if (classIdMap.containsKey(clazz.getName())) {
            return classIdMap.get(clazz.getName());
        } else {
            classIdMap.put(clazz.getName(), ++counter);
            return counter;
        }
    }
}

And then from your class you would do:

IdGenerator.getId(this.getClass());

The generated IDs might not be the same every time you run your app, depending on how it is structured. For example:

Scanner in = new Scanner(System.in);
if (in.nextInt() < 100) {
    System.out.println("a = " + new Aclass().id); // a = 1
    System.out.println("b = " + new Bclass().id); // b = 2
} else {
    System.out.println("b = " + new Bclass().id); // b = 1
    System.out.println("a = " + new Aclass().id); // a = 2
}

Upvotes: 1

Basil Bourque
Basil Bourque

Reputation: 339382

Class::getName

Each class in Java already carries a unique identifier: the fully-qualified name of the class. No need for you to add an identifier.

Access the fully-qualified name by calling Class::getName.

For a class:

String.class.getName()

"java.lang.String"

For an object (an instance), call Object::getClass, and then Class::getName. 

customer.getClass().getName()

com.example.invoicing.Customer

Upvotes: 1

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