Making class with makes subclass object unique with hashCode and equals

Question

I have LatLotoTicket.java :

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

public class LatLotoTicket {
    private static int counterID = 0;
    private final int id = counterID++;
    private final String address;
    private final Set<Integer> userNumbers;

    public LatLotoTicket(String address, Set<Integer> userNumbers) {
        this.address = address;
        this.userNumbers = userNumbers;
    }

    public LatLotoTicket(LatLotoTicket ticket) {
        this.address = ticket.address;
        this.userNumbers = ticket.getUserNumbers();
    }

    public String getAddress() {
        return address;
    }

    public Set<Integer> getUserNumbers() {
        return new HashSet<>(userNumbers);
    }

    @Override
    public boolean equals(Object o) {
        if(this == o) return true;
        if(o == null || getClass() != o.getClass()) return false;
        LatLotoTicket that = (LatLotoTicket) o;
        return Objects.deepEquals(this.id, that.id);
    }

    @Override
    public int hashCode() {
        return Objects.hashCode(id);
    }


}

I thought I could make a template class that extends this LatLotoTicket class. And make id unique there and give its own hashCode and equals methods, but I couldn't figure out how to make each template has its own static idCounter and id instances.

I tried to make template Class that has:

1. classIDCounter - with would assign id for every new class that extends this object.
2. classID - this would avoid comparing two different objects with same id .
3. IDCounter - counts individual objects id for each class separately
4. id - contains id for each object

Foo and Bar classes:

public class Foo extends Unique<Foo> {
   public Foo() {
       System.out.println("Object ID "super.id + " class id " + super.classID)
   }
}

public class Bar extends Unique<Bar> {
   public Foo() {
       System.out.println("Object ID "super.id + " class id " + super.classID)
   }
}

in main.java :

Foo foo1 = new Foo(); // Object ID 1 class id 1
Foo foo2 = new Foo(); // Object ID 2 class id 1
Bar bar1 = new Bar(); // Object ID 1 class id 2
Bar bar2 = new Bar(); // Object ID 2 class id 2
Foo foo3 = new Foo(); // Object ID 3 class id 1

I got this far(basically nothing) and now I don't know what to do. Unique.java:

import java.util.Objects;

public class Unique<Child> {
    private static int classIDCounter = 0;
    private final int classID = classIDCounter++;

    private static int IDCounter = 0;
    private final int id = IDCounter++;

    @Override
    public boolean equals(Object o) {
        if(this == o) return true;
        if(o == null || getClass() != o.getClass()) return false;
        Unique unique = (Unique) o;
        return Objects.deepEquals(classID, unique.classID);
    }

    @Override
    public int hashCode() {
        int result = classID;
        result = 31 * result + id;
        return result;
    }
}

How can I achieve this? And also is there built in class that does this?

I'm using JAVA 8.

Answer 1

Here it is.

public class Unique {

    private static final Map<Class, AtomicInteger> idCounters = new HashMap<>();
    private static final Map<Class, Integer> classIDs = new HashMap<>();
    private static int classIDCounters2 = 0;

    private final int id;

    public Unique() {
        this.id = nextIdFor(getClass());
    }

    private static int nextIdFor(Class<? extends Unique> aClass) {
        AtomicInteger counter = idCounters.get(aClass);
        if(counter == null){
            counter = new AtomicInteger(0);
            idCounters.put(aClass, counter);
        }
        return counter.incrementAndGet();
    }


    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Unique unique = (Unique) o;
        return getClassID() == unique.getClassID() && getId() == unique.getId();
    }

    @Override
    public int hashCode() {
        int result = getClassID();
        result = 31 * result + getId();
        return result;
    }

    public int getId() {
        return id;
    }

    public int getClassID() {
        Integer classId = classIDs.get(getClass());
        if(classId == null){
            classId = classIDCounters2++;
            classIDs.put(getClass(), classId);
        }
        return classId;
    }
}

But remember, It isn't thread safe. And its slow. And your better don't use it because it is smell bad.