Set for single instances of classes

Question

I have a Set of objects and I don't want to allow to store there more than one instance of any class. Which solution is the best:

• check if there is an instance of given class in a set (doesn't seem to be efficient)
• implement hashCode to return constant and equals to return o != null && o.getClass() == getClass()
• make own implementation of Set (almost like first option)
• anything else? maybe there is already such a set?

Answer 1

Guava provides ClassToInstanceMap, which is already checked for safety, tested, and the like. It's like a Map<Class<T>, T>, but it deals with all the type system stuff already.

Answer 2

I would use a Map<Class<? extends T>, T> for it.

---

Example implementation:

class SingleInstanceSet<T> {

    Map<Class<? extends T>, T> map = new HashMap<Class<? extends T>, T>();

    public boolean add(T o) {

        if (map.containsKey(o.getClass()))
            return false;

        map.put((Class<? extends T>) o.getClass(), o);
        return true;
    }

    public T get(Class<? extends T> klass) {
        return map.get(klass);
    }
}

Example usage:

public static void main(String[] args) {
    ClassSet<Object> instances = new ClassSet<Object>();

    instances.add("hello");
    instances.add(1234);
    instances.add("will not be added");

    System.out.println(instances.map);
}

Outputs something like:

{class java.lang.String=hello, class java.lang.Integer=1234}

Answer 3

You could invent implement a small wrapper class and store that instead of the instances:

 public class InstanceWrapper {
   private Object obj;
   public InstanceWrapper(Object obj) { this.obj = obj; }
   public Object getObject() { return obj; }

   @Override
   public boolean equals(Object other) {
     return this.getClass().equals(other.getClass());
   }

   @Override
   public int hashCode() {
     return this.getClass().hashCode());
   }
 }

As equality is defined via the Class instances, a set will only accept one instance of any class.

Answer 4

Use a HashMap. Before adding an object, get its class (using reflection) and check whether it is already a key in the HashMap.