singleton and inheritance in Java

Question

I have a base class that captures some functionality common to two classes. In other words, I can create one base class and make these two classes subclasses of that base cl

Answer 1

I had a similar requirement: I had multiple cache maps with repeating methods and members, so I've created an abstract class like:

public abstract class AbstractCache {

    protected final Map cache;

    protected AbstractCache() {
        this.cache = getDefaultExpiringMap(TimeUnit.HOURS.toMillis(4));
    }

    public Map getCache() {
        return cache;
    }

    public T getAll(String id) {
        return cache.get(id);
    }
}

Then I've extended this class and created a singleton instance:

public final class FooCache extends AbstractCache> {

    public static final FooCache INSTANCE = new FooCache();

    private FooCache() {
        super();
    }

    public void add(String fooId, Integer value) {
        cache.computeIfAbsent(fooId, k -> new HashSet<>()).add(value);
    }
}

And the usage:

public static void main(String[] args) {
    FooCache.INSTANCE.add("a", 1);
    System.out.println(FooCache.INSTANCE.getAll("a"));
}