Singletonizer

From APIDesign

Singletonizer is a simplification of Factory by use of Generics. It is very useful pattern for creating ProviderAPI. On the other hand, it is not that useful for ClientAPI, as its use requires an OpenClass concept.

Instead of having one class representing a Factory which creates another class representing the data, one can move all the methods into one class:

Code from Digestor.java:

public abstract class Digestor<Data> {
   protected abstract byte[] digest(Data data);
   protected abstract Data create(String algorithm); 
   protected abstract void update(Data data, ByteBuffer input);
 
}
 

This simplifies the life for the implementors of the Digestor class while retaining the same power as they had before. They have to subclass and implement just one type. Yet they can also have own data type. Moreover, due to use of Generics they do not need to deal with any casts or unsafe type checking. Or, just like in following example, one can reuse suitable existing data type:

Code from CountingDigestor.java:

public final class CountingDigestor extends Digestor<int[]> {
    @Override
    protected byte[] digest(int[] data) {
        int i = data[0];
        byte[] arr = { 
            (byte) (i & 255), 
            (byte) ((i >> 8) & 255), 
            (byte) ((i >> 16) & 255), 
            (byte) ((i >> 24) & 255) 
        };
        return arr;
    }
 
    @Override
    protected int[] create(String algorithm) {
        return "cnt".equals(algorithm) ? new int[1] : null;
    }
 
    @Override
    protected void update(int[] data, ByteBuffer input) {
        data[0] += input.remaining();
        input.position(input.position() + input.remaining());
    }
}
 

Singletonizer is useful when representing complex graphs or tree hierarchies. One can have a class in a ClientAPI that represents each graph's vertex. There can be tons of its instances. Yet plugging in behaviour for each instance can be quite simple. They can all delegate to single Singletonizer implementation which handles all their operation. Thus they are basically singletonizing behaviour of many instances to one - hence the name Singletonizer.

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