Question

I have already looked at this question enter link description here

however i cant seem to find the answer to WHY there is a composition between the two objects

The link to the uml is here:

enter link description here

Can anyone explain to me why there is a composition? or why there should be anything other than the generalization?

Answer 1

Because the point of the decorator pattern is to decorate a contained component:

public interface Logger {
    void print(String message);
}

public class ConsoleLogger implements Logger {
    @Override
    public void print(String message) {
        System.out.println(message);
    }
}

public class FileLogger implements Logger {
    @Override
    public void print(String message) {
        // TODO write the message to a file
    }
}

public WithCurrentDateLoggerDecorator implements Logger {
    private Logger delegate;

    public WithCurrentDateLoggerDecorator(Logger delegate) {
        this.delegate = delegate;
    }

    @Override
    public void print(String message) {
        message = new Date() + " - " + message;
        delegate.print(message);
    }
}

Using the above, the decorator can be used anywhere you would use any other logger, and the decorator can decorate any kind of logger (console, file, email, socket, whatever). It doesn’t care what the concrete logger does (print to the console or print to a file). All it does is decorate the concrete logger, by adding the current date to the message before letting the delegate logger print it.

If you had to do this without composition (or rather delegation), you would have to create a WithCurrentDateFileLogger and a WithCurrentDateConsoleLogger (add one subclass for each concrete logger). And every other decoration or concrete logger type would make the number of classes to create explode, due to the high number of combinations.