For example we can build a rectangle class using using the Method Chaining for setters. You can see the setters are a little bit different than regular setters:

class ChainedRectangle
{
    protected int x;
    public ChainedRectangle setX(int x) { this.x = x; return this; }

    protected int y;
    public ChainedRectangle setY(int y) { this.y = y; return this; }

    protected int width;
    public ChainedRectangle setWidth(int width) { this.width = width; return this; }

    protected int height;
    public ChainedRectangle setHeight(int height) { this.height = height; return this; }

    protected String color;
    public ChainedRectangle setColor(String color) { this.color = color; return this; }
}

Then if we need to build a rectangle we can write a single line for setting all the required values:

new         Rectangle().setX(10).setY(10).setWidth(13).setHeight(15).setColor("red");

Along with autocomplete option in most of todays IDEs method chaining might provide a suggestive way of doing some actions in less code.

For example we can write a simple class to build sql queries. Then we can invoke in a simple way.

public class Query
{
    protected String queryString = "";

    public Query select(String what)
    {
        queryString = "SELECT " + what;
    }

    public Query from(String from)
    {
        queryString = " FROM " + from;
    }

    public Query where(String where)
    {
        queryString = " WHERE " + where;
    }

    public ResultSet Invoke()
    {
        //... database invocation bla bla
        return result;
    }
}

Then, to build the query we invoke:

new Query().select("*").from("table").where("a=b").Invoke();

One of the problems of the Method Chaining pattern is the fact that it can not ensure the order of the invoked method. In order to do that each method can return a specific interface object which implements only the allowed methods.

For example we define an interface ISelectedQuery with a single method from, IFromQuery with a single method where. ISelectedQuery.from returns and object of type IFromQuery and IFromQuery returns a Query object. All the interfaces are implemented by the Query object. That way the order of invoking the methods is ensured.

Here is how the code looks like:

public interface ISelectedQuery
{
    IFromQuery from(String from);
}

public interface IFromQuery
{
    Query where(String where);
}

 public class Query implements ISelectedQuery, IFromQuery
{
    protected String queryString = "";

    public ISelectedQuery select(String what)
    {
        queryString = "SELECT " + what;
        return this;
    }

    public IFromQuery from(String from)
    {
        queryString = " FROM " + from;
        return this;
    }

    public Query where(String where)
    {
        queryString = " WHERE " + where;
        return this;
    }

    public ResultSet Invoke()
    {
        //... database invocation bla bla
        return result;
    }
}

The above code ensure that only the right methods can be invoked. Further more the syntax autocomplete option from most of today IDEs will show in the drop down only the right method(s).

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We need somehow to extend the structure of objects for accepting the visitors without changing them. A way of doing it would be to add a wrapper for the hierarchy classes which accepts visitors and duplicates the structure.

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The great idea to achieve it is to use delegation instead of inheritance. Let’s consider we already have to classes we want to extend. We can use the inheritance so our new class will extend the first class. For the second class we can not use inheritance again, because we already inherit the first one, on the other side we want to overwrite some method implementations. The trick is to create an inner class extending the second class, and to delegate the calls from the main outer class to the inner class.

The technique can be “scaled” for any number of classes. For each new class which we need to inherit, we need to create another inner class and delegate the request to it. The big drawback here is the amount of code that should be written for creating the inherited inner classes and the code for delegating all the requests which which in case of multiple inheritance are done by default.

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However there are 2 major drawbacks:
- All the subclasses implement a marker, and there is not option to unmark a subclass. For example if the base class is marked as Serializable all the subclasses will be marked as Serializable and there is no option to unmark them by “unimplementing” the Interface.
- It is dangerous to have several levels of markers (Let’s say we need XmlSerializable and BinarySerializable both extending Serializable). What is a class which is not XmlSerializable? Is it BinarySerializable or NotSerializable?

Starting with Java 1.5 annotations and attributes in .NET can be used instead of Marker Design Pattern. Annotations can be accessed using reflection and can be used to mark not only Classes but also Methods and Attributes.

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