Method Chaining

[tweet-dzone]Method chaining is a simple programming technique that can be implemented in almost any programming language. In simple words, it means that a method performs some operations on "this" object and then returns it so it can be used further more. It allows us to invoke several methods one after another, on one or different objects, usually written in the same line.

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).