I have been looking at how to improve my knowledge of Design Patterns and at the same time sharpen my Java 8 skills too. I have begun to realize that one can combine the two and also realized that Java 8 Lambdas are extremely powerful.
In this post, we are going to take a look at how the implementation of the Command Design Pattern can be simplified using Java 8 Lambdas.
I have written another blog where you can understand the essence of the Command Design Pattern.
The example we are going to refactor using Java 8 Lambdas is from here. This article on the tutorialspoint site refers to the typical way of implementing the pattern. You must take a look at it to get a basic understanding of the domain/problem at hand. The following is not a UML but I have shown the main components for the example mentioned.
In short , BuyStock and SellStock are 2 commands which implement the Order interface. Stock is the recipient and finally the Broker is the invoker. The Broker class deals only with the Order interface and has no idea about the concrete commands.
Let us implement the same using Java 8 Lambdas. The Order interface is a Functional interface and each ConcreteCommand above is really a function. Do we really a class to represent each Command ? Well, take a look at the client code now using Lambdas:
The Broker,Order interface remain the same and so does the Stock class. We can get rid of the individual Concrete Commands (BuyStock, SellStock) classes.
What we have done is actually passed behavior to the takeOrder method. This works as the Order interface has a method execute which takes no parameters and returns void. And hence () -> abcStock.buy() is a lambda expression which takes no parameters and returns void.
To understand this pattern better, let us extend our problem. Imagine that in the same system, we had to sell Bonds too. The process of buying/selling stocks is different from bonds. With the Broker and the Order interface, the system can easily be extended with minimal changes.
We added 2 more classes BuyBond and SellBond which implement the Order interface. These commands would invoke the Bond class( Receiver) which would perform the task of either buying or selling bonds. The Broker class remains untouched as it deals with only the Order Interface. Well, using Java 8 Lambdas, we don’t need the BuyBond or SellBond classes either.
The Command pattern can be implemented easily using Lambdas. One could argue that the Order interface has just 1 method and we haven’t considered the undo and redo commands in the Command Design pattern. We can always fall back on the typical implementation of the pattern in case we need an undo and redo in addition to the execute functionality.
Having said that, do we always need an Order interface which acts as the Command interface ? How about using the Runnable interface instead to run all the commands? It could affect the readability of the code a little bit but otherwise the use of Java 8 Lambdas really makes the implementation easy and powerful at the same time.