With Java 8 and the ability to pass a functional interface we can take a crack at another approach to building a nested fluent API. Fluent interface is a pattern, which usually relies on method chaining. In this post, I’ll use a Java library I recently open sourced to talk about both. Introduction. posted on Fri, Jan 03 '20 under tag: code. Builder Pattern, Fluent Programming. In software engineering, a fluent interface (as first coined by Eric Evans and Martin Fowler) is an implementation of an object-oriented API … Some good examples in the Java world are jOOQ or Apache Camel. Fluent Interface Definition by Wikipedia. Fluent is a pattern for adding method cascading to languages that don't have it, and combining cascading with chaining. Fluent Interface. Continuing on the example of the previous section, we’re trying to make our builder code better readable by applying a fluent interface to it. This construction is used quite often in frameworks which provide a fluent API. Typically, we could use a minimal example: In the JDK you can find the Calendar#getInstance() method that is similar to the previous example, among others. It’s important to understand that we’re not really changing any logic or behavior. Or in the front-end world, jQuery is a good example. If you're using tools that get flummoxed by setters that return values, Paulo's pattern … In this post on learning Java (and OOP) as a functional programming person, I look at the builder pattern, what problem it solves, and why it may be that way. It’s often repeated among experienced software professionals that code is read FAR more than it is written. You can certainly put a Fluent Interface on top of a Director/Builder pattern to make it read more – well – fluently and to emphasize the domain concepts (vs. the technical process of building and delegating). Fluent assertion pattern is supported by testing libraries. But also the stream API of Java 8 follows this style. The last method, build(), is a terminating method as it stops the fluent interface. Bad, Java! That would probably be an Expression Builder then. Libraries promoting the fluent builder pattern. I love fluent interfaces, and I love the builder pattern. Before Java 8, a nested fluent API was difficult to write as the following articles demonstrate (here, here, and here). Paulo Abrantes offers another way to make JavaBean setters fluent: define an inner builder class for each JavaBean. One of them is asserjJ that works very well with JUnit. Firstly it follows fluent builder pattern and allows to apply one assertion at a time. Further – in case of test failure – it facilitates writing one detailed message. Certain design decisions in Java … The Builder pattern is extremely popular in Java applications. I’d like to stress that Fluent Interfaces are not just Method Chaining. 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