Fluent interface

In software engineering, a fluent interface (as first coined by Eric Evans and Martin Fowler) is an implementation of an object oriented API that aims to provide for more readable code.

A fluent interface is normally implemented by using method cascading (concretely method chaining) to relay the instruction context of a subsequent call (but a fluent interface entails more than just method chaining ^[1]). Generally, the context is

• defined through the return value of a called method
• self-referential, where the new context is equivalent to the last context
• terminated through the return of a void context.

History[edit]

The term "fluent interface" was coined in late 2005, though this overall style of interface dates to the invention of method cascading in Smalltalk in the 1970s, and numerous examples in the 1980s. The most familiar is the iostream library in C++, which uses the << or >>operators for the message passing, sending multiple data to the same object and allowing "manipulators" for other method calls. Other early examples include the Garnet system (from 1988 in Lisp) and the Amulet system (from 1994 in C++) which used this style for object creation and property assignment.

Examples[edit]

Java[edit]

The jOOQ library models SQL as a fluent API in Java

Author a = AUTHOR.as("a");
create.selectFrom(a)
      .where(exists(selectOne()
                   .from(BOOK)
                   .where(BOOK.STATUS.eq(BOOK_STATUS.SOLD_OUT))
                   .and(BOOK.AUTHOR_ID.eq(a.ID))));

　　

The op4j library enables the use of fluent code for performing auxiliary tasks like structure iteration, data conversion, filtering, etc.

String[] datesStr = new String[] {"12-10-1492", "06-12-1978"};
...
List<Calendar> dates = 
    Op.on(datesStr).toList().map(FnString.toCalendar("dd-MM-yyyy")).get();

　　

The fluflu annotation processor enables the creation of a fluent API using Java annotations.

Also, the mock object testing library EasyMock makes extensive use of this style of interface to provide an expressive programming interface.

Collection mockCollection = EasyMock.createMock(Collection.class);
EasyMock.expect(mockCollection.remove(null)).andThrow(new NullPointerException()).atLeastOnce();

　　

In the Java Swing API, the LayoutManager interface defines how Container objects can have controlled Component placement. One of the more powerful LayoutManager implementations is the GridBagLayout class which requires the use of the GridBagConstraints class to specify how layout control occurs. A typical example of the use of this class is something like the following.

GridBagLayout gl = new GridBagLayout();
JPanel p = new JPanel();
p.setLayout( gl );
 
JLabel l = new JLabel("Name:");
JTextField nm = new JTextField(10);
 
GridBagConstraints gc = new GridBagConstraints();
gc.gridx = 0;
gc.gridy = 0;
gc.fill = GridBagConstraints.NONE;
p.add( l, gc );
 
gc.gridx = 1;
gc.fill = GridBagConstraints.HORIZONTAL;
gc.weightx = 1;
p.add( nm, gc );

　　

This creates a lot of code and makes it difficult to see what exactly is happening here. The Packer class, visible at http://java.net/projects/packer/, provides a Fluent mechanism for using this class so that you would instead write:

JPanel p = new JPanel();
Packer pk = new Packer( p );
 
JLabel l = new JLabel("Name:");
JTextField nm = new JTextField(10);
 
pk.pack( l ).gridx(0).gridy(0);
pk.pack( nm ).gridx(1).gridy(0).fillx();

　　

There are many places where Fluent APIs can greatly simplify how software is written and help create an API language that helps users be much more productive and comfortable with the API because the return value of a method always provides a context for further actions in that context.