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An interface is a description of a set of methods that
conforming implementing classes must have.
Under the hood, an interface is like a class with some
All its methods must be abstract instance methods, no static methods allowed. However you don’t declare the
methods abstract. That is implicit.
Yet, all the variables defined in an interface
must be staticfinal, i.e. constants. Happily the
values need not be known at compile time. You can do some
computation at class load time to compute the values. The variables need not be
just simple ints and Strings. They can be any type. Normally you leave off the
static final as it is implicit.
No static initialiser blocks. You must do your
initialisations in one line per variable. And, of course, no static initialiser helper methods defined in the interface. If you need them, them must be defined outside the
The throws clauses about Exceptions in the interface’s methods must exactly match the
throws clauses of the implementing methods.
Common interfaces include
java.util.Map and java.util.Set.
extend or implement?
a class extends another class.
a class implements one or more interfaces.
an interface extends one of more interfaces.
methods are always instance methods. To use them, there must be some associated
Object that implements the interface. You can’t
instantiate an interface directly, but you can
instantiate a class that implements an interface. References to an Object can by
via the class name, via one of its superclass names, or
one of its interface names.
In return for these restrictions, interfaces
possess a feature that classes do not — they can be multiply inherited. A new
class can extend only one superclass to inherit methods
and variables from, however, a class can implement
multiple interfaces. An interface can extend several other interfaces. An interface enforces how some of your methods must look to the outside
world. It says nothing about how you implement them. In contrast, a class usually brings with it some default implementations.
All methods in an interface are implicitly declared public and abstract. All variables in an interface must be constants. They are implicitly declared
The Business Metaphor
You won’t see many interfaces in
small programs. However, they are invaluable in large projects with many classes for
managing the complicated interactions between the classes. Managing a large set of
classes is a bit like managing a giant bureaucracy. You need formal job descriptions
and formal descriptions of who reports what information to whom, and who can ask what
services of whom. If you try to manage large collections of interacting classes
without using interfaces, every class becomes inextricably intertwined with all the
other classes and you can’t change anything without breaking everything. You
also get into chicken and egg problems. How can I test (or even compile) my code
without all the other classes I interact with fully functional? You can’t keep
track of the flow of information. You have no idea how the whole thing works, even if
I will develop this metaphor that will only gradually come clear over the years as
you experiment with interfaces. An interface is like a
rôle description. A person in a bureaucracy may play several rôle s:
filing clerk, keeper of petty cash, manager of the coffee machine. You are restricted
to a subset of her total capabilities when you interact with her in each of her
For someone to temporarily take over as manager of the coffee machine, all they
need assume is the one rôle, not all the capabilities of the usual
multi-talented coffee lady. The rôle description describes the
minimal capabilities for someone to pinch hit as a coffee machine
manager. Even a male can be the coffee lady.
With clearly defined rôle s, it is easy to ask, do you have the capabilities
to pinch hit as coffee lady? You don’t have to check out the necessary skills
one by one. An interface is like a skill set
In a hierarchical business organisation, each person has a formal set of
responsibilities, information to be passed to superiors, information to be gleaned
from underlings, commands taken from superiors, and commands given to
underlings… Most people in an organisation play many rôle s, depending
on whom they are talking to. Underlings are severely restricted in the services they
can demand of the president. Interfaces formally define the precise set of services
each class must provide in a given rôle . rôle s simplify interactions.
The requestor knows what services are guaranteed to be provided, and interfaces
prevent the requestor from asking for services outside the formally agreed on
rôle, even if the requestee is capable of providing them.
With interfaces, it makes who fulfills a rôle
interchangeable. Nobody informally comes to rely on the additional petty cash skills
of the original coffee lady, unless they formally interacted with
the coffee lady also in the rôle of petty cash manager. Different people could
pinch hit for a vacationing coffee lady, one taking over her coffee lady rôle
and one taking over her petty cash manager rôle . Interfaces allow a class to
say to other classes, if you want to interact with me in rôle A, you must be
capable of assuming rôle s B, C and D. It makes it much simpler to plug classes
together if all you have to do is find a matching set of compatible rôle s
(interfaces). If they won’t fit, it is clear what minimal work
you would have to do to upgrade the skill set (methods), to allow a class to act in a
new rôle, and hence to allow it to interact with some other particular
Rôle descriptions, like interfaces, can be nested (multiply inherited). The
rôle description for a telephone supervisor naturally assumes capability of
also acting as an operator, and also the rôle of petty cash manager.
Interfaces are a discipline to keep you from using any more of a class’s
methods than you absolutely have to. They thus make it much easier to replace a
class with some other mininmally functional class to fulfil some subset of its
There are some dummy marker interfaces without
any methods that don’t require you to write any code to implement them. They
include Serializable (note the American z spelling)
and Cloneable. All you do is add the line implementsSerializable to the class
definition line, and presto, your class can partake in pickling.
Interfaces are the duct tape of Java. Imagine that
JApplet had been developed totally independently of
Applet. (It wasn’t. JApplet is a subclass of Applet.) How could
you write some code that worked with both Applet and
JApplet objects, letting you get at the methods the two
have in common? In other words, how can you treat Objects
identically when they belong to similar classes, with many identically named methods,
but where that set of common methods is not reflected in a common base class?
You could use Java’s duct tape — the interface. You would define an interface say GeneralApplet with some of
the methods common to Applet and JApplet. You would then modify both Applet
and JApplet to implement your GeneralAppletinterface. This is very
easy to do, just add the words implementsGeneralApplet to the class definition line of both Applet and JApplet. You now have a
GeneralApplet reference that could be either an
Applet or JApplet decided
You would have glued together classes that came from quite different ancestries. If
necessary, you can mask slight differences in the method signatures of the two
classes with some interface wrapper methods.
Hymn of Praise
By using rôle s you describe the bare bones of
the interactions. You make clear what skills a class has are and are not necessary to
fulfill a given rôle . This pruning out all unessential skills from
consideration make the interactions much easier to understand. Avoiding interfaces is
a great way to write unmaintainable code.
You will find yourself using interfaces more and more as you work on complex
projects. They let you simplify the possible interactions between groups of classes
to just what is needed. Interfaces formalise, document and enforce the interactions
between classes. They make code components more independent and reusable. Pieces of
code can be written even when the code it needs to interact with is not written yet
or is currently not compilable. In GUI (Graphic User Interface)
s, where you have panels within panels communicating, you will find it helps
immensely to define an interface for each parent
listener callback delegate, even when there is only one caller class.
Interfaces give a much finer degree of control of how classes interact than the
public/ private scheme.
Eclipse refactoring leaves array
constants behind when you ask it to move them off to an interface. However, you can move them manually yourself. It is
perfectly ok to have array constants in an interface.
It is considered bad form to write
an interface with nothing but staticfinal constants it in. An
interface should have at least one abstract method. If all you want is a collection of constants, use
an ordinary class with import static
Interfaces have yet another use — in
writing easy-to-maintain code. Let us assume you created an ArrayList, then later had to change your code so that you used a
Vector instead, and then later realised a LinkedList would be the best way to handle it. You would have to
change every ArrayList cast and reference to Vector then later to LinkedList. You
might accidentally change some unrelated ArrayList. If
instead you used the interfaceList as your reference type, which these three classes share in
common, the only thing that would have to change is the newArrayList( 300); All the rest could stay as List. Let me put it yet another way. If you wrote a method that
expected a List parameter, it would happily work with
either an ArrayList, Vector or
LinkedList. You can write more generic code using
Reusable Code, Plug-Compatible
Yet another advantage of using
interfaces is it makes the type system less prissy when it does not really matter.
Anything that can implement List is roughly
interchangeable. You can work with a mixture of Vectors,
and OSVectorLists (ObjectStore persistent arrays), etc.
and for the most part, all just comes out in the wash in the interactions. It is as
though everything were a one-size-fits-all List.
When you use an interface, you lower the bar to
potential class interactions, and hence reusability. You demand fit only where fit
Yet another advantage of using
interfaces comes when you have a large project. If you modify a method, that class
has to be recompiled, but just in case you changed one of the method signatures, all
the code that calls that method has to be recompiled too. If you use an interface, and don’t change the interface (which lives in a separate file from the implementations),
changing just the interface implementation classes,
nothing need be recompiled, so long as those classes accessed via the interface name rather than specific class name, e.g. by using
factories and interface reference names. This is another
example of the adage All computer programs can be solved by
adding sufficient layers of indirection. In fact you can write and compile
code to call methods in an interface specification even
before anyone has written any implementations.
What You Don’t Specify
You can’t specify
static methods in an interface. All interface methods are
instance methods. Even if you could, how would you invoke the static method? There would be no way to invoke it via its
interface name, (How would the compiler know which
implementation version you wanted?) so there would be not much point in it. I think a
static declaration still might be useful as to the implementor of an interface to remind him to supply a useful static method to be invoked explicitly via its class name.
You don’t need to specify public in the
interface method definitions. That is presumed. However,
when you go to implement the interface, you need to
specifically spell out the public, so I find it easier to
specify public in the interface to make them more useful templates.
IDEs (Integrated Development Environments)
today are getting smarter and will insert it for you when you use wizards to generate
a skeleton implementation of an interface.
I don’t recommend this, but you can also leave off the static finals on your interface
You don’t specify synchronized in your
interface method declarations. This is an implementation
List of all Interfaces in the JDK (Java Development Kit)
Here is a list of all the interfaces in JDK1.8.0_20Last revised/verified:2014-05-04
by extracting the interface names from the all-classes.html