Interfaces in java

There are a number of situations in software engineering when it is important for disparate groups of programmers to agree to a “contract” that spells out how their software interacts. Each group should be able to write their code without any knowledge of how the other group’s code is written. Generally speaking, interfaces are such contracts.

For example, imagine a futuristic society where computer-controlled robotic cars transport passengers through city streets without a human operator. Automobile manufacturers write software (Java, of course) that operates the automobile – stop, start, accelerate, turn left, and so forth. Another industrial group, electronic guidance instrument manufacturers, make computer systems that receive GPS (Global Positioning System) position data and wireless transmission of traffic conditions and use that information to drive the car.

The auto manufacturers must publish an industry-standard interface that spells out in detail what methods can be invoked to make the car move (any car, from any manufacturer). The guidance manufacturers can then write software that invokes the methods described in the interface to command the car. Neither industrial group needs to know how the other group’s software is implemented. In fact, each group considers its software highly proprietary and reserves the right to modify it at any time, as long as it continues to adhere to the published interface.
Interfaces in Java
In the Java programming language, an interface is a reference type, similar to a class, that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated – they can only be implemented by classes or extended by other interfaces. Extension is discussed later in this lesson.

Defining an interface is similar to creating a new class:

Public interface OperateCar {

// constant declarations,

if any

// method signatures
int turn(Direction direction, // An enum with values RIGHT, LEFT
double radius, double startSpeed, double endSpeed);
int changeLanes(Direction direction, double startSpeed, double endSpeed);
int signalTurn(Direction direction, boolean signalOn);
int getRadarFront(double distanceToCar, double speedOfCar);
int getRadarRear(double distanceToCar, double speedOfCar);
// more method signatures

Note that the method signatures have no braces and are terminated with a semicolon.

To use an interface, you write a class that implements the interface. When an instantiable class implements an interface, it provides a method body for each of the methods declared in the interface. For example,

Public class OperateBMW760i implements OperateCar {

// the OperateCar method signatures, with implementation –
// for example:
int signalTurn(Direction direction, boolean signalOn) {
//code to turn BMW’s LEFT turn indicator lights on
//code to turn BMW’s LEFT turn indicator lights off
//code to turn BMW’s RIGHT turn indicator lights on
//code to turn BMW’s RIGHT turn indicator lights off

// other members, as needed – for example, helper classes
// not visible to clients of the interface


In the robotic car example above, it is the automobile manufacturers who will implement the interface. Chevrolet’s implementation will be substantially different from that of Toyota, of course, but both manufacturers will adhere to the same interface.

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Interfaces in java