Java interfaces

Interfaces
What is an Interface?
Creating an Interface
Implementing an Interface
What is Marker Interface?

Inheritance
OOP allows you to derive new classes from existing
classes. This is called inheritance.
Inheritance is an important and powerful concept in
Java. Every class you define in Java is inherited from
an existing class.
Sometimes it is necessary to derive a subclass from
several classes, thus inheriting their data and methods.
In Java only one parent class is allowed.
With interfaces, you can obtain effect of multiple
inheritance.

An interface is a classlike construct that contains only
constants and abstract methods.
Cannot be instantiated. Only classes that implements
interfaces can be instantiated. However, final
public static variables can be defined to interface
types.
Why not just use abstract classes? Java does not permit
multiple inheritance from classes, but permits
implementation of multiple interfaces.

Protocol for classes that completely separates
specification/behaviour from implementation.

One class can implement many interfaces

One interface can be implemented by many classes

By providing the interface keyword, Java allows you
to fully utilize the "one interface, multiple methods"
aspect of polymorphism.

Why an Interface?
Normally, in order for a method to be called from one
class to another, both classes need to be present at
compile time so the Java compiler can check to ensure
that the method signatures are compatible.
In a system like this, functionality gets pushed up
higher and higher in the class hierarchy so that the
mechanisms will be available to more and more
subclasses.
Interfaces are designed to avoid this problem.
Interfaces are designed to support dynamic method
resolution at run time.

Why an Interface?
Interfaces are designed to avoid this problem.
They disconnect the definition of a method or set of
methods from the inheritance hierarchy.
Since interfaces are in a different hierarchy from classes,
it is possible for classes that are unrelated in terms of the
class hierarchy to implement the same interface.
This is where the real power of interfaces is realized.

File InterfaceName.java:
modifier interface InterfaceName
{
constants declarations;
methods signatures;
}
Modifier is public or not used.

File ClassName.java:
modifier Class ClassName implements InterfaceName
{
methods implementation;
}
If a class implements an interface, it should override all
the abstract methods declared in the interface.

public interface MyInterface
{
public void aMethod1(int i); // an abstract methods
public void aMethod2(double a);
...
public void aMethodN();

}

public Class MyClass implements MyInterface
{
public void aMethod1(int i) { // implementaion }
public void aMethod2(double a) { // implementaion }
...
public void aMethodN() { // implementaion }
}

Creating a Multiple Interface
modifier interface InterfaceName1 {
methods1 signatures;
}
modifier interface InterfaceName2 {
methods2 signatures;
}
...
modifier interface InterfaceNameN {
methodsN signatures;
}

Creating a Multiple Interface
If a class implements a multiple interfaces, it should
override all the abstract methods declared in all the
interfaces.
public Class ClassName implements InterfaceName1,
InterfaceName2, …, InterfaceNameN
{
methods1 implementation;
methods2 implementation;
...
methodsN implementation;
}

Example of Creating an Interface

// This interface is defined in
// java.lang package
public interface Comparable
{
public int compareTo(Object obj);
}

Comparable Circle
public interface Comparable
{
public int compareTo(Object obj);
}
public Class Circle extends Shape
implements Comparable {
public int compareTo(Object o) {
if (getRadius() > ((Circle)o).getRadius())
return 1;
else if (getRadius()<((Circle)o).getRadius())
return -1;
else
return 0;
}
}

Comparable Cylinder
public Class Cylinder extends Circle
implements Comparable {
public int compareTo(Object o) {
if(findVolume() > ((Cylinder)o).findVolume())
return 1;
else if(findVolume()<((Cylinder)o).findVolume())
return -1;
else
return 0;
}
}

Generic findMax Method
public class Max
{
// Return the maximum between two objects
public static Comparable findMax(Comparable ob1,
Comparable ob2)
{
if (ob1.compareTo(ob2) > 0)
return ob1;
else
return ob2;
}
}

Access via interface reference
You can declare variables as object references that use an interface
rather than a class type. Any instance of any class that implements
the declared interface can be stored in such a variable.
Example: Comparable circle;
When you call a method through one of these references, the
correct version will be called based on the actual instance of the
interface being referred to.
Example: circle = Max.findMax(circle1, circle2);

Using Interface
This is one of the key features of interfaces.
The method to be executed is looked up dynamically at
run time, allowing classes to be created later than the
code which calls methods on them.
The calling code can dispatch through an interface
without having to know anything about the "callee."

Using Interface
Objective: Use the findMax() method to find a
maximum circle between two circles:
Circle circle1 = new Circle(5);
Circle circle2 = new Circle(10);
Comparable circle = Max.findMax(circle1, circle2);
System.out.println(((Circle)circle).getRadius());
// 10.0
System.out.println(circle.toString());
// [Circle] radius = 10.0

Interfaces vs. Abstract Classes
♦ In an interface, the data must be constants;
an abstract class can have all types of data.
♦ Each method in an interface has only a
signature without implementation;
♦ An abstract class can have concrete
methods. An abstract class must contain at
least one abstract method or inherit from
another abstract method.

Interfaces vs. Abstract Classes,
cont.

Since all the methods defined in an interface
are abstract methods, Java does not require
you to put the abstract modifier in the
methods in an interface, but you must put the
abstract modifier before an abstract
method in an abstract class.

Interfaces & Classes
Interface1_2 Interface2_2

Interface1_1 Interface1 Interface2_1

Object Class1 Class2

• One interface can inherit another by use of the keyword extends.
The syntax is the same as for inheriting classes.
• When a class implements an interface that inherits another
interface, it must provide implementations for all methods
defined within the interface inheritance chain.

Interfaces & Classes, cont.
public Interface1_1 { … } Interface1_2 Interface2_2

public Interface1_2 { … } Interface1_1 Interface1 Interface2_1

public Interface2_1 { … }
public Interface2_2 { … } Object Class1 Class2

public Interface1 extends Interface1_1, Interface1_2
{...}

public abstract Class Class1 implements Interface1
{... }

public Class Class2 extends Class1 implements
Interface2_1, Interface2_2 {...}

Interfaces vs. Absract Classes
♦ A strong is-a relationship that clearly describes a parent-child
relationship should be modeled using classes.
Example: a staff member is a person.

♦ A weak is-a relationship, also known as a-kind-of
relationship, indicates that an object possesses a certain
property.
Example: all strings are comparable, so the String class
implements the Comparable interface.

♦ The interface names are usually adjectives. You can also use
interfaces to circumvent single inheritance restriction if multiple
inheritance is desired. You have to design one as a superclass,
and the others as interfaces.

The Cloneable Interfaces
Marker Interface: An empty interface.
A marker interface does not contain constants
or methods, but it has a special meaning to the
Java system. The Java system requires a class
to implement the Cloneable interface to
become cloneable.
public interface Cloneable
{
}

The Cloneable Interfaces
public Class Circle extends Shape
implements Cloneable {
public Object clone() {
try {
return super.clone()
}
catch (CloneNotSupportedException ex) {
return null;
}
}
------------------------------------------
Circle c1 = new Circle(1,2,3);
Circle c2 = (Circle)c1.clone();

This is shallow copying with super.clone() method.
Override the method for deep copying.

Java interfaces

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Java interfaces