Is abstraction possible without inheritance? This is my code
abstract class whatever
{
abstract void disp1();
abstract void disp2();
abstract void disp3();
}
class what {
void disp1()
{
System.out.println("This is disp1");
}
}
public class threeClasses {
public static void main (String args[])
{
what obj =new what();
obj.disp1();
}
}
Please note above, how i:
did not extend the class "what" from abstract class "whatever" and yet the code runs perfectly with no errors
Did not declare class "what" as abstract (since it's not declaring the other two methods disp2() and disp3())
I am very confused. Please help.
You aren't using whatever (and Java naming conventions should be respected). The idea behind an abstract class (and inheritance) is that there is an interface contract. Let's examine it with a more practical example,
abstract class Whatever {
abstract void disp1();
void disp2() {
System.out.println("disp2");
}
void disp3() {
System.out.println("disp3");
}
}
Then make What extend it. Override two methods for demonstration (the annotation is a useful compile time safety check)
class What extends Whatever {
#Override
void disp1() {
System.out.println("This is disp1");
}
#Override
void disp2() {
System.out.println("This is disp2");
}
}
Finally, invoke methods on a What instance through the Whatever contract
public static void main(String args[]) {
Whatever obj = new What();
obj.disp1();
obj.disp2();
obj.disp3();
}
Which outputs
This is disp1
This is disp2
disp3
Note that What is providing the implementation for disp1 and disp2 while Whatever provides disp3.
There is no relationship between your abstract class and your concrete class. Whatever your definition of "abstraction", it actually represents a relationship between types. The abstract keyword does not establish that relationship between classes, it represents that relationship, and not by itself. The relationship needs to be extended from both sides.
abstract is a declaration from one side about a promise that must be kept, for an inheriting type either to implement abstract methods or to ask for that promise from its inheriting types.
The other side makes the promise by being a class that inherits from the abstract type. Without inheritance, the concrete type loses the is-a connection.
You will get the compiler error you're complaining about missing if you correct one major mistake you made. You failed to use the #Override annotation. Always use the #Override annotation when you intend to override a method, or you will forever enjoy just the sort of bug you show here.
I think what he meant was if we can implement abstract class's method without inheriting abstract class.
You might be thinking if we can do it with composition/association/aggregation relation.
To that, I will answer: NO because you can't create an object of abstract class as in these relations you have to make object or reference of the object.
So, the only way to implement abstract methods is through inheritance.
I have a problem with my abstract class.
Here is my interface:
package dovilesUzduotis4;
import java.util.ArrayList;
public interface Interface1 {
void a(ArrayList<K> kM, String g);
}
and abstract class:
package dovilesUzduotis4;
import java.util.ArrayList;
public abstract class Service implements Interface1 {
public void iK(ArrayList<Ks> kM, String g){
K aK = new K(g);
kM.add(aK);
}
}
But when I try to use service.iK(kM,g); in main I get the error "service cannot be resolved". How can I correct that?
Please paste in the main method first.
My guess is you forgot to instantiate an object of the class:
Service service= new Service() { //create an object of the class
}; //brackets are there because the Service is abstract class and I am redefining it.
service.iK(kM, g); //invoke a method an that object
Now, I don't think that the Service class needs to be abstract. You render the class abstract if you expect a user to implement a method (or methods) of that class that is marked as abstract in a manner that suits his needs. Needless to say, I don't see any abstract method in your Service class.
So it comes to this:
if the class is NOT abstract, you instantiate it as:
Service service= new Service();
if the class is abstract, you must redefine it at place:
Service service= new Service() {
//here you could implement an abstract method or redefine an existing one
};
First of all, Java is case-sensitive meaning that service and Service are different things. The error you just got: service cannot be resolved says, that service class is expected, while you have Service class.
Remember, that class names should implement the CamelCase, while variable names should start with a small letter.
To call methods you must either:
Create an object and access its method
Or make the method static
In the first case, you have to implement a child class:
SubService extends Service {}
because service is abstract and is expected to be extended.
Then:
SubService service = new SubService();
service.method();
In the second case, you do not have to extend the class, static methods can be called.
public abstract class Service implements Interface1 {
public static void iK(ArrayList<Ks> kM, String g){ //static method
K aK = new K(g);
kM.add(aK);
}
}
Then:
Service.iK(arg0, arg1);
This topic is suitable only for deletion.
ArrayList operates on Ks type, and you guys are putting inside it an K type object...
You should extend this class, or make it not abstract (by implementing interface) if you want to instantiate it.
Your specific example seems to be related to some sort of webservice api. Without the backing code to the abstract class we can't really help you there.
I think we can start with some simple fundamentals related to interfaces and abstract classes, since that seems to be your question.
Abstract classes are classes that you cannot, by definition, create an instance of. What darijan did to "construct" and instance of the abstract class is he is creating an anonymous inner class for the Service abstract type.
Service service= new Service() { }; // notice the curly braces, this is an anonymous class definition
There are many different schools of thought and opinions related to best practices with abstract classes and interfaces. What we really are talking about here is the heart of OOP, in my opinion. Abstract classes are meant to provide APIs with or without concrete implementation, so that they may be overridden and specialized for a specific purpose.
This would be a decent example:
public class Car {
String make;
public car (String make) { this.make = make; }
}
public class Hondacar extends Car{
public Hondacar() { super("honda"); }
}
Now you have the definition of what states define a "Car" object, and then you specialize that into the definition of a "Hondacar".
Hopefully this makes sense.
Onto interfaces... Interfaces are declarations of a public API. They are a "contract" that implementing classes must abide by. A class that implements an interface must, by definition, implement all methods on that interface. YOU CAN THINK of an interface as an abstract class with only abstract methods, where classes that subclass that abstract class will need to override every method on that supertype(this draws parallels to the "#override" annotation on implemented interface methods) though many will probably discourage this way of thought. I am not sure what you are trying to do with your specific example since it does not have any names that I can even draw inference from so I can't really help you there.
So drawing on the whole car example, a similar design would be:
interface Car {
String getMake();
}
class HondaCar implements Car {
private static final make = "honda";
#override
public String getMake() { return HondaCar.make; }
}
You can see how the interface does not provide any sort of implementation at all, it merely defines the public API that an implementing class must offer.
if we have 2 class Zoo & Moo as follows:
public class zoo {
String superString="super";
private String coolMethod(){
return "Zoo method";
}
}
public class Moo extends zoo{
public void useMyCoolMethod(){
zoo z=new zoo();
System.out.println(superString);//1
System.out.println(z.superString);//2
}
public static void main(String[]args){
new Moo().useMyCoolMethod();
}
}
at 1 & 2 we print the value of the String in the super class through inheritance and access, the question is , what is the benefit of Access although i can do the same thing by inheritance ? knowing that the Access approach isn't allowed if the two classes is in diff packages
By accessing through the object ref you are modifying the referenced object's state, by inheritance you are modifying the object you are currently in ( this reference ). So there is actually no benefit aside from encapsulation at class level and package level through out the access modifiers and that sort of thing, you just use it depending on the behavior you want for you're code, or in this case, how restrictive to modify the state of objects depending on the context.
But aside from that, i'm not sure if there is anything else.
Regarding your example there is no benefit at all in "access" the method, if you really wants to print "super" twice.
But normally OO programs contains more than two classes and the authors tries to modularize the code with defined interfaces (aka public methods or an API). Create modules with inheritance only is very hard and create clumsy code. So objects will need other instances and call there method by "access".
In line 1, you are using inheritance, i.e. you have a class animal and you have method move() which moves the animal using his four leg. But in case of Kangaroo, you want to use most of the feature of animal class but want to change how it moves based on the fact that it jumps and uses it hind leg for movement.
In Line 2, you are using composition, i.e. when you want to create car, you will need different component and they will interact with each other for that car to function. Here you can not inherit from GearBox or Engine but you have to use them as part of Car (or what you are calling access).
In the end its the relationship between Zoo and Moo will decide what method you want to use
Using this access and inheritance is same when you do Not want to modify the content of the inherited memeber..
eg:
public class A {
String s = "Hello";
}
public class B extends A{
String s = "Hi;
System.out.println(s); // Will print the s in class B
System.out.println(new A().s); // Will print the s in class A
}
Now as String s has no modifier, it is considered having Default modifier, that means it can be accessed by classes only with in its own package.
If you use protected access modifier, then you Need to extend the class and then use the inherited members, but you can Not use the protected member by creating an instance of the class that holds it and then use dot operator to access it, this will Not work..
eg :
package com.go;
public class A {
protected String s= "Hi";
}
package com.fo;
public class B extends A {
System.out.println(s); // This will work
A a = new A();
System.out.println(a.s); // This will NOT work
}
The access (default or package-private) would be useful if Moo was in the same package an didn't extend Zoo.
First of all, I think is a good practice to maintain class attributes with private visibility, and access them through getters and setters. In second place, you are not accessing the attribute by inheritance, you are creating an instance of Zoo, and accessing the superString attribute because of it package visibility, you could also access them from another class of the package that don't even extends Zoo class (and that's generally not a good thing) In third place, you don't need to create an instance of the super class to access his public or default attribute, you could simply do:
System.out.println(this.superString)
wich is absolutly the same as (if not local variable or parameter declared with the same name):
System.out.println(superString)
In conclusion, having default or public attributes, let client classes access them (read and write) without the class could do nothing about it, and this could bring side effects for the methods of the class that use those attributes.
Example 2 you have a separate instance of your zoo object, which is a bit weird, but since the method will always return the same thing there isn't much difference. If you changed your method to be based on constructor input or something you could see a difference in the 2.
public class zoo {
public String superString;
public zoo (String _superstring) {
superString = _superstring;
}
}
public class Moo extends zoo{
public void useMyCoolMethod(){
zoo z=new zoo("string1");
System.out.println(superString);//1
System.out.println(z.superString);//2
}
public Moo (String _superstring) {
superString = _superstring;
}
public static void main(String[]args){
new Moo("string2").useMyCoolMethod();
}
}
Will return
string2
string1
So lets say I have this interface:
public interface IBox
{
public void setSize(int size);
public int getSize();
public int getArea();
//...and so on
}
And I have a class that implements it:
public class Rectangle implements IBox
{
private int size;
//Methods here
}
If I wanted to use the interface IBox, i can't actually create an instance of it, in the way:
public static void main(String args[])
{
Ibox myBox=new Ibox();
}
right? So I'd actually have to do this:
public static void main(String args[])
{
Rectangle myBox=new Rectangle();
}
If that's true, then the only purpose of interfaces is to make sure that the class which implements an interface has got the correct methods in it as described by an interface? Or is there any other use of interfaces?
Interfaces are a way to make your code more flexible. What you do is this:
Ibox myBox=new Rectangle();
Then, later, if you decide you want to use a different kind of box (maybe there's another library, with a better kind of box), you switch your code to:
Ibox myBox=new OtherKindOfBox();
Once you get used to it, you'll find it's a great (actually essential) way to work.
Another reason is, for example, if you want to create a list of boxes and perform some operation on each one, but you want the list to contain different kinds of boxes. On each box you could do:
myBox.close()
(assuming IBox has a close() method) even though the actual class of myBox changes depending on which box you're at in the iteration.
What makes interfaces useful is not the fact that "you can change your mind and use a different implementation later and only have to change the one place where the object is created". That's a non-issue.
The real point is already in the name: they define an interface that anyone at all can implement to use all code that operates on that interface. The best example is java.util.Collections which provides all kinds of useful methods that operate exclusively on interfaces, such as sort() or reverse() for List. The point here is that this code can now be used to sort or reverse any class that implements the List interfaces - not just ArrayList and LinkedList, but also classes that you write yourself, which may be implemented in a way the people who wrote java.util.Collections never imagined.
In the same way, you can write code that operates on well-known interfaces, or interfaces you define, and other people can use your code without having to ask you to support their classes.
Another common use of interfaces is for Callbacks. For example, java.swing.table.TableCellRenderer, which allows you to influence how a Swing table displays the data in a certain column. You implement that interface, pass an instance to the JTable, and at some point during the rendering of the table, your code will get called to do its stuff.
One of the many uses I have read is where its difficult without multiple-inheritance-using-interfaces in Java :
class Animal
{
void walk() { }
....
.... //other methods and finally
void chew() { } //concentrate on this
}
Now, Imagine a case where:
class Reptile extends Animal
{
//reptile specific code here
} //not a problem here
but,
class Bird extends Animal
{
...... //other Bird specific code
} //now Birds cannot chew so this would a problem in the sense Bird classes can also call chew() method which is unwanted
Better design would be:
class Animal
{
void walk() { }
....
.... //other methods
}
Animal does not have the chew() method and instead is put in an interface as :
interface Chewable {
void chew();
}
and have Reptile class implement this and not Birds (since Birds cannot chew) :
class Reptile extends Animal implements Chewable { }
and incase of Birds simply:
class Bird extends Animal { }
The purpose of interfaces is polymorphism, a.k.a. type substitution. For example, given the following method:
public void scale(IBox b, int i) {
b.setSize(b.getSize() * i);
}
When calling the scale method, you can provide any value that is of a type that implements the IBox interface. In other words, if Rectangle and Square both implement IBox, you can provide either a Rectangle or a Square wherever an IBox is expected.
Interfaces allow statically typed languages to support polymorphism. An Object Oriented purist would insist that a language should provide inheritance, encapsulation, modularity and polymorphism in order to be a fully-featured Object Oriented language. In dynamically-typed - or duck typed - languages (like Smalltalk,) polymorphism is trivial; however, in statically typed languages (like Java or C#,) polymorphism is far from trivial (in fact, on the surface it seems to be at odds with the notion of strong typing.)
Let me demonstrate:
In a dynamically-typed (or duck typed) language (like Smalltalk), all variables are references to objects (nothing less and nothing more.) So, in Smalltalk, I can do this:
|anAnimal|
anAnimal := Pig new.
anAnimal makeNoise.
anAnimal := Cow new.
anAnimal makeNoise.
That code:
Declares a local variable called anAnimal (note that we DO NOT specify the TYPE of the variable - all variables are references to an object, no more and no less.)
Creates a new instance of the class named "Pig"
Assigns that new instance of Pig to the variable anAnimal.
Sends the message makeNoise to the pig.
Repeats the whole thing using a cow, but assigning it to the same exact variable as the Pig.
The same Java code would look something like this (making the assumption that Duck and Cow are subclasses of Animal:
Animal anAnimal = new Pig();
duck.makeNoise();
anAnimal = new Cow();
cow.makeNoise();
That's all well and good, until we introduce class Vegetable. Vegetables have some of the same behavior as Animal, but not all. For example, both Animal and Vegetable might be able to grow, but clearly vegetables don't make noise and animals cannot be harvested.
In Smalltalk, we can write this:
|aFarmObject|
aFarmObject := Cow new.
aFarmObject grow.
aFarmObject makeNoise.
aFarmObject := Corn new.
aFarmObject grow.
aFarmObject harvest.
This works perfectly well in Smalltalk because it is duck-typed (if it walks like a duck, and quacks like a duck - it is a duck.) In this case, when a message is sent to an object, a lookup is performed on the receiver's method list, and if a matching method is found, it is called. If not, some kind of NoSuchMethodError exception is thrown - but it's all done at runtime.
But in Java, a statically typed language, what type can we assign to our variable? Corn needs to inherit from Vegetable, to support grow, but cannot inherit from Animal, because it does not make noise. Cow needs to inherit from Animal to support makeNoise, but cannot inherit from Vegetable because it should not implement harvest. It looks like we need multiple inheritance - the ability to inherit from more than one class. But that turns out to be a pretty difficult language feature because of all the edge cases that pop up (what happens when more than one parallel superclass implement the same method?, etc.)
Along come interfaces...
If we make Animal and Vegetable classes, with each implementing Growable, we can declare that our Cow is Animal and our Corn is Vegetable. We can also declare that both Animal and Vegetable are Growable. That lets us write this to grow everything:
List<Growable> list = new ArrayList<Growable>();
list.add(new Cow());
list.add(new Corn());
list.add(new Pig());
for(Growable g : list) {
g.grow();
}
And it lets us do this, to make animal noises:
List<Animal> list = new ArrayList<Animal>();
list.add(new Cow());
list.add(new Pig());
for(Animal a : list) {
a.makeNoise();
}
The advantage to the duck-typed language is that you get really nice polymorphism: all a class has to do to provide behavior is provide the method. As long as everyone plays nice, and only sends messages that match defined methods, all is good. The downside is that the kind of error below isn't caught until runtime:
|aFarmObject|
aFarmObject := Corn new.
aFarmObject makeNoise. // No compiler error - not checked until runtime.
Statically-typed languages provide much better "programming by contract," because they will catch the two kinds of error below at compile-time:
// Compiler error: Corn cannot be cast to Animal.
Animal farmObject = new Corn();
farmObject makeNoise();
--
// Compiler error: Animal doesn't have the harvest message.
Animal farmObject = new Cow();
farmObject.harvest();
So....to summarize:
Interface implementation allows you to specify what kinds of things objects can do (interaction) and Class inheritance lets you specify how things should be done (implementation).
Interfaces give us many of the benefits of "true" polymorphism, without sacrificing compiler type checking.
Normally Interfaces define the interface you should use (as the name says it ;-) ). Sample
public void foo(List l) {
... do something
}
Now your function foo accepts ArrayLists, LinkedLists, ... not only one type.
The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class! Sample:
class Test extends Foo implements Comparable, Serializable, Formattable {
...
}
is possible but
class Test extends Foo, Bar, Buz {
...
}
is not!
Your code above could also be: IBox myBox = new Rectangle();. The important thing is now, that myBox ONLY contains the methods/fields from IBox and not the (possibly existing) other methods from Rectangle.
I think you understand everything Interfaces do, but you're not yet imagining the situations in which an Interface is useful.
If you're instantiating, using and releasing an object all within a narrow scope (for example, within one method call), an Interface doesn't really add anything. Like you noted, the concrete class is known.
Where Interfaces are useful is when an object needs to be created one place and returned to a caller that may not care about the implementation details. Let's change your IBox example to an Shape. Now we can have implementations of Shape such as Rectangle, Circle, Triangle, etc., The implementations of the getArea() and getSize() methods will be completely different for each concrete class.
Now you can use a factory with a variety of createShape(params) methods which will return an appropriate Shape depending on the params passed in. Obviously, the factory will know about what type of Shape is being created, but the caller won't have to care about whether it's a circle, or a square, or so on.
Now, imagine you have a variety of operations you have to perform on your shapes. Maybe you need to sort them by area, set them all to a new size, and then display them in a UI. The Shapes are all created by the factory and then can be passed to the Sorter, Sizer and Display classes very easily. If you need to add a hexagon class some time in the future, you don't have to change anything but the factory. Without the Interface, adding another shape becomes a very messy process.
you could do
Ibox myBox = new Rectangle();
that way you are using this object as Ibox and you don't care that its really Rectangle .
WHY INTERFACE??????
It starts with a dog. In particular, a pug.
The pug has various behaviors:
public class Pug {
private String name;
public Pug(String n) { name = n; }
public String getName() { return name; }
public String bark() { return "Arf!"; }
public boolean hasCurlyTail() { return true; } }
And you have a Labrador, who also has a set of behaviors.
public class Lab {
private String name;
public Lab(String n) { name = n; }
public String getName() { return name; }
public String bark() { return "Woof!"; }
public boolean hasCurlyTail() { return false; } }
We can make some pugs and labs:
Pug pug = new Pug("Spot");
Lab lab = new Lab("Fido");
And we can invoke their behaviors:
pug.bark() -> "Arf!"
lab.bark() -> "Woof!"
pug.hasCurlyTail() -> true
lab.hasCurlyTail() -> false
pug.getName() -> "Spot"
Let's say I run a dog kennel and I need to keep track of all the dogs I'm housing. I need to store my pugs and labradors in separate arrays:
public class Kennel {
Pug[] pugs = new Pug[10];
Lab[] labs = new Lab[10];
public void addPug(Pug p) { ... }
public void addLab(Lab l) { ... }
public void printDogs() { // Display names of all the dogs } }
But this is clearly not optimal. If I want to house some poodles, too, I have to change my Kennel definition to add an array of Poodles. In fact, I need a separate array for each kind of dog.
Insight: both pugs and labradors (and poodles) are types of dogs and they have the same set of behaviors. That is, we can say (for the purposes of this example) that all dogs can bark, have a name, and may or may not have a curly tail. We can use an interface to define what all dogs can do, but leave it up to the specific types of dogs to implement those particular behaviors. The interface says "here are the things that all dogs can do" but doesn't say how each behavior is done.
public interface Dog
{
public String bark();
public String getName();
public boolean hasCurlyTail(); }
Then I slightly alter the Pug and Lab classes to implement the Dog behaviors. We can say that a Pug is a Dog and a Lab is a dog.
public class Pug implements Dog {
// the rest is the same as before }
public class Lab implements Dog {
// the rest is the same as before
}
I can still instantiate Pugs and Labs as I previously did, but now I also get a new way to do it:
Dog d1 = new Pug("Spot");
Dog d2 = new Lab("Fido");
This says that d1 is not only a Dog, it's specifically a Pug. And d2 is also a Dog, specifically a Lab.
We can invoke the behaviors and they work as before:
d1.bark() -> "Arf!"
d2.bark() -> "Woof!"
d1.hasCurlyTail() -> true
d2.hasCurlyTail() -> false
d1.getName() -> "Spot"
Here's where all the extra work pays off. The Kennel class become much simpler. I need only one array and one addDog method. Both will work with any object that is a dog; that is, objects that implement the Dog interface.
public class Kennel {
Dog[] dogs = new Dog[20];
public void addDog(Dog d) { ... }
public void printDogs() {
// Display names of all the dogs } }
Here's how to use it:
Kennel k = new Kennel();
Dog d1 = new Pug("Spot");
Dog d2 = new Lab("Fido");
k.addDog(d1);
k.addDog(d2);
k.printDogs();
The last statement would display:
Spot Fido
An interface give you the ability to specify a set of behaviors that all classes that implement the interface will share in common. Consequently, we can define variables and collections (such as arrays) that don't have to know in advance what kind of specific object they will hold, only that they'll hold objects that implement the interface.
A great example of how interfaces are used is in the Collections framework. If you write a function that takes a List, then it doesn't matter if the user passes in a Vector or an ArrayList or a HashList or whatever. And you can pass that List to any function requiring a Collection or Iterable interface too.
This makes functions like Collections.sort(List list) possible, regardless of how the List is implemented.
This is the reason why Factory Patterns and other creational patterns are so popular in Java. You are correct that without them Java doesn't provide an out of the box mechanism for easy abstraction of instantiation. Still, you get abstraction everywhere where you don't create an object in your method, which should be most of your code.
As an aside, I generally encourage people to not follow the "IRealname" mechanism for naming interfaces. That's a Windows/COM thing that puts one foot in the grave of Hungarian notation and really isn't necessary (Java is already strongly typed, and the whole point of having interfaces is to have them as largely indistinguishable from class types as possible).
Don't forget that at a later date you can take an existing class, and make it implement IBox, and it will then become available to all your box-aware code.
This becomes a bit clearer if interfaces are named -able. e.g.
public interface Saveable {
....
public interface Printable {
....
etc. (Naming schemes don't always work e.g. I'm not sure Boxable is appropriate here)
the only purpose of interfaces is to make sure that the class which implements an interface has got the correct methods in it as described by an interface? Or is there any other use of interfaces?
I am updating the answer with new features of interface, which have introduced with java 8 version.
From oracle documentation page on summary of interface :
An interface declaration can contain
method signatures
default methods
static methods
constant definitions.
The only methods that have implementations are default and static methods.
Uses of interface:
To define a contract
To link unrelated classes with has a capabilities (e.g. classes implementing Serializable interface may or may not have any relation between them except implementing that interface
To provide interchangeable implementation e.g. strategy pattern
Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces
Organize helper methods in your libraries with static methods ( you can keep static methods specific to an interface in the same interface rather than in a separate class)
Some related SE questions with respect to difference between abstract class and interface and use cases with working examples:
What is the difference between an interface and abstract class?
How should I have explained the difference between an Interface and an Abstract class?
Have a look at documentation page to understand new features added in java 8 : default methods and static methods.
The purpose of interfaces is abstraction, or decoupling from implementation.
If you introduce an abstraction in your program, you don't care about the possible implementations. You are interested in what it can do and not how, and you use an interface to express this in Java.
If you have CardboardBox and HtmlBox (both of which implement IBox), you can pass both of them to any method that accepts a IBox. Even though they are both very different and not completely interchangable, methods that don't care about "open" or "resize" can still use your classes (perhaps because they care about how many pixels are needed to display something on a screen).
Interfaces where a fetature added to java to allow multiple inheritance. The developers of Java though/realized that having multiple inheritance was a "dangerous" feature, that is why the came up with the idea of an interface.
multiple inheritance is dangerous because you might have a class like the following:
class Box{
public int getSize(){
return 0;
}
public int getArea(){
return 1;
}
}
class Triangle{
public int getSize(){
return 1;
}
public int getArea(){
return 0;
}
}
class FunckyFigure extends Box, Triable{
// we do not implement the methods we will used the inherited ones
}
Which would be the method that should be called when we use
FunckyFigure.GetArea();
All the problems are solved with interfaces, because you do know you can extend the interfaces and that they wont have classing methods... ofcourse the compiler is nice and tells you if you did not implemented a methods, but I like to think that is a side effect of a more interesting idea.
Here is my understanding of interface advantage. Correct me if I am wrong.
Imagine we are developing OS and other team is developing the drivers for some devices.
So we have developed an interface StorageDevice. We have two implementations of it (FDD and HDD) provided by other developers team.
Then we have a OperatingSystem class which can call interface methods such as saveData by just passing an instance of class implemented the StorageDevice interface.
The advantage here is that we don't care about the implementation of the interface. The other team will do the job by implementing the StorageDevice interface.
package mypack;
interface StorageDevice {
void saveData (String data);
}
class FDD implements StorageDevice {
public void saveData (String data) {
System.out.println("Save to floppy drive! Data: "+data);
}
}
class HDD implements StorageDevice {
public void saveData (String data) {
System.out.println("Save to hard disk drive! Data: "+data);
}
}
class OperatingSystem {
public String name;
StorageDevice[] devices;
public OperatingSystem(String name, StorageDevice[] devices) {
this.name = name;
this.devices = devices.clone();
System.out.println("Running OS " + this.name);
System.out.println("List with storage devices available:");
for (StorageDevice s: devices) {
System.out.println(s);
}
}
public void saveSomeDataToStorageDevice (StorageDevice storage, String data) {
storage.saveData(data);
}
}
public class Main {
public static void main(String[] args) {
StorageDevice fdd0 = new FDD();
StorageDevice hdd0 = new HDD();
StorageDevice[] devs = {fdd0, hdd0};
OperatingSystem os = new OperatingSystem("Linux", devs);
os.saveSomeDataToStorageDevice(fdd0, "blah, blah, blah...");
}
}