Programming abstractions in java
Именно из-за такой «непонятки», когда объекту неясно, какое поведение он должен выбрать, создатели Java отказались от множественного наследования Даже не знаю, думаю пример не очень в статье, так как в большинстве источников сказано, что больше проблемы вызывают как раз обращение к переменным, при множественном наследовании. По сути те же интерфейсы могу создать неопределенную ситуацию с методами, но их(интерфейсы) все же добавили в язык.
У меня возник вопрос. Вот мы не можем наследовать от нескольких классов, потому что может возникнуть ситуация, когда в этих классах усть одинаковые названия методов с разной реализацией. Но что произойдёт, если мы реализуем несколько интерфейсов, у которых также есть одинаковые названия методов да ещё и с разной дефолтной реализацией?
На самом деле — и это очень важная особенность — класс является абстрактным, если хотя бы один из его методов является абстрактным. Хоть один из двух, хоть один из тысячи методов — без разницы. ORACLE: An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
Про отказ от множественного наследования в Java есть и другое мнение — такое наследование сильно усложняет иерархию классов и она становится большой головной болью при поддержании и развитии программы + усиливаются связанность и зависимости кода . С другой стороны, немножественное Наследование сильно ограничивает Полиморфизм, т.к. в таком случае полиморфными могут быть только дочки одного потомка. Но в Java есть концепция Интерфейса, которая сильно ослабляет зависимость Полиморфизма от Наследования. Благодаря Интерфейсу классы становятся полиморфными независимо от их родителя, и мы получаем Полиморфизм без ограничений Наследования.
На самом деле очень даже сомнительное доказательство того, что от множественного наследования отказались именно по той причине, которая указана в статье т.к следующий код:
public class Solution < public static void main(String[] args) < >public abstract class Human implements CanRun, CanSwim < >public interface CanRun < default void run()< System.out.println("run1"); >> public interface CanSwim < default void run()< System.out.println("run2"); >> > даже не компилируется выделяя класс Human и подсвечивает следующую ошибку: «Human inherits unrelated defaults for run() from types CanRun and CanSwim» Думаю то же самое могло бы писаться при попытке множественного наследования, если бы в двух разных родителях был одинаковый метод. Поэтому слабо верится. То же касается и переменных. Сейчас можно создавать переменные в интерфейсах, и если создать две переменных с одинаковым именем, то при имплементации и попытке обращения к таким переменным компилятор выдаст ошибку. Так что учитывая то, что сейчас в интерфейсе можно делать то же самое что и в классе, то можно сказать что в джаве есть множественное наследование.
Abstraction in Java
In Java, abstraction captures only those details about a class that are relevant to the current context. For example, a java.util.Map stores key-value pairs and exposes two methods get() and put() to store and retrieve key-value pairs. This is, in fact, the only information we need if we want to use the Map in an application. How the Map works inside, we are not required to know it to use. This is very much an example of abstraction in Java.
Take a more real-life example of abstraction which can be a TV remote. You know that when you press any button on the remote, some function is applied on the television e.g. change the channel, change the volume level etc. You are not required to know how internally remote works, to use it properly. It is an example of abstraction.
In object-oriented programming theory, abstraction involves defining objects representing abstract “actors” that can perform work, report on and change their state, and “communicate” with other objects in the system.
Abstraction in any programming language works in many ways. It can be seen from creating subroutines to defining interfaces for making low-level language calls.
Some abstractions try to limit the breadth of concepts a programmer needs, by completely hiding the abstractions they in turn are built on, e.g. design patterns.
Typically abstraction can be seen in two ways:
Data abstraction is the way to create complex data types and expose only meaningful operations to interact with the data type while hiding all the implementation details from outside works.
The benefit of this approach involves the capability of improving the implementation over time e.g. solving performance issues if any. The idea is that such changes are not supposed to impact client code since they involve no difference in abstract behavior.
Any software consists of numerous statements written in any programming language. Most of the time, statements are similar and repeated over places multiple times.
Control abstraction is the process of identifying all such statements and exposing them as a unit of work. We normally use this feature when we create a function to perform any work.
3. How to Achieve Abstraction in Java?
As abstraction is one of the core principles of Object-oriented programming practices and Java follows all OOPs principles, abstraction is one of the major building blocks of java language.
In Java, abstraction is achieved by interfaces and abstract classes. Interfaces allow you to abstract the implementation completely, while abstract classes allow partial abstraction as well.
- Data abstraction spans from creating simple data objects to complex collection implementations such as HashMap or HashSet .
- Similarly, control abstraction can be seen from defining simple function calls to complete open-source frameworks. control abstraction is the main force behind structured programming.
3.1. Abstraction using Interfaces
Let’s see one more example of abstraction in Java using interfaces. In this example, I am creating various reports which can be run on demand at any time during the application’s lifetime. As a consumer of the report, a class needs not to know the internals of the report’s run(), it only should execute this method and the report will be executed.
public interface Report < Listrun(ReportContext reportContext); > public class ReportContext < //fields > public class EmployeeReport implements Report < @Override public Listrun(ReportContext reportContext) < //Custom Logic System.out.println("Executing employee report"); return null; >>public class SalaryReport implements Report < @Override public Listrun(ReportContext reportContext) < //Custom logic System.out.println("Executing salary report"); return null; >>Now execute the reports with run() method.
Executing employee report Executing salary report4. Difference between Encapsulation and Abstraction
Encapsulation is realization of the desired abstraction .
Abstraction is more about hiding the implementation details. In Java, abstraction is achieved through abstract classes and interfaces.
Encapsulation is about wrapping the implementation (code) and the data it manipulates (variables) within the same class. A Java class, where all instance variables are private, and only the methods within the class can manipulate those variables, is an example of an encapsulated class.
If you want to read more about abstract classes and interfaces in Java, follow my next post Exploring interfaces and abstract classes in java.
Abstract Methods and Classes
An abstract class is a class that is declared abstract it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed.
An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon), like this:
abstract void moveTo(double deltaX, double deltaY);
If a class includes abstract methods, then the class itself must be declared abstract , as in:
public abstract class GraphicObject < // declare fields // declare nonabstract methods abstract void draw(); >
When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract .
Note: Methods in an interface (see the Interfaces section) that are not declared as default or static are implicitly abstract, so the abstract modifier is not used with interface methods. (It can be used, but it is unnecessary.)
Abstract Classes Compared to Interfaces
Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation. However, with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods. With interfaces, all fields are automatically public, static, and final, and all methods that you declare or define (as default methods) are public. In addition, you can extend only one class, whether or not it is abstract, whereas you can implement any number of interfaces.
Which should you use, abstract classes or interfaces?
- Consider using abstract classes if any of these statements apply to your situation:
- You want to share code among several closely related classes.
- You expect that classes that extend your abstract class have many common methods or fields, or require access modifiers other than public (such as protected and private).
- You want to declare non-static or non-final fields. This enables you to define methods that can access and modify the state of the object to which they belong.
- You expect that unrelated classes would implement your interface. For example, the interfaces Comparable and Cloneable are implemented by many unrelated classes.
- You want to specify the behavior of a particular data type, but not concerned about who implements its behavior.
- You want to take advantage of multiple inheritance of type.
An example of an abstract class in the JDK is AbstractMap , which is part of the Collections Framework. Its subclasses (which include HashMap , TreeMap , and ConcurrentHashMap ) share many methods (including get , put , isEmpty , containsKey , and containsValue ) that AbstractMap defines.
An example of a class in the JDK that implements several interfaces is HashMap , which implements the interfaces Serializable , Cloneable , and Map . By reading this list of interfaces, you can infer that an instance of HashMap (regardless of the developer or company who implemented the class) can be cloned, is serializable (which means that it can be converted into a byte stream; see the section Serializable Objects), and has the functionality of a map. In addition, the Map interface has been enhanced with many default methods such as merge and forEach that older classes that have implemented this interface do not have to define.
Note that many software libraries use both abstract classes and interfaces; the HashMap class implements several interfaces and also extends the abstract class AbstractMap .
An Abstract Class Example
In an object-oriented drawing application, you can draw circles, rectangles, lines, Bezier curves, and many other graphic objects. These objects all have certain states (for example: position, orientation, line color, fill color) and behaviors (for example: moveTo, rotate, resize, draw) in common. Some of these states and behaviors are the same for all graphic objects (for example: position, fill color, and moveTo). Others require different implementations (for example, resize or draw). All GraphicObject s must be able to draw or resize themselves; they just differ in how they do it. This is a perfect situation for an abstract superclass. You can take advantage of the similarities and declare all the graphic objects to inherit from the same abstract parent object (for example, GraphicObject ) as shown in the following figure.
Classes Rectangle, Line, Bezier, and Circle Inherit from GraphicObject
First, you declare an abstract class, GraphicObject , to provide member variables and methods that are wholly shared by all subclasses, such as the current position and the moveTo method. GraphicObject also declares abstract methods for methods, such as draw or resize , that need to be implemented by all subclasses but must be implemented in different ways. The GraphicObject class can look something like this:
abstract class GraphicObject < int x, y; . void moveTo(int newX, int newY) < . >abstract void draw(); abstract void resize(); >
Each nonabstract subclass of GraphicObject , such as Circle and Rectangle , must provide implementations for the draw and resize methods:
class Circle extends GraphicObject < void draw() < . >void resize() < . >> class Rectangle extends GraphicObject < void draw() < . >void resize() < . >>
When an Abstract Class Implements an Interface
In the section on Interfaces , it was noted that a class that implements an interface must implement all of the interface’s methods. It is possible, however, to define a class that does not implement all of the interface’s methods, provided that the class is declared to be abstract . For example,
abstract class X implements Y < // implements all but one method of Y >class XX extends X < // implements the remaining method in Y >
In this case, class X must be abstract because it does not fully implement Y , but class XX does, in fact, implement Y .
Class Members
An abstract class may have static fields and static methods. You can use these static members with a class reference (for example, AbstractClass.staticMethod() ) as you would with any other class.