Can you implement an abstract class java

Can you implement an abstract class 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.

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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.

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  Abstract Class in Java

  

  While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

  Abstract class in Java is similar to interface except that it can contain default method implementation. An abstract class can have an abstract method without body and it can have methods with implementation also. abstract keyword is used to create a abstract class and method. Abstract class in java can’t be instantiated. An abstract class is mostly used to provide a base for subclasses to extend and implement the abstract methods and override or use the implemented methods in abstract class.

  Abstract Class in Java

  

  Here is a simple example of an Abstract Class in Java.

  package com.journaldev.design; //abstract class public abstract class Person < private String name; private String gender; public Person(String nm, String gen)< this.name=nm; this.gender=gen; >//abstract method public abstract void work(); @Override public String toString() < return "Name="+this.name+"::Gender="+this.gender; >public void changeName(String newName) < this.name = newName; >> 

  Notice that work() is an abstract method and it has no-body. Here is a concrete class example extending an abstract class in java.

  package com.journaldev.design; public class Employee extends Person < private int empId; public Employee(String nm, String gen, int id) < super(nm, gen); this.empId=id; >@Override public void work() < if(empId == 0)< System.out.println("Not working"); >else < System.out.println("Working as employee!!"); >> public static void main(String args[]) < //coding in terms of abstract classes Person student = new Employee("Dove","Female",0); Person employee = new Employee("Pankaj","Male",123); student.work(); employee.work(); //using method implemented in abstract class - inheritance employee.changeName("Pankaj Kumar"); System.out.println(employee.toString()); >> 

  Note that subclass Employee inherits the properties and methods of superclass Person using inheritance in java. Also notice the use of Override annotation in Employee class. Read more for why we should always use Override annotation when overriding a method.

  Abstract class in Java Important Points

  1. abstract keyword is used to create an abstract class in java.
  2. Abstract class in java can’t be instantiated.
  3. We can use abstract keyword to create an abstract method, an abstract method doesn’t have body.
  4. If a class have abstract methods, then the class should also be abstract using abstract keyword, else it will not compile.
  5. It’s not necessary for an abstract class to have abstract method. We can mark a class as abstract even if it doesn’t declare any abstract methods.
  6. If abstract class doesn’t have any method implementation, its better to use interface because java doesn’t support multiple class inheritance.
  7. The subclass of abstract class in java must implement all the abstract methods unless the subclass is also an abstract class.
  8. All the methods in an interface are implicitly abstract unless the interface methods are static or default. Static methods and default methods in interfaces are added in Java 8, for more details read Java 8 interface changes.
  9. Java Abstract class can implement interfaces without even providing the implementation of interface methods.
  10. Java Abstract class is used to provide common method implementation to all the subclasses or to provide default implementation.
  11. We can run abstract class in java like any other class if it has main() method.

  That’s all for an abstract class in Java. If I missed anything important, please let us know through comments.

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