Public abstract class android

Абстрактные классы и методы

Класс, содержащий абстрактные методы, называется абстрактным классом. Такие классы помечаются ключевым словом abstract.

Абстрактный метод не завершён. Он состоит только из объявления и не имеет тела:

По сути, мы создаём шаблон метода. Например, можно создать абстрактный метод для вычисления площади фигуры в абстрактном классе Фигура. А все другие производные классы от главного класса могут уже реализовать свой код для готового метода. Ведь площадь у прямоугольника и треугольника вычисляется по разным алгоритмам и универсального метода не существует.

Если вы объявляете класс, производный от абстрактного класса, но хотите иметь возможность создания объектов нового типа, вам придётся предоставить определения для всех абстрактных методов базового класса. Если этого не сделать, производный класс тоже останется абстрактным, и компилятор заставит пометить новый класс ключевым словом abstract.

Можно создавать класс с ключевым словом abstract даже, если в нем не имеется ни одного абстрактного метода. Это бывает полезным в ситуациях, где в классе абстрактные методы просто не нужны, но необходимо запретить создание экземпляров этого класса.

В тоже время абстрактный класс не обязательно должен иметь только абстрактные методы. Напомню ещё раз, что если класс содержит хотя бы один абстрактный метод, то он обязан быть сам абстрактным.

Создавать объект на основе абстрактного класса нельзя.

Абстрактный класс не может содержать какие-либо объекты, а также абстрактные конструкторы и абстрактные статические методы. Любой подкласс абстрактного класса должен либо реализовать все абстрактные методы суперкласса, либо сам быть объявлен абстрактным. Короче, я сам запутался. Пойду лучше кота поглажу.

Я вернулся. Давайте напишем пример для абстрактного класса.

Допустим, мы хотим создать абстрактный класс СферическийКонь и не менее идиотский класс СферическийКоньВВакууме, наследующий от первого класса.

Когда вы напишете такой код, то студия подчеркнёт второй класс красной волнистой линией и предложит реализовать обязательный метод, который определён в абстрактном классе.

Соглашаемся и дописываем в созданную заготовку свой код для метода.

В главной активности напишем код для щелчка кнопки.

Обратите внимание, что абстрактный класс может содержать не только абстрактные, но и обычные методы.

Раннее мы создавали класс Фигура, у которого был метод вычисления площади фигуры. Метод ничего не делал, так как невозможно вычислить площадь неизвестной фигуры. Поэтому, этот метод можно сделать абстрактным, а в классах, производных от Фигуры, переопределить данный метод.

Фигура — это абстрактное понятие и мы не можем создать универсальный метод для вычисления площади. Поэтому мы создаём другой класс Треугольник и пишем код, вычисляющий площадь треугольника (загляните в учебник геометрии). Также вы можете создать новый класс Прямоугольник и написать свой код для вычисления площади этой фигуры.

Вам вряд ли придётся часто создавать абстрактные классы для своих приложений, но встречаться с ними вы будете постоянно, например, классы AsyncTask, Service и др.

Кот — понятие тоже абстрактное. А вот ваш любимец Васька или Мурзик уже конкретный шерстяной засранец. По запросу «abstract cat» мне выдало такую картинку. Всегда интересовал вопрос, где художники берут такую травку? (это был абстрактный вопрос).

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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:

If a class includes abstract methods, then the class itself must be declared abstract , as in:

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 .

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.
• Consider using interfaces if any of these statements apply to your situation:
  • 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:

Each nonabstract subclass of GraphicObject , such as Circle and Rectangle , must provide implementations for the draw and resize methods:

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,

In this case, class X must be abstract because it does not fully implement Y , but class XX does, in fact, implement Y .

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Let’s practice interface and abstract class in android (java)

Halima Akhter Mitu

Sep 9, 2019 · 3 min read

Polymorphism is one of the important features of object-oriented programming. It means taking different roles in different environments. For example, a human can play different character s/he can be a child, father/mother, employee, etc. Abstract and interface come from this concept.

Abstract class and interface are the most used attributes in android. When we create an activity it indirectly extends an abstract class called Context and when we implement the interface View.OnClickListener it forces us to override the onClick function. These are the example of abstract class and interface which are innate attributes in android.

So, w h at are we gonna learn in this blog?

We gonna learn what is an abstract class and interface, why interface or abstract class need to be used, how to create our own interface and abstract and last but not the least how to use them.

A class which has “abstract” declaration after access modifier (public, protected, private) is called an abstract class. An abstract class can have any kind of method with the method body, it also can have methods with the only function declaration, but those methods must be declared as an abstract method. When we extend an abstract class all the abstract methods must be overridden.

public abstract class MyAbstractClass <

public void myFunctionOne()<

//Abstract methods don’t have any body

protected abstract void myFunctionWhichisAbstract(String s);

public class UseAbstractClass extends MyAbstractClass <

protected void myFunctionWhichisAbstract(String s) <

// use this in any override function of the activity

UseAbstractClass useAbstractClass = new UseAbstractClass();

The interface is a feature in java which declared as interface and by default it is public. Methods in an interface are declared by definition they don’t have a body. We can implement multiple interfaces. An interface also can implement single or multiple interfaces.

For example here is an interface called TaskComplete

public interface TaskComplete <

void doTask(String asyncClassName, int defaultValue);

We can use this interface in different ways. I am showing three of them.

* define this in outside of any overridden method

TaskComplete taskComplete = new TaskComplete() <

public void doTask(String asyncClassName, int defaultValue) <

//do your functionality

Then call this from any override function in your activity

//In my case, I called it in the onCreate override function of MainActivity.java

Sometimes we need to update UI after network call, this kind of functionality can be a great help on this.

In MainActivity.java class implement the TaskComplete interface then we bound to use doTask method.

public class MainActivity extends AppCompatActivity implements TaskComplete <

public void doTask(String asyncClassName, int defaultValue) <

// do your functionality

Make a class named Event

public class Event <

private TaskComplete mTaskComplete;

public void setOnTaskCompleteListener(TaskComplete listener) <

Then use this class’s object in MainActivity class

// in the onCreate function of MainActivity.java file

new Event().setOnEventListener(new OnEventListener() <

public void onEvent(String er) <

//do functionality here

I hope this information will be helpful for some of you.

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Abstract Class in Java with example

By Chaitanya Singh | Filed Under: OOPs Concept

A class that is declared using “abstract” keyword is known as abstract class. It can have abstract methods(methods without body) as well as concrete methods (regular methods with body). A normal class(non-abstract class) cannot have abstract methods. In this guide we will learn what is a abstract class, why we use it and what are the rules that we must remember while working with it in Java.

An abstract class can not be instantiated, which means you are not allowed to create an object of it. Why? We will discuss that later in this guide.

Why we need an abstract class?

Lets say we have a class Animal that has a method sound() and the subclasses(see inheritance) of it like Dog , Lion , Horse , Cat etc. Since the animal sound differs from one animal to another, there is no point to implement this method in parent class. This is because every child class must override this method to give its own implementation details, like Lion class will say “Roar” in this method and Dog class will say “Woof”.

So when we know that all the animal child classes will and should override this method, then there is no point to implement this method in parent class. Thus, making this method abstract would be the good choice as by making this method abstract we force all the sub classes to implement this method( otherwise you will get compilation error), also we need not to give any implementation to this method in parent class.

Since the Animal class has an abstract method, you must need to declare this class abstract.

Now each animal must have a sound, by making this method abstract we made it compulsory to the child class to give implementation details to this method. This way we ensures that every animal has a sound.

Abstract class Example

Hence for such kind of scenarios we generally declare the class as abstract and later concrete classes extend these classes and override the methods accordingly and can have their own methods as well.

Abstract class declaration

An abstract class outlines the methods but not necessarily implements all the methods.

Rules

Note 1: As we seen in the above example, there are cases when it is difficult or often unnecessary to implement all the methods in parent class. In these cases, we can declare the parent class as abstract, which makes it a special class which is not complete on its own.

A class derived from the abstract class must implement all those methods that are declared as abstract in the parent class.

Note 2: Abstract class cannot be instantiated which means you cannot create the object of it. To use this class, you need to create another class that extends this this class and provides the implementation of abstract methods, then you can use the object of that child class to call non-abstract methods of parent class as well as implemented methods(those that were abstract in parent but implemented in child class).

Note 3: If a child does not implement all the abstract methods of abstract parent class, then the child class must need to be declared abstract as well.

Do you know? Since abstract class allows concrete methods as well, it does not provide 100% abstraction. You can say that it provides partial abstraction. Abstraction is a process where you show only “relevant” data and “hide” unnecessary details of an object from the user.

Interfaces on the other hand are used for 100% abstraction (See more about abstraction here).
You may also want to read this: Difference between abstract class and Interface in Java

Why can’t we create the object of an abstract class?

Because these classes are incomplete, they have abstract methods that have no body so if java allows you to create object of this class then if someone calls the abstract method using that object then What would happen?There would be no actual implementation of the method to invoke.
Also because an object is concrete. An abstract class is like a template, so you have to extend it and build on it before you can use it.

Example to demonstrate that object creation of abstract class is not allowed

As discussed above, we cannot instantiate an abstract class. This program throws a compilation error.

Note: The class that extends the abstract class, have to implement all the abstract methods of it, else you have to declare that class abstract as well.

Abstract class vs Concrete class

A class which is not abstract is referred as Concrete class. In the above example that we have seen in the beginning of this guide, Animal is a abstract class and Cat , Dog & Lion are concrete classes.

Key Points:

1. An abstract class has no use until unless it is extended by some other class.
2. If you declare an abstract method in a class then you must declare the class abstract as well. you can’t have abstract method in a concrete class. It’s vice versa is not always true: If a class is not having any abstract method then also it can be marked as abstract.
3. It can have non-abstract method (concrete) as well.

I have covered the rules and examples of abstract methods in a separate tutorial, You can find the guide here: Abstract method in Java
For now lets just see some basics and example of abstract method.
1) Abstract method has no body.
2) Always end the declaration with a semicolon(;).
3) It must be overridden. An abstract class must be extended and in a same way abstract method must be overridden.
4) A class has to be declared abstract to have abstract methods.

Note: The class which is extending abstract class must override all the abstract methods.

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