Java abstract class type
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.
Abstract Classes in Java
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We rely on other people’s code in our own work. Every day.
It might be the language you’re writing in, the framework you’re building on, or some esoteric piece of software that does one thing so well you never found the need to implement it yourself.
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Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.
The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.
Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.
Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:
DbSchema is a super-flexible database designer, which can take you from designing the DB with your team all the way to safely deploying the schema.
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The Kubernetes ecosystem is huge and quite complex, so it’s easy to forget about costs when trying out all of the exciting tools.
To avoid overspending on your Kubernetes cluster, definitely have a look at the free K8s cost monitoring tool from the automation platform CAST AI. You can view your costs in real time, allocate them, calculate burn rates for projects, spot anomalies or spikes, and get insightful reports you can share with your team.
Connect your cluster and start monitoring your K8s costs right away:
We’re looking for a new Java technical editor to help review new articles for the site.
1. Overview
There are many cases when implementing a contract where we want to postpone some parts of the implementation to be completed later. We can easily accomplish this in Java through abstract classes.
In this tutorial, we’ll learn the basics of abstract classes in Java, and in what cases they can be helpful.
2. Key Concepts for Abstract Classes
Before diving into when to use an abstract class, let’s look at their most relevant characteristics:
- We define an abstract class with the abstract modifier preceding the class keyword
- An abstract class can be subclassed, but it can’t be instantiated
- If a class defines one or more abstract methods, then the class itself must be declared abstract
- An abstract class can declare both abstract and concrete methods
- A subclass derived from an abstract class must either implement all the base class’s abstract methods or be abstract itself
To better understand these concepts, we’ll create a simple example.
Let’s have our base abstract class define the abstract API of a board game:
public abstract class BoardGame < //. field declarations, constructors public abstract void play(); //. concrete methods >Then, we can create a subclass that implements the play method:
public class Checkers extends BoardGame < public void play() < //. implementation >>3. When to Use Abstract Classes
Now, let’s analyze a few typical scenarios where we should prefer abstract classes over interfaces and concrete classes:
- We want to encapsulate some common functionality in one place (code reuse) that multiple, related subclasses will share
- We need to partially define an API that our subclasses can easily extend and refine
- The subclasses need to inherit one or more common methods or fields with protected access modifiers
Let’s keep in mind that all these scenarios are good examples of full, inheritance-based adherence to the Open/Closed principle.
Moreover, since the use of abstract classes implicitly deals with base types and subtypes, we’re also taking advantage of Polymorphism.
Note that code reuse is a very compelling reason to use abstract classes, as long as the “is-a” relationship within the class hierarchy is preserved.
And Java 8 adds another wrinkle with default methods, which can sometimes take the place of needing to create an abstract class altogether.
4. A Sample Hierarchy of File Readers
To understand more clearly the functionality that abstract classes bring to the table, let’s look at another example.
4.1. Defining a Base Abstract Class
So, if we wanted to have several types of file readers, we might create an abstract class that encapsulates what’s common to file reading:
public abstract class BaseFileReader < protected Path filePath; protected BaseFileReader(Path filePath) < this.filePath = filePath; >public Path getFilePath() < return filePath; >public List readFile() throws IOException < return Files.lines(filePath) .map(this::mapFileLine).collect(Collectors.toList()); >protected abstract String mapFileLine(String line); >Note that we’ve made filePath protected so that the subclasses can access it if needed. More importantly, we’ve left something undone: how to actually parse a line of text from the file’s contents.
Our plan is simple: while our concrete classes don’t each have a special way to store the file path or walk through the file, they will each have a special way to transform each line.
At first sight, BaseFileReader may seem unnecessary. However, it’s the foundation of a clean, easily extendable design. From it, we can easily implement different versions of a file reader that can focus on their unique business logic.
4.2. Defining Subclasses
A natural implementation is probably one that converts a file’s contents to lowercase:
public class LowercaseFileReader extends BaseFileReader < public LowercaseFileReader(Path filePath) < super(filePath); >@Override public String mapFileLine(String line) < return line.toLowerCase(); >>Or another might be one that converts a file’s contents to uppercase:
public class UppercaseFileReader extends BaseFileReader < public UppercaseFileReader(Path filePath) < super(filePath); >@Override public String mapFileLine(String line) < return line.toUpperCase(); >>As we can see from this simple example, each subclass can focus on its unique behavior without needing to specify other aspects of file reading.
4.3. Using a Subclass
Finally, using a class that inherits from an abstract one is no different than any other concrete class:
@Test public void givenLowercaseFileReaderInstance_whenCalledreadFile_thenCorrect() throws Exception < URL location = getClass().getClassLoader().getResource("files/test.txt") Path path = Paths.get(location.toURI()); BaseFileReader lowercaseFileReader = new LowercaseFileReader(path); assertThat(lowercaseFileReader.readFile()).isInstanceOf(List.class); >For simplicity’s sake, the target file is located under the src/main/resources/files folder. Hence, we used an application class loader for getting the path of the example file. Feel free to check out our tutorial on class loaders in Java.
5. Conclusion
In this quick article, we learned the basics of abstract classes in Java, and when to use them for achieving abstraction and encapsulating common implementation in one single place.
As usual, all the code samples shown in this tutorial are available over on GitHub.
Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.
The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.
Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.
Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes: