Public abstract int java
Class declarations define new reference types and describe how they are implemented (§8.1).
A top level class is a class that is not a nested class.
A nested class is any class whose declaration occurs within the body of another class or interface.
This chapter discusses the common semantics of all classes — top level (§7.6) and nested (including member classes (§8.5, §9.5), local classes (§14.3) and anonymous classes (§15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.
A named class may be declared abstract (§8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses. A class may be declared final (§8.1.1.2), in which case it cannot have subclasses. If a class is declared public , then it can be referred to from other packages. Each class except Object is an extension of (that is, a subclass of) a single existing class (§8.1.4) and may implement interfaces (§8.1.5). Classes may be generic (§8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.
Classes may be decorated with annotations (§9.7) just like any other kind of declaration.
The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (§8.1.6). The scope (§6.3) of a member (§8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (§6.6) public , protected , or private . The members of a class include both declared and inherited members (§8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.
Field declarations (§8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (§8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.
Member class declarations (§8.5) describe nested classes that are members of the surrounding class. Member classes may be static , in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).
Member interface declarations (§8.5) describe nested interfaces that are members of the surrounding class.
Method declarations (§8.4) describe code that may be invoked by method invocation expressions (§15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of a class type. A method whose declaration does not indicate how it is implemented must be declared abstract . A method may be declared final (§8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (§8.4.3.4). A synchronized method (§8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (§14.19), thus allowing its activities to be synchronized with those of other threads (§17 (Threads and Locks)).
Method names may be overloaded (§8.4.9).
Instance initializers (§8.6) are blocks of executable code that may be used to help initialize an instance when it is created (§15.9).
Static initializers (§8.7) are blocks of executable code that may be used to help initialize a class.
Constructors (§8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (§8.8.8).
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.
Public abstract int java
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.