Any non-abstract subclass of GraphicObject, such as Circle and Rectangle, must provide implementations for the draw and resize methods: When an abstract class is subclassed, the subclass typically provides implementations for all abstract methods of its parent class. However, if this is not the case, the subclass must also be declared abstract. As the Microsoft interface documentation states: An abstract method is a method declared without implementation (without braces and followed by a semicolon), as follows: In the section on interfaces, it was noted that a class that implements an interface must implement all methods of the interface. However, it is possible to define a class that does not implement all the methods of the interface, provided that the class is declared abstract. For example, consider the following Java program that demonstrates the use of an abstract keyword with classes and methods. (3), (6). Both are legal collective declarations. (1) is erroneous because a class cannot be abstract and definitive “there would be no way to use such a class”. (2) is incorrect because interfaces and classes cannot be marked as static. (4) and (5) are incorrect because classes and interfaces cannot be marked as protected. If a class contains abstract methods, the class itself must be declared abstract, as follows: Which of the following methods does an abstract method declare in an abstract Java class? First, you declare an abstract class, GraphicObject, to provide member variables and methods that are fully 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, which must be implemented by all subclasses, but must be implemented in different ways. The GraphicObject class may resemble the following: An abstract class is a class declared abstract, which may or may not contain abstract methods. Abstract classes cannot be instantiated, but they can be subordinate. In an object-oriented drawing application, you can draw circles, rectangles, lines, Bézier curves, and many other graphic objects. These objects all share specific states (such as position, alignment, line color, fill color) and behaviors (such as move, rotate, resize, draw). Some of these states and behaviors are the same for all graphic objects (for example, position, fill color, and moveTo). Others require different implementations (such as resizing or drawing). All GraphicObjects must be able to draw or resize. They differ only in the way they do it. This is a perfect situation for an abstract superclass.
You can take advantage of similarities and declare that all graphic objects inherit from the same abstract parent object (for example, GraphicObject), as shown below. Note: Although abstract classes cannot be used to instantiate objects, they can be used to create object references because the Java approach to runtime polymorphism is implemented through the use of superclass references. Therefore, it must be possible to create a reference to an abstract class so that it can be used to refer to a subclass object. Which of the following class definitions defines a legal abstract class? An abstract class is a special type of class that cannot be instantiated. An abstract class is designed to be inherited by subclasses that implement or replace its methods. In other words, abstract classes are partially implemented or not implemented at all. You can have functionality in your abstract classroom – the methods of an abstract class can be both abstract and concrete. An abstract class can have constructors, it is an essential difference between an abstract class and an interface. You can use abstract classes to design components and specify a certain level of general functionality that must be implemented by derived classes. In this case, class X must be abstract because it does not fully implement Y, but class XX actually implements Y. When designing applications, it is important to know when to use an abstract class and when to use an interface. While abstract classes and interfaces may seem similar in some ways, there are important differences that determine which is the best choice for what you want to achieve.
In this blog post, I will discuss these differences and how to decide when to use which one. As you can see, there is no method text. Any concrete class (that is.class without an abstract keyword) that extends an abstract class must replace all abstract methods of the class. Important rules for abstract methods: An abstract class can have static fields and static methods. You can use a class reference (for example, use AbstractClass.staticMethod()) as with any other class. 1. Final summary class Test {}2. public static interface Test {}3. last public class Test {}4. Protected abstract class Test {}5. protected interface Test {}6. abstract public class Test {} Given the following code: public class School{ public abstract double numberOfStudent(); } Which of the following statements is true? Abstract classes are similar to interfaces.
You cannot instantiate them, and they can contain a combination of methods declared with or without implementation. However, abstract classes allow you to declare fields that are not static and definitive, and to define public, protected, and private concrete methods. With interfaces, all fields are automatically public, static, and definitive, and all methods you declare or define (as default methods) are public. In addition, you can only extend one class, whether abstract or not, while you can implement any number of interfaces. Which one should you use, abstract classes or interfaces? Abstract classes give you the ability to use some concrete methods and other methods that derived classes must implement. Conversely, if you use interfaces, you must implement all methods of the class that extends the interface. An abstract class is a good choice if you have plans for future expansion, for example.