Deeply understand and apply Java abstract classes

1. Introduction

In Java object-oriented programming, abstract class is a very important concept. It allows us to define a class that contains some abstract methods and non-abstract methods, but cannot be instantiated by itself. Abstract classes are mainly used to create class hierarchies and provide common properties and methods for subclasses. This article will introduce in detail the basic concepts, features and application scenarios of Java abstract classes.

2. Basic concepts of abstract classes

1. Definition

In Java, useabstractThe class that is modified by keywords is called abstract classes. Abstract classes cannot be instantiated, that is, they cannot be used.newKeywords create objects of abstract class. But abstract classes can contain abstract methods and non-abstract methods.

2. Abstract method

The abstract method is to useabstractA method with keyword modification without method body. Abstract methods must be defined in abstract classes, and subclasses must implement all abstract methods in abstract classes (unless subclasses are also abstract classes).

3. Example

// Define an abstract classabstract class Animal {
    // Abstract method    abstract void makeSound();
    // Non-abstract method    void eat() {
        ("The animal eats.");
    }
}

3. Characteristics of abstract classes

1. Cannot be instantiated

Abstract classes cannot be instantiated, that is, they cannot be used.newKeywords create objects of abstract class. This is because abstract classes usually contain some abstract methods that are not implemented, so objects cannot be created directly.

2. Can contain abstract methods and non-abstract methods

Abstract classes can contain abstract methods and non-abstract methods. Abstract methods must be implemented in subclasses, while non-abstract methods can be used directly in abstract classes.

3. Subclasses must implement all abstract methods in abstract classes (unless subclasses are also abstract classes)

If a class inherits an abstract class, it must implement all abstract methods in the abstract class (unless the subclass is also an abstract class). Otherwise, the compiler will report an error.

4. It can be used as a superclass of other classes

Abstract classes can be used as superclasses of other classes, providing common properties and methods. In this way, subclasses can inherit the properties and methods of abstract classes and add their own properties and methods.

4. Abstract application scenarios

1. Define standard implementation of interfaces

Abstract classes can be used when you need to define a standard implementation of an interface. Abstract classes can contain some common non-abstract methods that can provide default implementations for subclasses. Subclasses only need to implement the methods defined in the interface.

2. Implementation details of hidden classes

In some cases, we may want to hide some implementation details of the class and expose only the necessary interfaces for external use. At this time, you can use abstract classes to define the interface and implement some common methods in the abstract classes. Subclasses can inherit abstract classes and implement methods defined in interfaces, but cannot access private properties and methods in abstract classes.

3. Implement template method mode

A template method pattern is a behavioral design pattern that defines the skeleton of an algorithm in a method and allows subclasses to provide implementations for one or more steps. In Java, abstract classes can be used to implement template method patterns. An abstract class can define a template method with multiple steps and call some abstract methods in it. Subclasses can inherit abstract classes and implement these abstract methods, thereby providing a concrete implementation of the algorithm.

5. Summary

Java abstract classes are a very important concept. They allow us to define classes that contain abstract methods and non-abstract methods, and provide common properties and methods for subclasses. Abstract classes cannot be instantiated, but can be used as superclasses for other classes. A subclass must implement all abstract methods in an abstract class (unless the subclass is also an abstract class). Abstract classes have extensive applications in defining standard implementations of interfaces, implementation details of hidden classes, and implementation template method patterns.

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