The usage and function of abstract keyword in PHP

Usage and function of abstract keyword in PHP

In PHP language, abstract is a keyword used to declare an abstract class or abstract method. Abstract classes refer to classes that cannot be instantiated and are mainly used to define classes that have common characteristics but no specific implementation; abstract methods refer to methods that only have method declarations but no specific implementation. This article will introduce the usage and function of abstract keyword in detail.

1. Declaration and use of abstract classes

In PHP, declare an abstract class by using the abstract keyword. Abstract classes cannot be instantiated, only inherited. Abstract classes are generally used to define some common methods and properties, but cannot be implemented concretely. The declaration of an abstract class is as follows:

abstract class AbstractClass {
    // 抽象方法
    abstract protected function someMethod();

    // 普通方法
    public function anotherMethod() {
        echo "这是一个普通方法。";
    }
}

As can be seen from the above code, an abstract class can contain abstract methods and ordinary methods. Among them, abstract methods are declared using the abstract keyword. There is no specific implementation code, only the declaration of the method. The ordinary methods have specific implementation codes.

The use of abstract classes is achieved through inheritance. For example:

class ConcreteClass extends AbstractClass {
    // 实现抽象方法
    protected function someMethod() {
        echo "抽象方法的实现。";
    }
}

After the abstract method in the abstract class is implemented in the subclass, the subclass can be instantiated. Subclasses can continue to extend methods in abstract classes, or they can override methods in abstract classes.

2. Declaration and use of abstract methods

Abstract methods refer to methods that only have method declarations but no specific implementation. Abstract methods are also declared using abstract keyword. The main purpose of using abstract methods is to force subclasses to implement the method. The abstract method is declared as follows:

abstract protected function someMethod();

As can be seen from the above code, the abstract method only has the method name and parameter list, and no specific implementation code.

If a class contains one or more abstract methods, then the class must be declared as an abstract class. Moreover, subclasses that inherit an abstract class must implement all abstract methods in the abstract class before they can be instantiated.

For example, in the above abstract class AbstractClass, someMethod() is an abstract method. In the subclass ConcreteClass, we must implement the someMethod() method to be instantiated.

class ConcreteClass extends AbstractClass {
    // 实现抽象方法
    protected function someMethod() {
        echo "抽象方法的实现。";
    }
}

Through the above method, we can see that abstract methods can be implemented by subclasses or overridden by subclasses.

3. The role of abstract keyword

1. Forcing subclasses to implement abstract methods: By using the abstract keyword, you can require subclasses to implement the abstract methods declared in the abstract class. This ensures that subclasses have certain behavioral specifications.
2. Prevent an abstract class from being instantiated: By declaring a class as an abstract class, you can prevent the class from being instantiated. This avoids using the class in unsuitable conditions.
3. Provide templates for public behaviors: abstract classes can contain some common methods and attributes, and define templates for some behaviors. Subclasses can inherit these common methods and attributes, and implement or rewrite them according to specific needs. method.

4. Notes

1. Abstract classes cannot be instantiated. If you try to instantiate an abstract class, it will cause the PHP interpreter to throw a fatal error.
2. After a subclass inherits an abstract class, it must implement all abstract methods in the abstract class. Otherwise, the subclass must also be declared abstract.
3. Abstract methods cannot have specific implementation code, only method declarations.

Summary:

By using the abstract keyword, abstract classes and abstract methods in PHP can implement code specifications and constraints. Abstract classes define some common methods and attributes, which have nothing to do with specific implementations. Subclasses can ensure the consistency and scalability of concrete classes by inheriting abstract classes and implementing abstract methods. At the same time, abstract classes can also provide a template to provide public behavior specifications for subclasses. In actual development, rational use of abstract can improve code reusability and maintainability.

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