PL/I 语言 基类与派生类关系

PL/I阿木 发布于 3 天前 4 次阅读


阿木博主一句话概括:PL/I语言中的基类与派生类关系实现与探讨

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本文旨在探讨PL/I语言中基类与派生类的关系,通过代码示例展示如何定义基类和派生类,以及如何实现多态性。PL/I是一种历史悠久的高级程序设计语言,它支持面向对象编程(OOP)的概念,包括类和对象。本文将深入分析PL/I中的类继承、方法重写和多态性,并通过实例代码进行说明。

一、
PL/I(Programming Language One)是一种多用途的高级程序设计语言,它结合了多种编程语言的特点,包括COBOL、FORTRAN和ALGOL。尽管PL/I在21世纪的现代编程语言中不再流行,但它仍然在一些遗留系统中发挥着重要作用。PL/I支持面向对象编程,允许开发者定义基类和派生类,实现代码的重用和扩展。

二、基类与派生类的定义
在PL/I中,基类和派生类的定义是通过使用`CLASS`语句来实现的。以下是一个简单的基类和派生类的定义示例:

pl/i
IDENTIFICATION DIVISION.
PROGRAM-ID. BASE-CLASS-EXAMPLE.

CLASS BASE-CLASS.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 BASE-ATTRIBUTES.
05 BASE-VALUE PIC 9(5).

PROCEDURE DIVISION.
DISPLAY "Base class attributes: " BASE-VALUE.

CLASS DERIVED-CLASS INHERITS BASE-CLASS.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 DERIVED-ATTRIBUTES.
05 DERIVED-VALUE PIC 9(5).

PROCEDURE DIVISION.
DISPLAY "Derived class attributes: " DERIVED-VALUE.
CALL 'DISPLAY-BASE-ATTRIBUTES' USING BASE-ATTRIBUTES.
END BASE-CLASS.

PROCEDURE DIVISION.
DISPLAY "Creating a derived class instance."
PERFORM CREATE-INSTANCE.
STOP RUN.

CREATE-INSTANCE.
DECLARE BASE-CLASS INSTANCE BASE-INSTANCE.
DECLARE DERIVED-CLASS INSTANCE DERIVED-INSTANCE.
INITIALIZE BASE-INSTANCE, DERIVED-INSTANCE.
SET BASE-INSTANCE TO BASE-CLASS.
SET DERIVED-INSTANCE TO DERIVED-CLASS.
SET DERIVED-INSTANCE TO BASE-INSTANCE.
SET DERIVED-VALUE OF DERIVED-INSTANCE TO 12345.
CALL 'DISPLAY-DERIVED-ATTRIBUTES' USING DERIVED-ATTRIBUTES.
END CREATE-INSTANCE.

在这个例子中,我们定义了一个基类`BASE-CLASS`和一个派生类`DERIVED-CLASS`。派生类通过`INHERITS`关键字继承了基类的属性和方法。

三、方法重写
在面向对象编程中,派生类可以重写基类的方法。以下是一个方法重写的示例:

pl/i
CLASS BASE-CLASS.
PROCEDURE DIVISION.
DISPLAY "Base class method called."
END BASE-CLASS.

CLASS DERIVED-CLASS INHERITS BASE-CLASS.
PROCEDURE DIVISION.
DISPLAY "Derived class method called."
END DERIVED-CLASS.

在这个例子中,`DERIVED-CLASS`重写了`BASE-CLASS`中的方法。当创建派生类的实例并调用该方法时,将执行派生类中的方法。

四、多态性
多态性是面向对象编程的核心概念之一。在PL/I中,多态性可以通过使用类型转换来实现。以下是一个多态性的示例:

pl/i
PROCEDURE DIVISION.
DECLARE BASE-CLASS INSTANCE BASE-INSTANCE.
DECLARE DERIVED-CLASS INSTANCE DERIVED-INSTANCE.
INITIALIZE BASE-INSTANCE, DERIVED-INSTANCE.
SET BASE-INSTANCE TO BASE-CLASS.
SET DERIVED-INSTANCE TO DERIVED-CLASS.
SET DERIVED-INSTANCE TO BASE-INSTANCE.
SET DERIVED-VALUE OF DERIVED-INSTANCE TO 12345.
CALL 'DISPLAY-ATTRIBUTES' USING BASE-INSTANCE.
CALL 'DISPLAY-ATTRIBUTES' USING DERIVED-INSTANCE.
END PROCEDURE.

DISPLAY-ATTRIBUTES.
DECLARE BASE-CLASS INSTANCE USING BASE-INSTANCE.
DISPLAY "Attributes: " BASE-VALUE.
END DISPLAY-ATTRIBUTES.

在这个例子中,我们定义了一个`DISPLAY-ATTRIBUTES`过程,它接受一个`BASE-CLASS`实例作为参数。由于`DERIVED-CLASS`是`BASE-CLASS`的子类型,我们可以将派生类实例传递给这个过程,从而实现多态性。

五、结论
PL/I语言支持面向对象编程的概念,包括基类与派生类的关系。通过使用`CLASS`语句,我们可以定义基类和派生类,并通过继承、方法重写和多态性来实现代码的重用和扩展。尽管PL/I在现代编程语言中不再流行,但它仍然在一些遗留系统中发挥着重要作用,理解其面向对象编程的概念对于维护和更新这些系统至关重要。

本文通过代码示例展示了PL/I中基类与派生类的关系,包括定义、方法重写和多态性的实现。这些概念对于任何学习面向对象编程的开发者来说都是基础,即使在非PL/I的语言中也是如此。