This indicates that the BankAccount class is an abstract class and the withdrawal method is an abstract operation.
这表示，BankAccount类是一个抽象类，而withdrawal方法是抽象的操作。
Abstract: Operation structure is one of the most important parts of construction industry.
业务结构是建筑业产业结构中非常重要的组成部分之一。
Abstract: Operation structure is one of the most important parts of construction industry.
业务结构是建筑业产业结构中非常重要的组成部分之一。
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