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The principle of opening and closing means opening to expansion and closing to modification. When the program needs to be expanded, the original code cannot be modified, but the original code should be expanded to achieve a hot plug effect. Therefore, a summary of one sentence is: in order to make the program scalable, easy to maintain and upgrade. To achieve this effect, we need to use interfaces and abstract classes, etc. We will mention this point in the later specific design.曹参突然觉得自己肩上的任务沉重了许多。停了一会，郑氏又道：淼淼会医术，会弹琴，茶饭也好。Although there are many unpredictable potential causes for new attacks, according to our experience, the following two events may trigger the occurrence of new attacks:尹旭斜躺在大石上向下瞧去，当先一人赫然正是苏岸，嘴角露出一丝淡淡的笑意。只因为任何人都知道，世上绝没有一个少女能抵挡江枫的微微一笑，也绝没有一个英雄能抵挡燕南天的轻轻一剑。An NBA game lasts 48 minutes and consists of four quarters with 12 minutes each.