开闭原则（Open-Closed Principle）

• 概念：指一个软件实体（类、模块、函数）应该对扩展开放、对修改关闭
• 强调：用抽象构建框架，用实现扩展细节
• 提高软件系统的可复用性及可维护性

开闭原则的核心思想就是面向抽象编程
**

事例一：

public interface IFinance{
    Double getRate();
}

public class Self implements IFinance{
    private Double rate;
    public Self(Double rate){
        this.rate = rate;
    }
    public Double getRate(){
        return this.rate;
    }
}

public class Unite implements IFinance{
    private Double rate;
    public Self(Double rate){
        this.rate = rate;
    }
    public Double getRate(){
        return this.rate + 100;
    }
}

public class BLG extends Self{
    public BLG(Double rate){
        super(rate);
    }
    public Double getBLGRate(){
        return super.getRate() * 0.55555;
    }
}