[Java知识库]Java多if-else转换为表驱动优化

背景：
if else 嵌套层数套多，导致复杂度过高，因此采用采用表驱动的方式进行优化，目前只知道这种方法。

public class ThroughDate {
    private static final int MONTH_5 = 5;
    private static final int MONTH_6 = 6;
    private static final int MONTH_7 = 7;

    public static void main(String[] args) {
        int date = 4;
        throughMonth(date);
    }

    private static void throughMonth(int date) {
        if (date < MONTH_5) {
            throughMonth5(date);
        }
        if (date < MONTH_6) {
            throughMonth6(date);
        }
        if (date < MONTH_7) {
            throughMonth7(date);
        }
    }

    private static void throughMonth5(int date) {
        System.out.println("走过了5月份：" +  date);
    }

    private static void throughMonth6(int date) {
        System.out.println("走过了6月份：" +  date);
    }

    private static void throughMonth7(int date) {
        System.out.println("走过了7月份：" +  date);
    }
} 

该例子的典型应用是安卓的数据库升级。
优化方式如下：

public class ThroughDate2 {
    private static final int MONTH_5 = 5;
    private static final int MONTH_6 = 6;
    private static final int MONTH_7 = 7;

    public static void main(String[] args) {
        int date = 1;
        int newDate = 8;

        Map<Integer, Function<Integer, Void>> dates = throughMonth();
        for (int i = date + 1; i < date + 1 + dates.size(); i++) {
            System.out.println("i: " + i);
            dates.get(i).apply(newDate);
        }
    }

    private static Map<Integer, Function<Integer, Void>> throughMonth() {
        Map<Integer, Function<Integer, Void>> dates = new LinkedHashMap<>();   // 使用排序的哈希算法
        dates.put(MONTH_5, throughMonth5);
        dates.put(MONTH_6, throughMonth6);
        dates.put(MONTH_7, throughMonth7);

        return dates;
    }

    private static final Function<Integer, Void> throughMonth5 = (Integer date) -> {
        System.out.println("走过了5月份：" +  date);
        return null;
    };

    private static final Function<Integer, Void> throughMonth6 = (Integer date) -> {
        System.out.println("走过了6月份：" +  date);
        return null;
    };

    private static final Function<Integer, Void> throughMonth7 = (Integer date) -> {
        System.out.println("走过了7月份：" +  date);
        return null;
    };
}