序言
本文通过Java示例代码片段展示了常用重构原则和技巧，供初级开发人员参考。精致的代码能够清楚传达作者的意图，精致的代码是最好的注释，精致的代码非常容易维护和扩展。程序员阅读精致的代码如同大众欣赏优美的散文一样享受。
16 减少重复计算
16.1 重构前
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if(list != null && list.size() > 0){ for(int i = 0; i < list.size(); i++){ //skip... } }
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16.2 重构后
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if(list != null){ for(int i = 0, len = list.size(); i < len; i++){ //skip... } }
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17 何时需要何时创建
17.1 重构前
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public static Date parseDate(String date) throws ParseException { SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
if ((date == null) || (date.equals(""))) { return null; } else { try { return formatter.parse(date); } catch (ParseException pe) { throw new ParseException(pe.getMessage(), pe.getErrorOffset()); } } }
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17.2 重构后
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public static Date parseDate(String date) throws ParseException { if ((date == null) || (date.equals(""))) { return null; } else { try { SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd"); return formatter.parse(date); } catch (ParseException pe) { throw new ParseException(pe.getMessage(), pe.getErrorOffset()); } } }
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18 利用已有的计算结果
18.1 重构前
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public static final String DAY = "DAY"; public static final String MONTH = "MONTH"; public static final String YEAR = "YEAR"; public static final String HOUR = "HOUR"; public static final String MINUTE = "MINUTE"; public static final String SECOND = "SECOND"; public static final String WEEK = "WEEK";
public long toMilliseconds(String unit) { if (unit == null) { return 0L; } else if (unit.equals(SECOND)) { return 1 * 1000L; } else if (unit.equals(MINUTE)) { return 60 * 1000L; } else if (unit.equals(HOUR)) { return 60 * 60 * 1000L; } else if (unit.equals(DAY)) { return 24 * 60 * 60 * 1000L; } else if (unit.equals(WEEK)) { return 7 * 24 * 60 * 60 * 1000L; } else if (unit.equals(MONTH)) { return 30 * 24 * 60 * 60 * 1000L; } else if (unit.equals(YEAR)) { return 365 * 24 * 60 * 60 * 1000L; } else { return 0L; } }
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18.2 重构后
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public class Unit { private static final long SECOND_MILLIS = 1000; private static final long MINUTE_MILLIS = 60 * SECOND_MILLIS; private static final long HOUR_MILLIS = 60 * MINUTE_MILLIS; private static final long DAY_MILLIS = 24 * HOUR_MILLIS; private static final long WEEK_MILLIS = 7 * DAY_MILLIS; private static final long MONTH_MILLIS = 30 * DAY_MILLIS; private static final long YEAR_MILLIS = 365 * DAY_MILLIS; private static final long CENTURY_MILLIS = 100 * YEAR_MILLIS;
static Map<String, Unit> units = new HashMap<String, Unit>();
public static final Unit SECOND = new Unit(SECOND_MILLIS, "SECOND"); public static final Unit MINUTE = new Unit(MINUTE_MILLIS, "MINUTE"); public static final Unit HOUR = new Unit(HOUR_MILLIS, "HOUR"); public static final Unit DAY = new Unit(DAY_MILLIS, "DAY"); public static final Unit WEEK = new Unit(WEEK_MILLIS, "WEEK"); public static final Unit MONTH = new Unit(MONTH_MILLIS, "MONTH"); public static final Unit YEAR = new Unit(YEAR_MILLIS, "YEAR"); public static final Unit CENTURY = new Unit(CENTURY_MILLIS, "CENTURY");
static { units.put(SECOND.name, SECOND); units.put(MINUTE.name, MINUTE); units.put(HOUR.name, HOUR); units.put(DAY.name, DAY); units.put(WEEK.name, WEEK); units.put(MONTH.name, MONTH); units.put(YEAR.name, YEAR); units.put(CENTURY.name, CENTURY); }
private long millis; private String name;
private Unit(long millis, String name) { this.millis = millis; this.name = name; }
public long getMillis() { return millis; }
public String getName() { return name; }
public String toString() { StringBuffer buffer = new StringBuffer(); buffer.append(this.getName()); buffer.append("["); buffer.append(this.getMillis()); buffer.append("]"); return buffer.toString(); } }
public long toMilliseconds(Unit unit) { return unit.getMillis(); }
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19 替换switch结构
19.1 重构前
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public boolean isLeap(int year) { return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); }
public static int getMonthDays(int year, int month) { int numberDays = 0;
switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12: numberDays = 31; break;
case 4: case 6: case 9: case 11: numberDays = 30; break;
case 2: numberDays = isLeap(year) ? 29 : 28; break; }
return numberDays; }
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19.2 重构后
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public boolean isLeap(int year) { return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0); }
private int getFebruaryDays(int year) { return this.isLeap(year) ? 29 : 28; }
public int getMonthDays(int year, int month) { int[] months = new int[] { 31, this.getFebruaryDays(year), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
return months[month]; }
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20 避免对参数赋值
20.1 重构前
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public Date getStartTime(Date date) { date.setMinutes(fromMinute); date.setDate(fromHour); date.setDate(fromHour); date.setSeconds(0);
return date; }
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20.2 重构后
public Date getStartTime(final Date date) {
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
calendar.set(Calendar.HOUR_OF_DAY, fromHour);
calendar.set(Calendar.MINUTE, fromMinute);
calendar.set(Calendar.SECOND, 0);
return calendar.getTime();
}
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