在调用类似类 Java 时摆脱 if/else

Question

我遇到了我想要并且需要摆脱一些 if else 情况的问题。我在我的项目中得到了以下代码：

if (ar[4].equals("week")) {

    WeekThreshold wt = new WeekThreshold();
    firstTime = unparsedDate.format(wt.getStartDate().getTime());
    secondTime = unparsedDate.format(wt.getEndDate().getTime());

} else if (ar[4].equals("month")) {

    MonthThreshold mt = new MonthThreshold();
    firstTime = unparsedDate.format(mt.getStartDate().getTime());
    secondTime = unparsedDate.format(mt.getEndDate().getTime());

} else if (ar[4].equals("quarter")) {

    quarterThreshold();

} else if (ar[4].equals("year")) {

    YearThreshold yt = new YearThreshold();
    firstTime = unparsedDate.format(yt.getStartDate().getTime());
    secondTime = unparsedDate.format(yt.getEndDate().getTime());
}

这三个 classes WeekThreshold、MonthThreshold 和 YearThreshold 从 AbstractThreshold class 扩展，它们从日历中获取日期，但这并不重要。方法quarterThreshold()比较特殊，可以留在那里。但是我怎样才能摆脱 if else 块并有一个语句来调用不同的 classes?

编辑：忘了说，需要调用的classes来自数组ar[]的各种类型。如果数组ar[4]是月份，则必须调用MonthThreshold等

Answer 1

多种可能性... XYZThreshold classes 是否具有通用接口，如 Threshold？然后你可以用它分配一个变量，例如...

Threshold threshold = null;
if ((ar[4].equals("week")) {
  threshold = new WeekThreshold();
} else ... {

}

firstTime = unparsedDate.format(threshold.getStartDate().getTime());
secondTime = unparsedDate.format(threshold.getEndDate().getTime());

这将是第一步。如果你愿意，你可以，例如，使用枚举来存储你的阈值：

enum Thresholds {
  WEEK("week") {

     public Threshold getThreshold() {
           return new WeekThreshold();
     }
  },
  etc.

  private String period;

  private Thresholds(String period) {
    this.period = period;
  }

  public abstract Threshold getThreshold();

  //  ...add a static class to iterate and search by period, 
  // ...so you can write Threshold threshold = Thresholds.getByPeriod("week").getThreshold();
}

使用枚举是个人喜好，当然，您可以使用普通的 classes 做同样的事情，或者简单地将用于阈值选择的 if 块放入单独的 class .

Answer 2

你可以像这样在外部合并公共代码（unparsedDate.format(...)）：

AbstractThreshold at = null;
switch(ar[4]) {
case "week":
    at = new WeekThreshold();
    break;
case "month":
    at = new MonthThreshold();
    break;
case "year":
    at = new YearThreshold();
    break;
case "quarter":
    quarterThreshold();
    break;
}
if(at != null) {
    firstTime = unparsedDate.format(at.getStartDate().getTime());
    secondTime = unparsedDate.format(at.getEndDate().getTime());
}

当然，过度设计的版本是可能的。这里只是说明如何使用 Java-8 功能实现它：

// Map can be initialized only once, then used many times
Map<String, Supplier<AbstractThreshold>> thresholdSuppliers = new HashMap<>();
thresholdSuppliers.put("week", WeekThreshold::new);
thresholdSuppliers.put("month", MonthThreshold::new);
thresholdSuppliers.put("year", YearThreshold::new);

AbstractThreshold at = thresholdSuppliers.getOrDefault(ar[4], () -> null).get();
if(at != null) {
    firstTime = unparsedDate.format(at.getStartDate().getTime());
    secondTime = unparsedDate.format(at.getEndDate().getTime());
} else if(ar[4].equals("quarter"))
    quarterThreshold();
}

Answer 3

你可以使用 switch 语句

String typeOfDay;
     switch (dayOfWeekArg) {
         case "Monday":
             typeOfDay = "Start of work week";
             break;
         case "Tuesday":
         case "Wednesday":
         case "Thursday":
             typeOfDay = "Midweek";
             break;
         case "Friday":
             typeOfDay = "End of work week";
             break;
         case "Saturday":
         case "Sunday":
             typeOfDay = "Weekend";
             break;
         default:
             throw new IllegalArgumentException("Invalid day of the week: " + dayOfWeekArg);
     }

您可以将我从 java 文档中窃取的示例替换为您自己的代码

switch(periodType){
    case "week":
      WeekThreshold wt = new WeekThreshold();
    break; // add your other cases
}
firstTime = unparsedDate.format(wt.getStartDate().getTime());
secondTime = unparsedDate.format(wt.getEndDate().getTime());

Answer 4

这里可以好好利用工厂模式

class ThresholdFactory
{
  public static AbstractThreshold getThreshold(String criteria)
  {
    if ( criteria.equals("week") )
      return new WeekThreshold();
    if ( criteria.equals("month") )
      return new MonthThreshold();
    if ( criteria.equals("year") )
      return new YearThreshold();

    return null;
  }
}

其余代码如下所示：

AbstractThreshold at = ThresholdFactory.getThreshold(ar[4]);
if(at != null){
  firstTime = unparsedDate.format(at.getStartDate().getTime());
  secondTime = unparsedDate.format(at.getEndDate().getTime());
} else {
   quarterThreshold();
}

Answer 5

这里有一个如何使用接口和工厂设计模式的例子如果您的多个实现者共享公共代码，请让它们都扩展一个实现该接口的抽象 class。通过接口引用你的方法是个好主意，而不是具体的 class 以利用多态性......见下面的代码......

public class Example {

    public static void main(String[] args) {

        String[] intervals = {"week", "week", "quarter", "month", "year", "week"}; 

        IThreshold[] objects = new IThreshold[intervals.length];

        // Create your objects using Factory pattern
        for(int index = 0; index < intervals.length; index++) {
            objects[index] = ThresholdFactory.createInstance(intervals[index]);
        }

        // Now iterate through your objects and refer to them through a common interface
        for(IThreshold object : objects) {
            int start = object.getFirstTime();
            int end = object.getFirstTime();
        }
    }
}

interface IThreshold {
    public int getFirstTime();
    public int getLastTime();
}


abstract class AbstractThreshold implements IThreshold {

    @Override
    public int getFirstTime() {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public int getLastTime() {
        // TODO Auto-generated method stub
        return 0;
    }

}

class WeekThreshold extends AbstractThreshold {}
class MonthThreshold extends AbstractThreshold {}
class QuarterThreshold extends AbstractThreshold {}
class YearThreshold extends AbstractThreshold {}

class ThresholdFactory {

    public static final IThreshold createInstance(String interval) {
        IThreshold instance = null;

        if(interval.equals("week")){
            instance = new WeekThreshold();
        } 
        else if(interval.equals("month")){
            instance = new MonthThreshold();
        } 
        else if(interval.equals("quarter")){
            instance = new QuarterThreshold();
        } 
        else {
            if(interval.equals("year")){
                instance = new YearThreshold();
            }
        }
        return instance;
    }
}

Answer 6

首先创建门槛工厂，

static enum ThresholdsFactory {


        week(new WeekThreshold()), month(new MonthThreshold())/* etc */;

        static private Map<String,ThresholdsFactory> lookup = new HashMap<String, ThresholdsFactory>();
        static{
            for(ThresholdsFactory val :  ThresholdsFactory.values()){
            lookup.put(val.name(), val);
            }
        }

        public AbstractThreshold threshold;

        public static ThresholdsFactory find(String name){
            return lookup.get(name);
        }

        ThresholdsFactory(AbstractThreshold th) {
            threshold = th;

} }

现在您需要做的就是

AbstractThreshold th = ThresholdsFactory.find(ar[4]);

if (th!=null){
    firstTime = unparsedDate.format(th.getStartDate().getTime());
    secondTime = unparsedDate.format(th.getEndDate().getTime());
}

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