如何按升序和降序对哈希图数据进行排序

Question

我有一个HashMap<String, List<String>> myData，我希望能够将我的HashMap按升序排序，然后将其放入一个变量中，我将使用该变量进行迭代并添加到excel文件中。我也想按降序排序，然后在一个变量中获得结果，迭代它，并将数据添加到excel文件中。我的HashMap看起来像这样：Aug - 19, {"11", "12"} July- 19, {"01", "22"} Jun - 19, {"77", "02"} May - 19, {"99", "42"}关键字是日期。值只是一个字符串列表。我需要检索包含按升序排序的数据的变量，并在另一个变量中检索按降序排序的数据。非常感谢你的帮助。

Answer 1

首先，将map中的键转换为更自然的格式，即日期。然后将它们存储在TreeMap中，这是一个按排序顺序存储键的关联容器。

使用parse(String)获取Date对象，然后使用format(Date)获取该对象的字符串形式。

public class DateAsTreeMapKey {
    public static void main(String[] args) {
        HashMap<String, List<String>> myData = new HashMap<>();

        String format = "MMM - yy";
        myData.put("Aug - 19", Arrays.asList("11", "12"));
        myData.put("July - 19", Arrays.asList("01", "22")); // try out July also
        myData.put("Jun - 19", Arrays.asList("77", "02"));
        myData.put("May - 19", Arrays.asList("99", "42"));      
        printDataInAscendingOrder(getDataInSortedForm(myData, format), format);
        printDataInDescendingOrder(getDataInSortedForm(myData, format), format);

    }
    public static TreeMap<Date, List<String>> getDataInSortedForm(HashMap<String, List<String>> myData, String format) {
        TreeMap<Date, List<String>> mySortedData = new TreeMap<>();
        for (Map.Entry<String, List<String>> entry : myData.entrySet()) {
            String monthYear = entry.getKey();
            List<String> data = entry.getValue();
            try {
                Date date = new SimpleDateFormat(format, Locale.ENGLISH).parse(monthYear);
                mySortedData.put(date, data);
            } catch (ParseException e) {
                e.printStackTrace();
            }
        }
        return mySortedData;
    }
    public static void printDataInAscendingOrder(TreeMap<Date, List<String>> mySortedData, String format) {
        System.out.println("Data in ascending order: ");
        for (Entry<Date, List<String>> entry : mySortedData.entrySet()) {
            System.out.println("Month '" + new SimpleDateFormat(format, Locale.ENGLISH).format(entry.getKey())
                    + "' has data as " + entry.getValue().toString());
        }
    }
    public static void printDataInDescendingOrder(TreeMap<Date, List<String>> mySortedData, String format) {
        System.out.println("Data in descending order: ");
        TreeMap<Date, List<String>> mySortedDataReversed = new TreeMap<>(Collections.reverseOrder());
        mySortedDataReversed.putAll(mySortedData);
        for (Entry<Date, List<String>> entry : mySortedDataReversed.entrySet()) {
            System.out.println("Month '" + new SimpleDateFormat(format, Locale.ENGLISH).format(entry.getKey())
                    + "' has data as " + entry.getValue().toString());
        }
    }
}

打印：

Data in ascending order: 
Month 'May - 19' has data as [99, 42]
Month 'Jun - 19' has data as [77, 02]
Month 'Jul - 19' has data as [01, 22]
Month 'Aug - 19' has data as [11, 12]
Data in descending order: 
Month 'Aug - 19' has data as [11, 12]
Month 'Jul - 19' has data as [01, 22]
Month 'Jun - 19' has data as [77, 02]
Month 'May - 19' has data as [99, 42]

Answer 2

这是一个java 1.8+流版本。它可以更高效，但这可能会使示例变得模糊，所以我把这个问题留给您。请注意，to排序和反向排序之间的唯一区别是.reversed()方法。

import org.junit.jupiter.api.Test;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class MapSorter {

    @Test
    public void test() {

        Map<String, List<String>> myData = new HashMap<>();

        myData.put("Aug - 19", Arrays.asList("11", "12"));
        myData.put("July - 19", Arrays.asList("01", "22")); // try out July also
        myData.put("Jun - 19", Arrays.asList("77", "02"));
        myData.put("May - 19", Arrays.asList("99", "42"));

        myData.keySet().stream()
          .map(SortableKey::new)
          .sorted(Comparator.comparingLong(SortableKey::getSortableKey))
          .map(SortableKey::getOriginalKey)
          .forEach(originalKey -> addToExcel(originalKey, myData));
        myData.keySet().stream()
          .map(SortableKey::new)
          .sorted(Comparator.comparingLong(SortableKey::getSortableKey)
                    .reversed())
          .map(SortableKey::getOriginalKey)
          .forEach(originalKey -> addToExcel(originalKey, myData));
    }

    private void addToExcel(String key, Map<String, List<String>> map) {
        System.out.println(key + " = " + map.get(key));
    }

    class SortableKey {

        private String dateStr;
        private long sortableKey;

        SortableKey(String dateStr) {
            this.dateStr = dateStr;
            try {
                sortableKey = new SimpleDateFormat("MMM - yy").parse(dateStr).getTime();
            } catch (ParseException e) {
                // some type of error handling
            }
        }

        long getSortableKey() {
            return sortableKey;
        }

        String getOriginalKey() {
            return dateStr;
        }
    }
}