英文:
How can I sort a Map<LocalDate, Integer>?
问题
我只想知道如何按时间顺序对一个Map<LocalDate，Integer>进行排序。
这是我代码的第一行：
Map<LocalDate, Integer> commitsPerDate = new HashMap<>();
然后，我会用任何值填充我的Map（但不是按时间顺序）。当我显示Map的值时，它是按照以下顺序显示的：
for(var item : commitsPerDate.entrySet()) {
System.out.println(item.getKey() + " = " + item.getValue());
}
2020-08-31 = 1
2020-09-30 = 3
2020-09-29 = 1
2020-09-28 = 5
2020-09-27 = 5
2020-08-27 = 4
2020-09-25 = 3
2020-10-21 = 1
2020-10-18 = 1
2020-10-17 = 5
2020-10-16 = 4
2020-10-15 = 5
2020-10-14 = 6
2020-09-14 = 1
2020-10-13 = 2
2020-09-13 = 2
我希望它按照时间顺序排序，以便显示顺序相同。
谢谢。
英文:
I just want to know how I can sort by chnrological order a Map<LocalDate, Integer>.
Here's the first line of my code :
Map<LocalDate, Integer> commitsPerDate = new HashMap<>();
Afterwards, I fill in my Map with any value (but not in chronological order).
And when I display the values of my Map, it is displayed in this order :
for(var item : commitsPerDate.entrySet()) {
System.out.println(item.getKey() + " = " + item.getValue());
}
2020-08-31 = 1
2020-09-30 = 3
2020-09-29 = 1
2020-09-28 = 5
2020-09-27 = 5
2020-08-27 = 4
2020-09-25 = 3
2020-10-21 = 1
2020-10-18 = 1
2020-10-17 = 5
2020-10-16 = 4
2020-10-15 = 5
2020-10-14 = 6
2020-09-14 = 1
2020-10-13 = 2
2020-09-13 = 2
And I would like it to be sorted in chronological order so that the display is in the same order.
Thank you.
答案1
得分: 2
正如Pshemo已经提到的，如果您希望通过键来对映射排序，您可以使用TreeMap代替HashMap。
示例：
import java.time.LocalDate;
import java.util.Map;
import java.util.TreeMap;
public class Main {
public static void main(String[] args) {
Map<LocalDate, Integer> commitsPerDate = new TreeMap<>();
commitsPerDate.put(LocalDate.parse("2020-08-31"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-30"), 3);
commitsPerDate.put(LocalDate.parse("2020-09-29"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-28"), 5);
System.out.println(commitsPerDate);
}
}
输出：
{2020-08-31=1, 2020-09-28=5, 2020-09-29=1, 2020-09-30=3}
按相反顺序：
import java.time.LocalDate;
import java.util.Collections;
import java.util.Map;
import java.util.TreeMap;
public class Main {
public static void main(String[] args) {
Map<LocalDate, Integer> commitsPerDate = new TreeMap<>(Collections.reverseOrder());
commitsPerDate.put(LocalDate.parse("2020-08-31"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-30"), 3);
commitsPerDate.put(LocalDate.parse("2020-09-29"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-28"), 5);
System.out.println(commitsPerDate);
}
}
输出：
{2020-09-30=3, 2020-09-29=1, 2020-09-28=5, 2020-08-31=1}
如果出于任何原因您必须使用HashMap，请参阅https://stackoverflow.com/questions/922528/how-to-sort-map-values-by-key-in-java。
英文:
As Pshemo has already mentioned, if you want the map to order elements by key then you can use TreeMap instead of HashMap.
Demo:
import java.time.LocalDate;
import java.util.Map;
import java.util.TreeMap;
public class Main {
public static void main(String[] args) {
Map<LocalDate, Integer> commitsPerDate = new TreeMap<>();
commitsPerDate.put(LocalDate.parse("2020-08-31"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-30"), 3);
commitsPerDate.put(LocalDate.parse("2020-09-29"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-28"), 5);
System.out.println(commitsPerDate);
}
}
Output:
{2020-08-31=1, 2020-09-28=5, 2020-09-29=1, 2020-09-30=3}
In the reverse order:
import java.time.LocalDate;
import java.util.Collections;
import java.util.Map;
import java.util.TreeMap;
public class Main {
public static void main(String[] args) {
Map<LocalDate, Integer> commitsPerDate = new TreeMap<>(Collections.reverseOrder());
commitsPerDate.put(LocalDate.parse("2020-08-31"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-30"), 3);
commitsPerDate.put(LocalDate.parse("2020-09-29"), 1);
commitsPerDate.put(LocalDate.parse("2020-09-28"), 5);
System.out.println(commitsPerDate);
}
}
Output:
{2020-09-30=3, 2020-09-29=1, 2020-09-28=5, 2020-08-31=1}
For any reason, if you have to use HashMap, check https://stackoverflow.com/questions/922528/how-to-sort-map-values-by-key-in-java
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