Java 以相反的顺序对 Set 进行排序
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Sort a Set in reverse order
提问by carlspring
Apologies for the newbie question, but what's the proper way to get a Set(say LinkedHashSet) in reverse order? For Collections there's Collections.reverse(Collection c), but how does one do it for a Setwith ordered elements (like a LinkedHashSet)?
为新手问题道歉，但是以相反的顺序获得Set（例如LinkedHashSet）的正确方法是什么？对于Collections 有Collections.reverse(Collection c)，但是如何为Set具有有序元素（如 a LinkedHashSet）的a 做到这一点？
采纳答案by Peter
Sets are not ordered in general, so to preserve the sorting, after sorting the set as a list, you would need to use a known iteration order implementation of Set, such as LinkedHashSet
集合一般不排序，所以为了保持排序，在将集合排序为列表后，您需要使用 Set 的已知迭代顺序实现，例如 LinkedHashSet
List list = new ArrayList(set);
Collections.sort(list, Collections.reverseOrder());
Set resultSet = new LinkedHashSet(list);
You could also use TreeSet with a comparator, but that is not as fast as the ArrayList method above.
您也可以将 TreeSet 与比较器一起使用，但这不如上面的 ArrayList 方法快。
回答by Daniel Williams
Check this out
看一下这个
http://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#descendingSet()
http://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html#descendingSet()
If you use a TreeSet you can get reverse order by calling descendingSet.
如果您使用 TreeSet，您可以通过调用descendingSet 获得相反的顺序。
回答by Ved Prakash
public class LargestArray {
public static void main(String[] args) {
ArrayList<Integer> al = new ArrayList<>();
Set<Integer> set = new TreeSet<>();
set.add(10);
set.add(20);
set.add(7);
set.add(4);
set.add(1);
set.add(2);
set.add(3);
set.add(4);
System.out.println("after Sorting");
for(int i : set) {
System.out.print(" " + i);
}
al.addAll(set);
set.clear();
Collections.reverse(al);
System.out.println();
System.out.println("After Reverse");
for (int i : al) {
System.out.print(" " + i);
}
}
}
output = after Sorting 1 2 3 4 7 10 20 After Reverse 20 10 7 4 3 2 1
输出 = 排序后 1 2 3 4 7 10 20 反向后 20 10 7 4 3 2 1
回答by Manjunatha B
I will explain you with an example. Comments are added in mid of the code for better understanding.
我会用一个例子来解释你。代码中间添加了注释，以便更好地理解。
public class ReverseLinkedHashSet {
public static void main(String[] args) {
// creating a LinkedHashSet object which is
// of type String or any. Will take a example of String.
HashSet<String> cars = new LinkedHashSet<String>();
// adding car elements to LinkedHashSet object as below
cars.add("Toyato");
cars.add("Hundai");
cars.add("Porshe");
cars.add("BMW");
// Iterating using enhanced for-loop to see the order.
System.out.println("Insertion Order: Iterating LinkedHashSet\n");
for(String car : cars) {
System.out.println(car);
// Output will be as below
//Toyato
//Hundai
//Porshe
//BMW
}
// Now convert to ArrayList to rearrange to reverse
// the linkedHashset
List<String> listOfCars = new ArrayList<String>(cars);
// to reverse LinkedHashSet contents
Collections.reverse(listOfCars);
// reverse order of LinkedHashSet contents
// can be done as below
System.out.println("\n\n\nReverse Order of LinkedHashSet\n");
for(String car : listOfCars) {
System.out.println(car);
// Output will be as below
//BMW
//Porshe
//Hundai
//Toyato
}
}
}
Also, I suggest not to use LinkedhashSetwithout a strong reason. For a complex application, it will reduce the performance. Use HashSetinstead.
另外，我建议不要在LinkedhashSet没有充分理由的情况下使用。对于复杂的应用程序，它会降低性能。使用HashSet来代替。
回答by bamossza
Java 8, I using solution below,
Java 8，我使用下面的解决方案，
Set<String> setTest = new HashSet<>();
setTest.add("1");
setTest.add("2");
setTest.add("3");
List<String> list = new ArrayList<>(setTest);
list.sort(Collections.reverseOrder());
Set<String> result = new LinkedHashSet<>(list);
for (String item: result) {
System.out.println("---> " + item);
}
Result:
---> 3
---> 2
---> 1
Work for me.
为我工作。
