java 如何在java中获得比较器的倒数
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How to get inverse of a comparator in java
提问by Abhinav Batra
In a method I receive a generic object E extends Comparable<E>as an argument. Now i want to create two priority queues.One which uses the comparatorused by E and other queue which uses the opposite of comparatorused by E(i.e. if E uses '<' then second queue must use '>=').
Please hep me how to create two such queues.
在一个方法中，我收到一个泛型object E extends Comparable<E>作为参数。现在我想创建两个优先级comparator队列。一个使用 E 使用的队列，另一个使用 E 使用的相反的队列comparator（即，如果 E 使用“<”，则第二个队列必须使用“>=”）。请教我如何创建两个这样的队列。
queue2=new PriorityQueue<E>(0,Collections.reverseOrder(e));
I am getting the error that reverseOrderis not applicable.
我收到reverseOrder不适用的错误。
please help
请帮忙
回答by casablanca
Look at Collections.reverseOrder.
回答by Bill
Your object Eextends java.lang.Comparable,but it is not a java.util.Comparator.
您的对象E扩展java.lang.Comparable,但它不是java.util.Comparator.
Create your first queue w/o a Comparator and you'll get the ordering in your compareTofunction, then create a java.util.Comparatorthat does the comparison in reverse (just call a.compareTo(b) and then negate the result) and create your second queue with that comparator.
使用 Comparator 创建您的第一个队列，您将获得compareTo函数中的排序，然后创建一个java.util.Comparator反向比较的队列（只需调用 a.compareTo(b) 然后否定结果）并使用它创建您的第二个队列比较器。
回答by Stefan Birkner
The single argument of Collections.reverseOrderis a Comparator and not a Collection. For your code simply use reverseOrder without an argument. You have to use a non-zero inital size, too. The following code will work.
Collections.reverseOrder的单个参数是 Comparator 而不是 Collection。对于您的代码，只需使用不带参数的 reverseOrder。您也必须使用非零初始大小。以下代码将起作用。
queue2=new PriorityQueue<E>(1, Collections.reverseOrder());
回答by Amit Deshpande
Below Program depicts how to do it.
下面的程序描述了如何做到这一点。
I have StringLengthComparatorwhich compares based on string length. Using Collections.reverseOrderI have created queue which is reverse ordered and another queue which is ordered correctly.
我有StringLengthComparator基于字符串长度的比较。使用Collections.reverseOrder我创建了反向排序的队列和另一个正确排序的队列。
import java.util.Collections;
import java.util.Comparator;
import java.util.PriorityQueue;
public class TestReverseorder {
public static void main(String[] args) {
Comparator<String> comparator = new TestReverseorder().new StringLengthComparator();
PriorityQueue<String> reverse = new PriorityQueue<String>(10,
Collections.reverseOrder(comparator));
PriorityQueue<String> queue = new PriorityQueue<String>(10,comparator);
queue.add("1");
queue.add("12");
queue.add("123");
reverse.add("1");
reverse.add("12");
reverse.add("123");
while (!queue.isEmpty()) {
System.out.println(queue.poll());
}
while (!reverse.isEmpty()) {
System.out.println(reverse.poll());
}
}
public class StringLengthComparator implements Comparator<String> {
@Override
public int compare(String x, String y) {
// Assume neither string is null. Real code should
// probably be more robust
if (x.length() < y.length()) {
return -1;
}
if (x.length() > y.length()) {
return 1;
}
return 0;
}
}
}
It will print output
它会打印输出
Normal Order:
1
12
123
Reverse Order:
123
12
1
