Collections.frequency返回0 -1值的ArrayList

Question

List line = [0, -1, 28, 0, 50, 0, -1, 75, 0] // pseudocode 
System.out.println(Collections.frequency(line, -1)); 

爲什麼Collections.frequency回報0當我檢查-1價值？

Answer 1

這是怎麼了，collections.frequency作品：

import java.util.Arrays; 
import java.util.Collections; 
import java.util.List; 

public class stackexample { 
    public static void main(String[] args) { 
     List<Integer> values = Arrays.asList(5, 0, 0, 2); 
     int occurrences = Collections.frequency(values, 0); 
     System.out.println("occurrences of zero is " + occurrences); //shows 0 but answer should be 2 
    } 
} 

在你的情況下，它返回零，因爲你的收藏中沒有-1可用

Answer 2

您在line中沒有任何值爲-1的項目。

Answer 3

根據Java文檔

Returns the number of elements in the specified collection equal to the specified object.  
More formally, returns the number of elements e in the collection such that (o == null ? e 
== null : o.equals(e)). 

所以沒有比賽那麼返回0，見下文

http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#frequency(java.util.Collection，java.lang.Object中）