Java Collections.singleton()用法及代码示例

java.util.Collections.singleton()方法是java.util.Collections类的方法。它在单个指定的元素上创建一个不可变的集合。此方法的一个应用是从Collection中删除一个元素，例如List和Set。

用法:

public static  Set singleton(T obj)
and
public static  List singletonList(T obj)
参数： obj:the sole object to be stored in
                  the returned list or set.
返回： an immutable list/set containing only the 
       specified object.

Example:
myList:{"Geeks", "code", "Practice", " Error",  "Java", 
          "Class", "Error", "Practice", "Java" }

To remove all "Error" elements from our list at once, we use 
singleton() method 
myList.removeAll(Collections.singleton("Error"));

After using singleton() and removeAll, we get following.
{"Geeks", "code", "Practice", "Java", "Class", "Practice", "Java" }

// Java program to demonstrate 
// working of singleton() 
import java.util.*; 
  
class GFG { 
    public static void main(String args[]) 
    { 
        String[] geekslist = { "1", "2", "4", "2", "1", "2", 
                               "3", "1", "3", "4", "3", "3" }; 
  
        // Creating a list and removing 
        // elements without use of singleton() 
        List geekslist1 = new ArrayList(Arrays.asList(geekslist)); 
        System.out.println("Original geeklist1:" + geekslist1); 
  
        geekslist1.remove("1"); 
        System.out.println("geekslist1 after removal of 1 without"
                           + " singleton " + geekslist1); 
        geekslist1.remove("1"); 
        System.out.println("geekslist1 after removal of 1 without"
                           + " singleton " + geekslist1); 
        geekslist1.remove("2"); 
        System.out.println("geekslist1 after removal of 2 without"
                           + " singleton " + geekslist1); 
  
        /* Creating another list and removng  
    its elements using singleton() method */
        List geekslist2 = new ArrayList(Arrays.asList(geekslist)); 
        System.out.println("\nOriginal geeklist2:" + geekslist2); 
  
        // Selectively delete "1" from 
        // all it's occurences 
        geekslist2.removeAll(Collections.singleton("1")); 
        System.out.println("geekslist2 after removal of 1 with "
                           + "singleton:" + geekslist2); 
  
        // Selectively delete "4" from 
        // all it's occurences 
        geekslist2.removeAll(Collections.singleton("4")); 
        System.out.println("geekslist2 after removal of 4 with "
                           + "singleton:" + geekslist2); 
  
        // Selectively delete "3" from 
        // all it's occurences 
        geekslist2.removeAll(Collections.singleton("3")); 
        System.out.println("geekslist2 after removal of 3 with"
                           + " singleton:" + geekslist2); 
    } 
}

输出：

Original geeklist1                 [1, 2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 1 without singleton [2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 1 without singleton [2, 4, 2, 2, 3, 1, 3, 4, 3, 3]
geekslist1 after removal of 2 without singleton [4, 2, 2, 3, 1, 3, 4, 3, 3]

Original geeklist2                 [1, 2, 4, 2, 1, 2, 3, 1, 3, 4, 3, 3]
geekslist2 after removal of 1 with singleton     [2, 4, 2, 2, 3, 3, 4, 3, 3]
geekslist2 after removal of 4 with singleton     [2, 2, 2, 3, 3, 3, 3]
geekslist2 after removal of 3 with singleton     [2, 2, 2]

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注：本文由纯净天空筛选整理自 Collections.singleton() method in Java with example。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。