Java Collections.singleton()方法及示例

java.util.Collections.singleton() 方法是一个java.util.Collections类的方法。它在一个单一的指定元素上创建一个不可变的集合。这个方法的一个应用是将一个元素从List和Set等集合中移除。

语法

public static  Set singleton(T obj)
and
public static  List singletonList(T obj)

参数： obj : 唯一的对象将被存储在返回列表或集合。
返回：一个只包含指定对象的不可变的列表/集合。指定的对象。

示例:

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

为了一次性从我们的列表中删除所有 “错误 “元素，我们使用

singleton()方法
myList.removeAll(Collections.singleton(“Error”))。

在使用singleton()和removeAll之后，我们得到了以下结果。
{“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 removing
    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 occurrences
        geekslist2.removeAll(Collections.singleton("1"));
        System.out.println("geekslist2 after removal of 1 with "
                           + "singleton:" + geekslist2);
 
        // Selectively delete "4" from
        // all it's occurrences
        geekslist2.removeAll(Collections.singleton("4"));
        System.out.println("geekslist2 after removal of 4 with "
                           + "singleton:" + geekslist2);
 
        // Selectively delete "3" from
        // all it's occurrences
        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]