Java中的HashSet removeAll()方法及示例

java.util.HashSet 类的 removeAll() 方法用于从此集合中删除包含在指定集合中的所有元素。
语法：

public boolean removeAll(Collection c)

参数： 该方法将 集合c 作为参数，其中包含要从此集合中删除的元素。
返回值： 如果该调用导致此集合更改，则该方法返回 true 。
异常： 如果此集合包含null元素且指定的集合不允许null元素（可选），或者指定的集合为null，则此方法将抛出 NullPointerException 。 下面是举例说明

示例1：

// Java program to demonstrate
// removeAll() method for Integer value
 
import java.util.*;
 
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
 
        try {
 
            // Creating object of HashSet<Integer>
            HashSet<Integer>
                arrset1 = new HashSet<Integer>();
 
            // Populating arrset1
            arrset1.add(1);
            arrset1.add(2);
            arrset1.add(3);
            arrset1.add(4);
            arrset1.add(5);
 
            // print arrset1
            System.out.println("HashSet before "
                               + "removeAll() operation : "
                               + arrset1);
 
            // Creating another object of  HashSet<Integer>
            HashSet<Integer>
                arrset2 = new HashSet<Integer>();
            arrset2.add(1);
            arrset2.add(2);
            arrset2.add(3);
 
            // print arrset2
            System.out.println("Collection Elements"
                               + " to be removed : "
                               + arrset2);
 
            // Removing elements from arrset
            // specified in arrset2
            // using removeAll() method
            arrset1.removeAll(arrset2);
 
            // print arrset1
            System.out.println("HashSet after "
                               + "removeAll() operation : "
                               + arrset1);
        }
 
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

HashSet before removeAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be removed : [1, 2, 3]
HashSet after removeAll() operation : [4, 5]

示例2： 与 NullPointerException 一起使用。

// Java program to demonstrate
// removeAll() method for Integer value

import java.util.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {

        try {

            // Creating object of HashSet<Integer>
            HashSet<Integer>
                arrset1 = new HashSet<Integer>();

            // Populating arrset1
            arrset1.add(1);
            arrset1.add(2);
            arrset1.add(3);
            arrset1.add(4);
            arrset1.add(5);

            // print arrset1
            System.out.println("HashSet before "
                                 + "removeAll() operation : "
                                 + arrset1);

            // Creating another object of  HashSet<Integer>
            HashSet<Integer>
                arrset2 = null;

            // print arrset2
            System.out.println("Collection Elements"
                                 + " to be removed : "
                                 + arrset2);

            System.out.println("\nTrying to pass "
                                 + "null as a specified element\n");

            // Removing elements from arrset
            // specified in arrset2
            // using removeAll() method
            arrset1.removeAll(arrset2);

            // print arrset1
            System.out.println("HashSet after "
                                 + "removeAll() operation : "
                                 + arrset1);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

HashSet before removeAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be removed : null

Trying to pass null as a specified element

Exception thrown : java.lang.NullPointerException