Java中的 HashSet removeAll() 方法与示例

Java.util.HashSet类的removeAll()方法用于从该集合中删除指定集合中包含的所有元素。
句法：

public boolean removeAll(Collection c)

参数：此方法将集合 c作为包含要从此集合中删除的元素的参数。
返回值：如果此集合因调用而更改，则此方法返回true 。
异常：如果此集合包含空元素并且指定的集合不允许空元素（可选），或者指定的集合为空，则此方法抛出NullPointerException 。
下面是说明removeAll()方法的示例。
示例 1：

Java

// 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
            HashSet
                arrset1 = new HashSet();
 
            // 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
            HashSet
                arrset2 = new HashSet();
            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：对于NullPointerException

Java

// 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
            HashSet
                arrset1 = new HashSet();
 
            // 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
            HashSet
                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