Java HashSet removeAll()方法及示例

java.util.HashSet 类的 removeAll() 方法用于从这个集合中移除所有包含在指定集合中的元素。

语法

public boolean removeAll(Collection c)

参数： 该方法接受 集合c 作为参数，其中包含要从这个集合中移除的元素。

返回值： 如果这个集合因调用而发生变化，则该方法返回 true 。

异常： 如果这个集合包含一个空元素，而指定的集合不允许空元素（可选），或者指定的集合为空，则该方法抛出 NullPointerException 。

以下是说明removeAll()方法的例子。

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]

例2： 对于NullPointerException

// 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