Java HashSet retainAll()方法及示例

java.util.HashSet 类的 retainAll() 方法是用来保留这个集合中包含在指定集合中的所有元素。

语法

public boolean retainAll(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保留的元素。

返回值： 如果这个集合在调用后发生了变化，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），或者指定的集合是空的，这个方法会抛出 NullPointerException 。

下面是一些例子来说明 retainAll() 方法。

例1 :

// Java program to demonstrate
// retainAll() 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 "
                               + "retainAll() 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 retained : "
                               + arrset2);
  
            // Removing elements from arrset
            // specified in arrset2
            // using retainAll() method
            arrset1.retainAll(arrset2);
  
            // print arrset1
            System.out.println("HashSet after "
                               + "retainAll() operation : "
                               + arrset1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

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

例2： 对于NullPointerException

// Java program to demonstrate
// retainAll() 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 "
                               + "retainAll() operation : "
                               + arrset1);
  
            // Creating another object of  HashSet<Integer>
            HashSet<Integer>
                arrset2 = null;
  
            // print arrset2
            System.out.println("Collection Elements"
                               + " to be retained : "
                               + arrset2);
  
            System.out.println("\nTrying to pass "
                               + "null as a specified element\n");
  
            // Removing elements from arrset
            // specified in arrset2
            // using retainAll() method
            arrset1.retainAll(arrset2);
  
            // print arrset1
            System.out.println("HashSet after "
                               + "retainAll() operation : "
                               + arrset1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

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

Trying to pass null as a specified element

Exception thrown : java.lang.NullPointerException