java.util.concurrent.CopyOnWriteArraySet类的retainAll()方法用于从该集合中保留指定集合中包含的所有元素。

用法:

public boolean retainAll(Collection c)

参数：此方法将集合c作为包含要从该集合保留的元素的参数。

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null元素并且指定的集合不允许null元素(可选)，或者指定的collection为null，则此方法引发NullPointerException。

以下示例说明了retainAll()方法。

示例1:

// Java program to demonstrate

// retainAll() method for Integer value

import java.util.concurrent.*;

public class GFG1 {

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

{

try {

// Creating object of CopyOnWriteArraySet

CopyOnWriteArraySet

set1 = new CopyOnWriteArraySet();

// Populating set1

set1.add(1);

set1.add(2);

set1.add(3);

set1.add(4);

set1.add(5);

// print set1

System.out.println("CopyOnWriteArraySet before "

+ "retainAll() operation : "

+ set1);

// Creating another object of  CopyOnWriteArraySet

CopyOnWriteArraySet

set2 = new CopyOnWriteArraySet();

set2.add(1);

set2.add(2);

set2.add(3);

// print set2

System.out.println("Collection Elements"

+ " to be retained : "

+ set2);

// Removing elements from set

// specified in set2

// using retainAll() method

set1.retainAll(set2);

// print set1

System.out.println("CopyOnWriteArraySet after "

+ "retainAll() operation : "

+ set1);

}

catch (NullPointerException e) {

System.out.println("Exception thrown : " + e);

}

}

}

输出：

CopyOnWriteArraySet before retainAll() operation : [1, 2, 3, 4, 5]

Collection Elements to be retained : [1, 2, 3]

CopyOnWriteArraySet after retainAll() operation : [1, 2, 3]

示例2:对于NullPointerException

// Java program to demonstrate

// retainAll() method for Integer value

import java.util.concurrent.*;

public class GFG1 {

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

{

try {

// Creating object of CopyOnWriteArraySet

CopyOnWriteArraySet

set1 = new CopyOnWriteArraySet();

// Populating set1

set1.add(1);

set1.add(2);

set1.add(3);

set1.add(4);

set1.add(5);

// print set1

System.out.println("CopyOnWriteArraySet before "

+ "retainAll() operation : "

+ set1);

// Creating another object of  CopyOnWriteArraySet

CopyOnWriteArraySet

set2 = null;

// print set2

System.out.println("Collection Elements"

+ " to be retained : "

+ set2);

System.out.println("\nTrying to pass "

+ "null as a specified element\n");

// Removing elements from set

// specified in set2

// using retainAll() method

set1.retainAll(set2);

// print set1

System.out.println("CopyOnWriteArraySet after "

+ "retainAll() operation : "

+ set1);

}

catch (NullPointerException e) {

System.out.println("Exception thrown : " + e);

}

}

}

输出：

CopyOnWriteArraySet 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