java.util.Set接口的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.*;

public class GFG1 {

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

{

try {

// Creating object of Set

Set 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("Set before retainAll() operation : "

+ arrset1);

// Creating another object of  Set

Set arrset2 = new HashSet();

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("Set after retainAll() operation : "

+ arrset1);

}

catch (NullPointerException e) {

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

}

}

}

输出：

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

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

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

示例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 Set

Set 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("Set before retainAll() operation : "

+ arrset1);

// Creating another object of Set

Set 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("Set after retainAll() operation : "

+ arrset1);

}

catch (NullPointerException e) {

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

}

}

}

输出：

Set 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