Java 中的 CopyOnWriteArraySet retainAll()方法，示例
原文:https://www . geeksforgeeks . org/copy onwriterarrayset-retain all-method-in-Java-with-example/
Java . util . concurrent . copy onwriterarrayset类的retainnal()方法用于从该集合中保留指定集合中包含的所有元素。
语法:
public boolean retainAll(Collection c)
参数:该方法以集合 c 为参数，包含从该集合中保留的元素。
返回值:如果该设置因调用而改变，则该方法返回真。
异常:如果该集合包含空元素，并且指定的集合不允许空元素(可选)，或者指定的集合为空，则该方法抛出空指针异常。
以下是说明retainal()方法的例子。
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<Integer>
CopyOnWriteArraySet<Integer>
set1 = new CopyOnWriteArraySet<Integer>();
// 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<Integer>
CopyOnWriteArraySet<Integer>
set2 = new CopyOnWriteArraySet<Integer>();
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);
}
}
}
Output:
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: 适用于空指针异常
// 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<Integer>
CopyOnWriteArraySet<Integer>
set1 = new CopyOnWriteArraySet<Integer>();
// 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<Integer>
CopyOnWriteArraySet<Integer>
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);
}
}
}
Output:
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
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