Java SortedSet retainAll()方法及示例

SortedSet 接口的 retainAll() 方法用于保留这个SortedSet中包含在指定集合中的所有元素。

语法

public boolean retainAll(Collection c)

参数： 该方法以 集合c 为参数，包含要从这个SortedSet中保留的元素。

返回值： 如果这个SortedSet由于调用而发生了变化，那么这个方法将返回 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 。

注意 ：SortedSet中的retainAll()方法是继承自Java中的Set接口。

下面是一些例子来说明retainAll()方法。

例1 :

// Java program to demonstrate// retainAll() method for Sorted set import java.util.*; public class GFG1 {    public static void main(String[] args)        throws Exception    {         try {             // Creating object of Set            SortedSet<Integer> arrset1                = new TreeSet<Integer>();             // 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            SortedSet<Integer> arrset2                = new TreeSet<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(                "Set after "                + "retainAll() operation : "                + arrset1);        }         catch (NullPointerException e) {            System.out.println(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]

例2： 对于NullPointerException

// Java program to demonstrate// retainAll() method for Sorted set import java.util.*; public class GFG1 {    public static void main(String[] args)        throws Exception    {        try {             // Creating object of SortedSet<Integer>            SortedSet<Integer> arrset1                = new TreeSet<Integer>();             // 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<Integer>            SortedSet<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(                "Set after "                + "retainAll() operation : "                + arrset1);        }         catch (NullPointerException e) {            System.out.println(e);        }    }}

输出

Set before retainAll() operation : [1, 2, 3, 4, 5]Collection Elements to be retained : nullTrying to pass null as a specified elementjava.lang.NullPointerException

参考资料 : https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#retainAll(java.util.Collection)