Java中的TreeSet retainAll()方法示例

java.util.TreeSet 类的 retainAll() 方法用于从此集合中保留包含在指定集合中的所有元素。

语法：

public boolean retainAll(Collection c)

参数： 该方法将包含要从此集合中保留的元素的集合c作为参数。

返回值： 如果通过调用此方法更改了此集合，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），或者指定的集合为null，则此方法将抛出 NullPointerException

以下是说明retainAll()方法的示例。

示例1：

//Java程序演示
//整数值的retainAll()方法
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
  
        try {
  
            //创建TreeSet<Integer>对象
            TreeSet<Integer>
                set1 = new TreeSet<Integer>();
  
            //填充set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);
  
            //打印set1
            System.out.println("retainAll()操作之前的TreeSet: "
                               + set1);
  
            //创建另一个TreeSet<Integer>对象
            TreeSet<Integer>
                set2 = new TreeSet<Integer>();
            set2.add(1);
            set2.add(2);
            set2.add(3);
  
            //打印set2
            System.out.println("要保留的集合元素: "
                               + set2);
  
            //使用retainAll()方法从集合中删除在set2中指定的元素。
            set1.retainAll(set2);
  
            //打印set1
            System.out.println("retainAll()操作之后的TreeSet: "
                               + set1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

retainAll()操作之前的TreeSet: [1, 2, 3, 4, 5]
要保留的集合元素: [1, 2, 3]
retainAll()操作之后的TreeSet: [1, 2, 3]

示例2： 对于NullPointerException

// Java程序演示了
// retainAll()方法用于整数值
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
  
        try {
  
            // 创建TreeSet<Integer>对象
            TreeSet<Integer>
                set1 = new TreeSet<Integer>();
  
            // 填充set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);
  
            // 打印set1
            System.out.println("TreeSet before "
                               + "retainAll() operation : "
                               + set1);
  
            // 创建另一个TreeSet<Integer>对象
            TreeSet<Integer>
                set2 = null;
  
            // 打印set2
            System.out.println("Collection Elements"
                               + " to be retained : "
                               + set2);
  
            System.out.println("\nTrying to pass "
                               + "null as a specified element\n");
  
            // 使用retainAll()方法从set2指定的集合中删除元素
            set1.retainAll(set2);
  
            // 打印set1
            System.out.println("TreeSet after "
                               + "retainAll() operation : "
                               + set1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

TreeSet 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