Java中的 TreeSet retainAll() 方法与示例

Java.util.TreeSet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。

句法：

public boolean retainAll(Collection c)

参数：此方法将集合 c作为参数，其中包含要从该集合中保留的元素。

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，则此方法抛出NullPointerException 。

下面是说明retainAll()方法的示例。

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 TreeSet
            TreeSet
                set1 = new TreeSet();
  
            // Populating set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);
  
            // print set1
            System.out.println("TreeSet before "
                               + "retainAll() operation : "
                               + set1);
  
            // Creating another object of  TreeSet
            TreeSet
                set2 = new TreeSet();
            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("TreeSet after "
                               + "retainAll() operation : "
                               + set1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

TreeSet before retainAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be retained : [1, 2, 3]
TreeSet 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 TreeSet
            TreeSet
                set1 = new TreeSet();
  
            // Populating set1
            set1.add(1);
            set1.add(2);
            set1.add(3);
            set1.add(4);
            set1.add(5);
  
            // print set1
            System.out.println("TreeSet before "
                               + "retainAll() operation : "
                               + set1);
  
            // Creating another object of  TreeSet
            TreeSet
                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("TreeSet after "
                               + "retainAll() operation : "
                               + set1);
        }
  
        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

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