Java Set retainAll()用法及代码示例

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<Integer> arrset1 = new HashSet<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 
            Set<Integer> arrset2 = new HashSet<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("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<Integer> 
            Set<Integer> arrset1 = new HashSet<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> 
            Set<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("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

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

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注：本文由纯净天空筛选整理自gopaldave大神的英文原创作品 Set retainAll() method in Java with Example。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。