Java 中的 HashSet retainAll()方法，示例

原文:https://www . geesforgeks . org/hashset-retain nal-method-in-Java-with-example/

java.util.HashSet 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.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {

        try {

            // Creating object of HashSet<Integer>
            HashSet<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("HashSet before "
                               + "retainAll() operation : "
                               + arrset1);

            // Creating another object of  HashSet<Integer>
            HashSet<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("HashSet after "
                               + "retainAll() operation : "
                               + arrset1);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

HashSet before retainAll() operation : [1, 2, 3, 4, 5]
Collection Elements to be retained : [1, 2, 3]
HashSet 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.*;

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {

        try {

            // Creating object of HashSet<Integer>
            HashSet<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("HashSet before "
                               + "retainAll() operation : "
                               + arrset1);

            // Creating another object of  HashSet<Integer>
            HashSet<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("HashSet after "
                               + "retainAll() operation : "
                               + arrset1);
        }

        catch (NullPointerException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

HashSet 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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