Add all elements from one collection to another using the addAll() method of the HashSet class

Use the addAll() method of the HashSet class to add all elements in a collection to another collection

HashSet is an implementation class in the Java collection framework. It inherits from AbstractSet and implements Set interface. HashSet is an unordered set based on a hash table, which does not allow duplicate elements. It provides many commonly used methods to operate elements in the collection, one of which is the addAll() method.

The addAll() method adds all elements in the specified collection to the current collection. This method accepts a Collection type parameter, which can be an instance object of List, Set or other collection class.

The following is an example that demonstrates how to use the addAll() method of HashSet to add all elements in one collection to another collection.

import java.util.HashSet;
import java.util.Set;

public class AddAllExample {
    public static void main(String[] args) {
        // 创建一个HashSet集合
        Set<String> set1 = new HashSet<String>();

        // 向集合set1中添加元素
        set1.add("apple");
        set1.add("banana");
        set1.add("orange");

        // 创建一个新的HashSet集合
        Set<String> set2 = new HashSet<String>();

        // 向集合set2中添加元素
        set2.add("grape");
        set2.add("kiwi");

        // 使用addAll()方法将set1中的所有元素添加到set2中
        set2.addAll(set1);

        // 输出set2中的所有元素
        for (String fruit : set2) {
            System.out.println(fruit);
        }
    }
}

In the above code, we first created two HashSet collections: set1 and set2. Then, all elements in set1 are added to set2 by calling the addAll() method of set2. Finally, we use an enhanced for loop to iterate through all elements in set2 and output them to the console.

Run the above code, the output result is as follows:

orange
kiwi
apple
banana
grape

You can see that the elements in set2 contain all the elements in set1. Note that the enhanced for loop does not guarantee the order of the elements when traversing the elements of the collection.

Using the addAll() method of HashSet can easily add all the elements in one collection to another collection, avoiding the trouble of manually traversing the collection and adding elements one by one. This is very useful in certain scenarios, such as merging elements from two collections, removing duplicates, etc.

It should be noted that the addAll() method will only add unique elements to the collection. If the collection already contains the element to be added, duplicate elements will not be added. This is exactly the characteristic of HashSet: it does not allow duplicate elements.

In short, the addAll() method of HashSet makes it easier and more efficient to add all elements in one collection to another collection. In the actual development process, we can use this method to process elements in the collection according to specific needs.

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