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Introduction

Merging collections is a common operation when working with data in Java. The Apache Commons Collections library provides a powerful utility class called CollectionUtils, which includes a collate method that can be used to merge two collections while sorting them. In this article, we'll explore how to merge two string lists using CollectionUtils.collate and print the merged result.

Prerequisites

Before we begin, ensure that you have included the Apache Commons Collections library in your project. You can add it either through Maven or Gradle.

Maven Dependency:

xml

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.4</version>
</dependency>

Gradle Dependency:

groovy

implementation 'org.apache.commons:commons-collections4:4.4'

Code Example: Merging Two Lists

Below is a simple example that demonstrates how to merge two string lists using the collate method from CollectionUtils. The merged list will be sorted automatically if the original lists are sorted.

java

import org.apache.commons.collections4.CollectionUtils;
import java.util.ArrayList;
import java.util.List;

public class MergeLists {
    public static void main(String[] args) {
        // Create two lists of strings
        List<String> list1 = new ArrayList<>();
        list1.add("apple");
        list1.add("banana");
        list1.add("cherry");

        List<String> list2 = new ArrayList<>();
        list2.add("date");
        list2.add("fig");
        list2.add("grape");

        // Merge the two lists using CollectionUtils.collate
        List<String> mergedList = CollectionUtils.collate(list1, list2);

        // Print the merged list elements
        for (String element : mergedList) {
            System.out.println(element);
        }
    }
}

Explanation

1. List Creation: We create two lists of strings, list1 and list2, and add some fruit names to them.
2. Merging with collate: The CollectionUtils.collate method is used to merge the two lists. It guarantees that the merged list will be sorted if both original lists are sorted.
3. Printing the Merged List: We then iterate over the merged list and print each element.

Output Example

The output of this code will be the merged list of strings, sorted in lexicographical order:

bash

apple
banana
cherry
date
fig
grape

Alternative: Merging Lists Without Sorting

If you don’t want the merged list to be sorted, you can simply use the addAll method instead of collate. Here's how you can concatenate the two lists while maintaining their original order:

java

List<String> mergedList = new ArrayList<>(list1);
mergedList.addAll(list2);

This will merge the two lists without sorting them.

Conclusion

The collate method from Apache Commons Collections is a great utility for merging two collections while ensuring they are sorted. It is particularly useful when you need to merge sorted data. However, if sorting isn't required, you can use the addAll method to concatenate the lists without altering the order of elements.

By including the Apache Commons Collections library and using CollectionUtils, you can simplify your collection management and improve the readability of your code.