Filtering and Collecting Map String, List Object Based on Conditions in Java

Learn how to filter and collect a `Map String, List Object ` in Java by applying specific conditions, like collecting persons above a particular age.
---
This video is based on the question https://stackoverflow.com/q/68299614/ asked by the user 'Pryda' ( https://stackoverflow.com/u/5537770/ ) and on the answer https://stackoverflow.com/a/68299712/ provided by the user 'ernest_k' ( https://stackoverflow.com/u/5761558/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Filter and collect a Map String, List Object to Map String, List Object based on a condition

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Filtering and Collecting Map<String, List<Object>> in Java

When working with data in Java, you may encounter scenarios where you need to filter collections based on specific criteria. One common example is filtering a map that holds lists of objects based on certain attributes of those objects. In this post, we’ll focus on how to filter a Map<String, List<Person>> to keep only the entries that satisfy a given condition regarding the Person objects' properties.

The Problem

Suppose you have a Map<String, List<Person>> where each key maps to a list of Person objects, and you want to collect only those Person entries that meet a specific condition; in our case, we want to filter in only those persons whose age is greater than 20.

Example Data

Consider the following map:

[[See Video to Reveal this Text or Code Snippet]]

After applying the filter condition, the expected result would be:

[[See Video to Reveal this Text or Code Snippet]]

The Solution

To achieve this, we will utilize Java Streams, which provide a powerful way to process collections. By leveraging streams, we can efficiently filter and collect the desired data with concise and readable code.

Step-by-Step Guide

Use Stream over EntrySet
Start by streaming the entries of the map (entrySet()), which gives you access to both keys and values in pairs.

Mapping Each Entry
For each entry, filter the list of Person objects using the defined condition (predicate), and collect the resulting filtered list.

Remove Empty Lists
After the filtering step, use another filter to remove any entries from the result that have empty lists as values.

Collect Final Results
Finally, collect the filtered entries back into a new map.

Implementation Code

Here’s the code that encapsulates the above logic:

[[See Video to Reveal this Text or Code Snippet]]

Important Notes

If you are using Java 8, replace Map.entry() with new AbstractMap.SimpleEntry<>() since the Map.entry() method was introduced in Java 9.

Always ensure that each filtering condition is appropriate for your data type to avoid runtime exceptions.

Conclusion

Filtering and collecting data from a Map<String, List<Object>> in Java can be streamlined using the power of streams. By understanding how to navigate collections and apply filtering conditions effectively, you can manage and manipulate your datasets with ease.

Try to utilize this approach in your projects, and you'll find that working with collections becomes much more manageable and efficient!

Feel free to ask any questions you may have or share your experiences in filtering data using Java Streams.

Видео Filtering and Collecting Map String, List Object Based on Conditions in Java канала vlogize