Скачать или смотреть How to Convert Map String, List Object to Map String, List ClassVariable Using Java Streams

Learn how to efficiently convert a `Map String, List Test ` to `Map String, List someObjectArray ` and `Map String, List someOtherObjectArray ` in Java using streams.
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This video is based on the question https://stackoverflow.com/q/77558908/ asked by the user 'iamrogue' ( https://stackoverflow.com/u/2091028/ ) and on the answer https://stackoverflow.com/a/77559116/ provided by the user 'Irremediable' ( https://stackoverflow.com/u/19648355/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Convert Map<String, List<Object>> to Map<String, List<ClassVariable>> Using Java Streams

In the world of Java programming, data manipulation is a common task that can often feel overwhelming. One frequent operation is converting data structures such as maps to different formats. A common scenario arises when dealing with a Map<String, List<Object>> and needing to convert it into more specific data types such as Map<String, List<ClassVariable>>.

In this guide, we will guide you through a scenario where we need to convert a Map<String, List<Test>> containing objects of a class Test into two separate maps that extract specific arrays within that class using Java Streams.

Introduction to the Problem

Consider a class Test defined as follows:

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

In this example:

Each Test object has a unique id and two arrays of TestObject1 and TestObject2.

We have a map of type Map<String, List<Test>>, where the key is the id of each Test instance.

Our goal is to create two new maps:

Map<String, List<TestObject1>> that collects all someObjectArray values.

Map<String, List<TestObject2>> that collects all someOtherObjectArray values.

The Requirements:

Transform the initial map of Test objects into two maps capturing the specific object arrays.

Utilize Java Streams for a declarative approach, which can enhance readability and maintainability.

Solution

Initial Setup

Starting with an initial map that represents our input:

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

First Map Creation: Extracting someObjectArray

To create the first map, we can leverage Java Streams to process entries in the initialMap:

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

Second Map Creation: Extracting someOtherObjectArray

Similarly, for the second map, we follow the same logic to extract the someOtherObjectArray:

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

Important Consideration

If you encounter an issue where .toList() does not resolve on the inner stream, you can simply substitute it with:

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

Performance Notes

While using Java Streams is syntactically elegant, please be aware of the performance implications when processing large datasets. In such scenarios, traditional loops might provide better performance:

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

Conclusion

Converting between different map formats in Java, especially when dealing with complex objects, can be streamlined using Java Streams. In our example, we successfully transformed a Map<String, List<Test>> into two distinct maps containing specific arrays. With the techniques outlined above, you should be well on your way to efficiently managing and processing your data structures in Java.

Implementing robust data manipulation strategies can not only improve your code's efficiency but also its clarity and maintainability.