How to Map Multiple Values to a Single DTO Using Java Streams

Learn how to effectively transform data from your Spring Boot application entities to a Data Transfer Object (DTO) using Java Streams for efficient data manipulation and mapping.
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Mapping Multiple Values to a Single DTO Using Java Streams

When developing applications with Spring Boot, particularly when dealing with complex entities, one common challenge is the need to map multiple related entities into a single Data Transfer Object (DTO). In this post, we'll dive into how to achieve this effectively using the power of Java Streams.

The Problem Statement

Imagine you have an entity structure consisting of Countries, States, and Towns. Your goal is to aggregate information from these entities into a cohesive DTO. The entities are structured as follows:

Country

String name

List<State> states

State

String name

Long population

List<Town> towns

Town

String name

Your objective is to create a CountryDTO that looks like this:

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

Here’s what you need to extract for each CountryDTO:

The name of the country

The total population summing all states in that country

The list of towns associated with the states in the country

The Solution

Using Java Streams, we can conveniently map the entities to a DTO. Here’s how you can accomplish this in a step-by-step manner.

Step 1: Stream Countries and Map to DTO

First, you need to create a stream of countries from your repository and process each Country entity:

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

Step 2: Calculating the Total Population

To calculate the sum of the population of all states within a country, you can use mapToInt in conjunction with sum:

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

Step 3: Flattening the List of Towns

To collect all towns associated with the states into a single list, you can use flatMap:

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

Full Implementation

Combining all these steps, here’s how the complete code snippet looks in action:

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

Conclusion

Using Java Streams to map multiple values to a single DTO is not only a clean approach but also an efficient way of handling complex entities. By leveraging methods like mapToInt and flatMap, you can gather and transform data seamlessly.

This technique proves to be invaluable when working with nested data structures in your Spring Boot applications, allowing you to build robust and maintainable code.

Feel free to experiment with this technique in your own projects and tailor the approach to fit your specific needs!