Java Stream implements CSV file data merging tutorial

This tutorial shows how to use Java Stream to combine data read from multiple CSV files. The focus is on maintaining read order, and an example based on city and country data is provided to show how to achieve data association and merging through `forEach` and `filter` operations.

In actual development, we often encounter the need to read data from multiple data sources (such as multiple CSV files) and associate and merge these data. This article will introduce a method to use Java Stream to achieve such data merging, especially when the original data order needs to be maintained.

scene description

Suppose we have two CSV files that store data for city (City) and country (Country). We need to associate city data and national data based on the country code (countryCode) in the city data, and merge the associated data. The goal is to add the corresponding country name to each city object and maintain the order of the city data in the original CSV file.

data model

First, define two Java classes to represent city and country data structures:

 import com.opencsv.bean.CsvBindByPosition;

public class City {

    @CsvBindByPosition(position = 0)
    private Integer id;

    @CsvBindByPosition(position = 1)
    private String name;

    @CsvBindByPosition(position = 2)
    private String countryCode;

    private String countryName;

    // Getters and setters
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCountryCode() {
        return countryCode;
    }

    public void setCountryCode(String countryCode) {
        this.countryCode = countryCode;
    }

    public String getCountryName() {
        return countryName;
    }

    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }

    @Override
    public String toString() {
        return "City{"  
                "id=" id  
                ", name='" name '\''  
                ", countryCode='" countryCode '\''  
                ", countryName='" countryName '\''  
                '}';
    }
}

 import com.opencsv.bean.CsvBindByPosition;

public class Country {

    @CsvBindByPosition(position = 0)
    private Integer id;

    @CsvBindByPosition(position = 1)
    private String name;

    @CsvBindByPosition(position = 2)
    private String code;

    // Getters and setters
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    @Override
    public String toString() {
        return "Country{"  
                "id=" id  
                ", name='" name '\''  
                ", code='" code '\''  
                '}';
    }
}

Data reading and merging

Next, show how to use Java Stream to read data in CSV files and merge city data and country data. Suppose we have read the city and country data from a CSV file and stored it in cities and countries lists respectively.

 import java.util.Arrays;
import java.util.List;

public class StreamMergeExample {

    public static void main(String[] args) {
        // Simulate data read from CSV List<country> countries = Arrays.asList(
                new Country(100, "Germany", "DE"),
                new Country(105, "France", "FR"),
                new Country(108, "Denmark", "DK")
        );

        List<city> cities = Arrays.asList(
                new City(1, "Berlin", "DE"),
                new City(2, "Munich", "DE"),
                new City(3, "Köln", "DE"),
                new City(4, "Paris", "FR"),
                new City(5, "Kopenhag", "DK")
        );

        // Use Stream to merge data cities.forEach(city -&gt; city.setCountryName(countries.stream()
                .filter(country -&gt; country.getCode().equals(city.getCountryCode()))
                .map(Country::getName)
                .findAny()
                .orElse(null)));

        //Print the merged city data cities.forEach(System.out::println);
    }
}</city></country>

Code explanation

1. cities.forEach(...) : Use the forEach method to iterate through the list of cities, ensuring that each city object is processed in the original order.
2. countries.stream() : Create a Stream of list of countries for each city.
3. .filter(country -> country.getCode().equals(city.getCountryCode())) : Use the filter method to filter out country objects that match the current city country code.
4. .map(Country::getName) : Use the map method to extract the country name.
5. .findAny().orElse(null) : Use the findAny method to get the first matching country name. If no match is found, null is returned.
6. city.setCountryName(...) : Set the obtained country name to the countryName attribute of the city object.

Things to note

• The above code uses the findAny() method. If you need to ensure that unique results are returned, and you know that the country code is unique in the list of countries, you can use the findFirst() method instead.
• If you need to handle more complex data association logic, such as one-to-many relationships, you may need to use other Stream operations such as flatMap.
• If the CSV file is very large, you can consider using paging reading to avoid loading a large amount of data into memory at once.

Summarize

This article describes a method to use Java Stream to merge data read from multiple CSV files. Through the forEach loop and Stream operation, data association and merging can be easily achieved while maintaining the order of the original data. This method is suitable for scenarios where multiple data sources need to be processed and data can be associated and merged.

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