How to Convert a List<Foo> to a Map<String, Map<String, List<String>>> in Java 8

How can I transform a List<Foo> into a Map<String, Map<String, List<String>>> using Java 8 features?

// Sample Code Snippet - See Detailed Explanation Below For Context

In Java 8, you can leverage the Stream API to convert a List<Foo> into a complex Map structure. This involves grouping elements by multiple criteria and collecting them into nested Maps and Lists efficiently.

import java.util.*;
import java.util.stream.Collectors;

public class Foo {
    private String key1;
    private String key2;
    private String value;

    public Foo(String key1, String key2, String value) {
        this.key1 = key1;
        this.key2 = key2;
        this.value = value;
    }

    public String getKey1() { return key1; }
    public String getKey2() { return key2; }
    public String getValue() { return value; }
}

public class Example {
    public static void main(String[] args) {
        List<Foo> listFoo = Arrays.asList(
            new Foo("key1", "subKey1", "value1"),
            new Foo("key1", "subKey2", "value2"),
            new Foo("key2", "subKey1", "value3")
        );

        Map<String, Map<String, List<String>>> result = 
            listFoo.stream()
                .collect(Collectors.groupingBy( 
                    Foo::getKey1,
                    Collectors.toMap(
                        Foo::getKey2,
                        Collectors.mapping(Foo::getValue, Collectors.toList())
                    )
                ));

        System.out.println(result);
    }
}

Causes

• Data needs to be organized in a more accessible format for retrieval or processing.
• Using Java Collections efficiently can lead to improved code readability and maintainability.

Solutions

• Use the Stream API's collect method to transform the List<Foo> into the desired Map structure.
• Utilize Collectors.groupingBy() to facilitate the hierarchical grouping of data.

Mistake: Incorrectly using the groupBy collector without properly defining the nested collector.

Solution: Always ensure that the appropriate nested Collectors are used, as shown in the code example.

Mistake: Failing to handle null values that may arise in the list or its attributes.

Solution: Implement null checks or use Optional to ensure safe data handling.