Java 8 Streams | Create a map from a Collection, List or Set

Java 8 Streams – Create a map from a collection, list or set

Preamble

With the introduction of streams in Java 8 the writing of Java code has changed. With Java 8 you can now transfer objects from a list to a map in a comfortable way. How to implement this I will show you in this tutorial.

Requirement

Java knowledge

Convert Collection, List or Set to Map with the Java 8 Streaming API

As a developer you sometimes face the problem that objects of a collection, list or set have to be transferred into a map. There are different reasons for this. One reason might be that the access to the elements of a map is much faster than the access to the elements of a list. In this case it makes sense to transfer the collection, list or set into a map in advance.

In this tutorial we assume that we have a list of the class Customer.java and that it has the properties id, firstname, lastname.

package de.ertantoker.tutorial.model;

public class Customer {
    private String id;
    private String firstname;
    private String lastname;


    public Customer(String id, String firstname, String lastname) {
        this.id = id;
        this.firstname = firstname;
        this.lastname = lastname;
    }

    public String getId() {
        return id;
    }

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

    public String getFirstname() {
        return firstname;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

    public String getLastname() {
        return lastname;
    }

    public void setLastname(String lastname) {
        this.lastname = lastname;
    }
}

With the Streamin API of Java 8 we can use streams and at the end of the processing we can transfer them via collectors into a target structure suitable for us. In this example we want to convert from a list to a map.

The following code shows how to transfer a List<Customer> to Map<String, Customer>.

package de.ertantoker.tutorial;

import de.ertantoker.tutorial.model.Customer;

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Java8ListToMap {

    public static void main(String[] args) {
        Customer customer1 = new Customer("1", "Albert", "Einstein");
        Customer customer2 = new Customer("2", "Otto", "Hahn");
        Customer customer3 = new Customer("3", "Isaac", "Newton");

        List customerList = Arrays.asList(customer1, customer2, customer3);

        Map<String, Customer> customerMap = customerList
                .stream()
                .collect(Collectors.toMap(Customer::getId, customer -> customer));
    }


}

The Result

We have now learned how to convert lists or sets into a map with Java 8 and Streams Collection. With the collector classes it is also possible to convert the list into a set or to convert the objects in the list into another class. The Streams API offers a lot of possibilities. How to convert the objects of a list into another class you can see in my next tutorial Java 8 Stream Object mapping.

I hope you enjoyed my little tutorial. If you should have questions, criticism or suggestions, then I would be pleased, if you would leave a comment.

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