Introduction

If you have been keeping up with the latest trends in Android development, you’ve probably heard of Realm. Realm is a lightweight database that can replace both SQLite and ORM libraries in your Android projects.

Compared to SQLite, Realm is faster and has lots of modern features, such as JSON support, a fluent API, data change notifications, and encryption support, all of which make life easier for Android developers.

In this quick tip, you are going to learn the basics of Realm for Android. In this tutorial, I will be useing Realm v0.84.1.

1. Adding Realm to a Project

To use Realm in an Android project, you need to add it as a compile dependency in the app module’s build.gradle file.

compile 'io.realm:realm-android:0.84.1'

2. Creating a Realm

A Realm is similar to a SQLite database. It has a file associated with it, which, once created, will persist on Android’s file system.

To create a new Realm, you can call the static Realm.getInstance method from inside any Activity.

Realm myRealm = Realm.getInstance(context);

Note that calling Realm.getInstance, without passing a RealmConfiguration to it, results in the creation of a Realm file called default.realm.

If you want to add another Realm to your app, you must use a RealmConfiguration.Builder object and give the Realm file a unique name.

Realm myOtherRealm =
        Realm.getInstance(
                new RealmConfiguration.Builder(context)
                        .name("myOtherRealm.realm")
                        .build()
);

3. Creating a RealmObject

Any JavaBean can be stored in a Realm once it extends the RealmObject class. If you are wondering what a JavaBean is, it is simply a Java class that is serializable, has a default constructor, and has getter/setter methods for its member variables. For example, instances of the following class can be easily stored in a Realm:

public class Country extends RealmObject {

    private String name;
    private int population;

    public Country() { }

    public String getName() {
        return name;
    }

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

    public int getPopulation() {
        return population;
    }

    public void setPopulation(int population) {
        this.population = population;
    }

}

If you want to use a member variable of a RealmObject as a primary key, you can use the @PrimaryKey annotation. For example, here’s how you would add a primary key called code to the Country class:

@PrimaryKey
private String code;

public String getCode() {
    return code;
}

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

4. Creating Transactions

While reading data from a Realm is very simple, as you will see in the next step, writing data to it is slightly more complex. Realm is ACID compliant and to ensure atomicity and consistency, Realm forces you to execute all write operations inside a transaction.

To start a new transaction, use the beginTransaction method. Similarly, to end the transaction, use the commitTransaction method.

Here’s how you would create and save an instance of the Country class:

myRealm.beginTransaction();

	// Create an object
	Country country1 = myRealm.createObject(Country.class); 

	// Set its fields
	country1.setName("Norway");
	country1.setPopulation(5165800);
	country1.setCode("NO");

myRealm.commitTransaction();

You might have noticed that country1 was not created using the constructor of the Country class. For a Realm to manage an instance of a RealmObject, the instance must be created using the createObject method.

If you must use the constructor, don’t forget to use the copyToRealm method of the relevant Realm object before you commit the transaction. Here’s an example:

// Create the object
Country country2 = new Country();
country2.setName("Russia");
country2.setPopulation(146430430);
country2.setCode("RU");

myRealm.beginTransaction();
	Country copyOfCountry2 = myRealm.copyToRealm(country2);
myRealm.commitTransaction();

5. Writing Queries

Realm offers a very intuitive and fluent API for creating queries. To create a query, use the where method of the relevant Realm object and pass the class of the objects you are interested in. After creating the query, you can fetch all results using the findAll method, which returns a RealmResults object. In the following example, we fetch and print all objects of type Country:

RealmResults<Country> results1 =
        myRealm.where(Country.class).findAll();

for(Country c:results1) {
    Log.d("results1", c.getName());
}

// Prints Norway, Russia

Realm offers several aptly named methods, such as beginsWith, endsWith, lesserThan and greaterThan, you can use to filter the results. The following code shows you how you can use the greaterThan method to fetch only those Country objects whose population is greater than 100 million:

RealmResults<Country> results2 =
        myRealm.where(Country.class)
                .greaterThan("population", 100000000)
                .findAll();

// Gets only Russia

If you want the results of the query to be sorted, you can use the findAllSorted method. As its arguments, it takes a String specifying the name of the field to sort by and a boolean specifying the sort order.

// Sort by name, in descending order
RealmResults<Country> results3 =
        myRealm.where(Country.class)
                .findAllSorted("name", false);

// Gets Russia, Norway

Conclusion

In this quick tip, you learned how to use Realm in an Android project. You saw how easy it is to create a Realm database, store data in it, and query it. To learn more about Realm for Android, you can go through its Java documentation.