Realm is substitute of SQLite and ORM libraries in your Android application. It is a lightweight database that can replace both SQLite and ORM libraries.

Installation:-
 

Step 1: Add the following class path dependency to the project level build.gradle file.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath "io.realm:realm-gradle-plugin:0.90.0"
    }
}

Step 2: Apply the realm-android plugin to the top of application/module level build.gradle file. 

apply plugin: 'realm-android'

From the above successfully integrated Realm in our application. Now you can use the awesome feature of Realm.

Step3: Set default configuration of Realm in application class so that you can use its instance in any activity

public class MyApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        RealmConfiguration realmConfiguration = new RealmConfiguration.Builder(this).build();
        Realm.setDefaultConfiguration(realmConfiguration);
    }
}

Step 4: Create RealM data models

Create a model class and extend with RealmObject. Any Model class or JavaBean can be stored in a Realm once it extends the RealmObject class.

public class EmployeeBean extends RealmObject {
    private String employeeName;
   
    private String employeeId;

    private long employeeSalary;

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public String getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(String employeeId) {
        this.employeeId = employeeId;
    }

    public long getEmployeeSalary() {
        return employeeSalary;
    }

    public void setEmployeeSalary(long employeeSalary) {
        this.employeeSalary = employeeSalary;
    }
}

Step 5: Obtain Realm instance in activity

mRealm = Realm.getDefaultInstance();

Step 6: Store data on Realm object

Syntax:-

// Obtain a Realm instance
Realm realm = Realm.getDefaultInstance();

realm.beginTransaction();

//... add or update objects here ...

realm.commitTransaction();

Add employee details on Realm like this

 mRealm.beginTransaction();
 EmployeeBean employee = mRealm.createObject(EmployeeBean.class);
 employee.setEmployeeId("EMP001");
 employee.setEmployeeName("Tarun Joshi");
 employee.setEmployeeSalary(100000);
 mRealm.commitTransaction();

You can also use Transaction blocks  which will automatically handle begin/commit, and cancel if an error happens. That means you don't need to add beginTransaction() and commitTransaction() everytime.

// Transaction block
mRealm.executeTransaction(new Realm.Transaction() {
     @Override
     public void execute(Realm realm) {
         EmployeeBean employee = mRealm.createObject(EmployeeBean.class);
         employee.setEmployeeId("EMP002");
         employee.setEmployeeName("Amit Rai");
         employee.setEmployeeSalary(60000);
     }
 });

If you want to do the above transaction in a background thread then use Asynchronous Transactions

mRealm.executeTransactionAsync(new Realm.Transaction() {
      @Override
      public void execute(Realm bgRealm) {
       EmployeeBean employee = bgRealm.createObject(EmployeeBean.class);
       employee.setEmployeeId("EMP003");
       employee.setEmployeeName("Devesh Bhandari");
       employee.setEmployeeSalary(75000);
      }
    }, new Realm.Transaction.OnSuccess() {
      @Override
      public void onSuccess() {
        // Transaction was a success.  
      }
    }, new Realm.Transaction.OnError() {
      @Override
      public void onError(Throwable error) {
        // Transaction failed and was automatically canceled.
      }
    });

Step 7: Retrieve result from Realm objects or how to make query using Realm

// query all employee
RealmResults<EmployeeBean> employees = mRealm.where(EmployeeBean.class).findAll();
for (EmployeeBean employee : employees) {
     Log.i(TAG, "EmployeeId: "+employee.getEmployeeId());
     Log.i(TAG, "EmployeeName: "+employee.getEmployeeName());
     Log.i(TAG, "EmployeeSalary: "+employee.getEmployeeSalary());
}
// retrieve all employees whose sal is greater than 50000
RealmResults<EmployeeBean> employees = mRealm.where(EmployeeBean.class).greaterThan("employeeSalary", 50000).findAll();

This is sufficient information to start with Realm. Please let me know in comments for further queries.

Happy coding :)