This gist provides a simple example of mocking a Realm database for android in java using Robolectric.

ActivityRealmRobolectricTest.java

package com.example;

import android.content.Intent;
import android.widget.Button;

import com.example.MockSupport;

import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PowerMockIgnore;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.rule.PowerMockRule;
import org.robolectric.Robolectric;
import org.robolectric.RobolectricGradleTestRunner;
import org.robolectric.Shadows;
import org.robolectric.annotation.Config;
import org.robolectric.shadows.ShadowActivity;
import org.robolectric.util.ActivityController;

import io.realm.Realm;

import static org.junit.Assert.*;

@RunWith(RobolectricGradleTestRunner.class)
@Config(constants = BuildConfig.class, sdk = 21)
@PowerMockIgnore({"org.mockito.*", "org.robolectric.*", "android.*"})
@PrepareForTest({Realm.class})
public class ActivityTest {

  Realm mockRealm;

  @Rule
  public PowerMockRule rule = new PowerMockRule();

  @Before
  public void setup() {
    mockRealm = MockSupport.mockRealm();
  }
  
  @Test
  public void realmShouldBeMocked() {
    assertThat(Realm.getDefaultInstance(), is(mockRealm));
  }
}

build.gradle

// Other stuff...

dependencies {
  testCompile 'junit:junit:4.12'
  testCompile "org.robolectric:robolectric:3.0"
  testCompile "org.mockito:mockito-core:1.10.19"
  testCompile 'org.robolectric:shadows-support-v4:3.0'

  testCompile "org.powermock:powermock-module-junit4:1.6.2"
  testCompile "org.powermock:powermock-module-junit4-rule:1.6.2"
  testCompile "org.powermock:powermock-api-mockito:1.6.2"
  testCompile "org.powermock:powermock-classloading-xstream:1.6.2"
}

MockRealm.java

package com.example;

import com.example.model.Address;
import com.example.model.Job;
import com.example.model.Offer;
import com.example.model.Tasker;

import org.powermock.api.mockito.PowerMockito;

import io.realm.Realm;

import static org.powermock.api.mockito.PowerMockito.mockStatic;
import static org.powermock.api.mockito.PowerMockito.when;

public class MockSupport {

  public static Realm mockRealm() {
    mockStatic(Realm.class);

    Realm mockRealm = PowerMockito.mock(Realm.class);

    when(mockRealm.createObject(Address.class)).thenReturn(new Address());
    when(mockRealm.createObject(Offer.class)).thenReturn(new Offer());
    when(mockRealm.createObject(Tasker.class)).thenReturn(new Tasker());
    when(mockRealm.createObject(Job.class)).thenReturn(new Job());

    when(Realm.getDefaultInstance()).thenReturn(mockRealm);

    return mockRealm;
  }
}

As stated in powermock/powermock#819, it's time to replace powermock-api-mockito with powermock-api-mockito2