Androidにmockitoをインストール

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.0"
    defaultConfig {
        applicationId "jp.co.test.myapplication"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })

    //3行追加
    androidTestCompile 'org.mockito:mockito-core:1.10.19'
    androidTestCompile 'com.google.dexmaker:dexmaker:1.2'
    androidTestCompile 'com.google.dexmaker:dexmaker-mockito:1.2'

    compile 'com.android.support:appcompat-v7:25.0.1'
    testCompile 'junit:junit:4.12'
}

ExampleInstrumentedTest.java

package jp.co.test.myapplication;

import android.content.Context;
import android.content.SharedPreferences;
import android.support.test.InstrumentationRegistry;
import android.support.test.runner.AndroidJUnit4;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;

import java.util.ArrayList;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

/**
 * Instrumentation test, which will execute on an Android device.
 *
 * @see <a href="http://d.android.com/tools/testing">Testing documentation</a>
 */
@RunWith(AndroidJUnit4.class)
public class ExampleInstrumentedTest {
  
    @Test
    public void testMockArray() throws Exception {
        ArrayList mockList = mock(ArrayList.class);
        // get(0)で神奈川県を返すようにモック化する
        when(mockList.get(0)).thenReturn("神奈川県");
        assertEquals("神奈川県", mockList.get(0));
    }

    @Test
    public void testPref() throws Exception {
        SharedPreferences sp = Mockito.mock(SharedPreferences.class);
        Mockito.doReturn("ポテト").when(sp).getString("food", "");
        assertEquals("ポテト", sp.getString("food", ""));
    }
}