Android AdMob Interstitial

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="test.admob.com.admobinterstitialtest">

    <!-- Include required permissions for Google Mobile Ads to run. -->
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">

        <!-- This meta-data tag is required to use Google Play Services. -->
        <meta-data
            android:name="com.google.android.gms.version"
            android:value="@integer/google_play_services_version" />

        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <!-- Include the AdActivity configChanges and theme. -->
        <activity
            android:name="com.google.android.gms.ads.AdActivity"
            android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"
            android:theme="@android:style/Theme.Translucent" />

    </application>

</manifest>

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 22
    buildToolsVersion "22.0.1"

    defaultConfig {
        applicationId "test.admob.com.admobinterstitialtest"
        minSdkVersion 15
        targetSdkVersion 22
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:22.2.1'
    compile 'com.google.android.gms:play-services-ads:8.3.0'
}

MainActivity.java

package test.admob.com.admobinterstitialtest;

import com.google.android.gms.ads.AdListener;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.InterstitialAd;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private InterstitialAd interstitial;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Create the interstitial.
        interstitial = new InterstitialAd(this);
        interstitial.setAdUnitId(getString(R.string.interstitial_ad_unit_id));
        loadinterstitial();

        // Listener for Ad
        interstitial.setAdListener(new AdListener()
        {
            // When Closed Ad, Load new Ad
            @Override
            public void onAdClosed()
            {
                super.onAdClosed();
                loadinterstitial();
            }
        });

    }

    private void loadinterstitial()
    {
        // Create ad request.
        AdRequest adRequest = new AdRequest.Builder().build();

        // Begin loading your interstitial.
        interstitial.loadAd(adRequest);
    }

    // Show Interstitial Ad
    private void showInterstitialAd()
    {
        // return, if Ad data is no loaded
        if (!interstitial.isLoaded()) {
            return;
        }

        // Show Ad
        interstitial.show();
    }

    // Back button
    @Override
    public void onBackPressed()
    {
        super.onBackPressed();

        // Make rand
        Random rnd = new Random();

        // Omikuji
        int Omikuji = rnd.nextInt(2);
        if (Omikuji == 0) {
            // Go to Show Interstitial Ad
            showInterstitialAd();
        }

    }

}

strings.xml

<resources>
    <string name="app_name">AdMobInterstitialTEST</string>

    <!-- Interstitial ad unit ID -->
    <string name="interstitial_ad_unit_id">ca-app-pub-3940256099942544/1033173712</string>
</resources>

not work for me

setAdUnitId
setAdListener
loadAd
isLoaded

error for me