Crash on Android 5 (API 21)

[FedericoMatera]

I added apply plugin: 'com.github.sgtsilvio.gradle.android-retrofix' in my project but if I try to connect via MQTT on Android 5 the app crashes with this error:

Caused by: java.lang.NoClassDefFoundError: com.hivemq.client.internal.mqtt.message.connect.mqtt3.-$$Lambda$Wn55JBsOAP1F2GOaN0qiXRejLL0
        at com.hivemq.client.internal.mqtt.message.connect.mqtt3.Mqtt3ConnectViewBuilder.simpleAuth(Mqtt3ConnectViewBuilder.java:80)

[SgtSilvio]

You have to set:

android {
    ...
    compileOptions {
        // enables lambdas, method references, default methods, static interface methods
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    ...
}

[FedericoMatera]

I had.
I have this config:
root:

buildscript {
    repositories {
        google()
        jcenter()
        maven { url 'https://maven.fabric.io/public' }
        gradlePluginPortal()
    }
    dependencies {
        ...
        classpath 'gradle.plugin.com.github.sgtsilvio.gradle:android-retrofix:0.2.1'
    }
}

app:

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt'
apply plugin: 'com.github.sgtsilvio.gradle.android-retrofix'

android {
   ...
   compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {
    ...
    implementation project(":data")
    implementation 'net.sourceforge.streamsupport:android-retrostreams:1.7.1'
    implementation 'net.sourceforge.streamsupport:android-retrofuture:1.7.1'
}

data:

apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt'

android {
   ...
   compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {
    ...
    implementation 'com.hivemq:hivemq-mqtt-client:1.1.1'
}

[SgtSilvio]

Did you add

dependencies {
    ...
    implementation 'net.sourceforge.streamsupport:android-retrostreams:1.7.1'
    implementation 'net.sourceforge.streamsupport:android-retrofuture:1.7.1'
}

to your app project?

[FedericoMatera]

yes, I updated the comment ^

[SgtSilvio]

The error is caused because the classes of the hivemq-mqtt-client dependency are not backported at all.
I think this is caused because of the hivemq-mqtt-client dependency being a transitive dependency in your case.
Can you try adding implementation 'com.hivemq:hivemq-mqtt-client:1.1.1' to your app dependencies to verify this?

[FedericoMatera]

Now it works! Thank you!