Is it possible to use the Java 8 Stream API on Android API < 24?

Yes, it is! Starting from AGP 4.0.0 we can use Stream API and some other Java 8+ APIs (e.g. java.time)

Minimal required changes to your app's build.gradle:

android {
  compileOptions {
    // Flag to enable support for the new language APIs
    coreLibraryDesugaringEnabled true
  }
}

dependencies {
  coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.0.9'
}

[original answer]

You can not use Java8 streams on API level < 24.

However, there are some libraries that backport some of the stream functionality

https://github.com/aNNiMON/Lightweight-Stream-API

https://github.com/konmik/solid

https://sourceforge.net/projects/streamsupport/ (mentioned by @sartorius in comment)

[update k3b 2019-05-23]

https://github.com/retrostreams/android-retrostreams is a spinoff from streamsupport which takes advantage of Android Studio 3.x D8 / desugar toolchain's capability to use interface default & static methods across Jar file boundaries. There are also links to other android-retroXXX ie for CompletableFuture.

[update aeracode 2020-07-24]

Good news, now we can use Java 8 Stream API and more without requiring a minimum API level.

Since release 8.2 of DexGuard it is possible to use the Java 8 streams API also on Android devices < API level 24. In order to do so, one needs to include the streamsupport library and DexGuard will translate all Java 8 stream API calls to the provided library. No additional handling is needed, and developers can simply code using the provided Java 8 streams API. Dependencies are also translated automatically, so it is possible to use libraries with Java 8 features also for Android development.

This feature will also be included in ProGuard in the near future, stay tuned.

Edit: Proguard 6.1.0 for which there already exists a beta version supports backporting Java 8 stream and time API.

Might be a little late to the party, but I just saw that If we use java8.util.stream.StreamSupport then we can run the same functional programming approach and use it prior to Api 24. Eg taken from web3j -

StreamSupport.stream(transactionReceipt.getLogs())
            .map(log -> extractEventParameters(event, log))
            .filter(Objects::nonNull)
            .collect(Collectors.toList());