ProGuard manual

Languages - Java

Java 8 language support¶

ProGuard can backport Java 8 language features to Dalvik and to older versions of Java, so you can use them in applications and libraries that work on all versions of Android.

The following language features are supported:

lambda expressions (closures)
method references
try-with-resources statements
static interface methods
default interface methods

Additions to the Java runtime library (like the stream API) can not be backported without additional runtime dependencies; see the next section.

In order to enable Java 8 language support, a Java 8 Compiler must be available when running gradle, i.e. by setting JAVA\_HOME. You need to configure the source and target compatibility settings in your build.gradle script:

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

If you want to target a minimum API level less than 19, you should add the following configuration to your proguard-project.txt:

-target 1.6

Java 8 stream API support¶

ProGuard can backport the Java 8 stream API to older versions of Android (< API 24), so you can use it in applications that work on all versions of Android.

The following additions to Java 8 can be backported and thus be used in an application:

classes added in java.util.function package
classes added in java.util.stream package
default and static methods added to interfaces in java.lang and java.util (e.g. Iterable, Collection)
static methods added to classes in java.lang (Integer, Long, Double) and java.util (Arrays)
classes added in java.util package (e.g. Optional, StringJoiner)
classes added/updated in java.util.concurrent package (e.g. ForkJoinPool)

The following methods can not be backported and using them will fail the build:

methods added to java.util.Random (e.g. ints, longs, doubles)
default method remove() in java.util.Iterator: classes that rely on a default implementation will not work

In order to use the Java 8 stream API and enable backporting in ProGuard, you have to specify a minimum compileSdkVersion of 24:

android {
    .....
    compileSdkVersion 24 // or a higher version
}

Furthermore, you need to add the streamsupport library as dependency to your project, e.g. like this:

repositories {
    mavenCentral()
}

dependencies {
    ...
    compile 'net.sourceforge.streamsupport:streamsupport:1.6.3'
}

If you use the Java 8 stream API in your project, but ProGuard can not backport to the included streamsupport library (e.g. due to an incompatible version), ProGuard issues warnings in the console log.

Note: ProGuard currently only supports the core streamsupport library. Additional modules (cfuture, atomic, flow, literal) are not yet backported. Please contact Guardsquare if you want to use them as well.

Java 8 time API support (JSR310)¶

ProGuard can backport the Java 8 time API to older versions of Android (< API 26), so you can use it in applications that work on all versions of Android.

The following additions to Java 8 can be backported and thus be used in an application:

classes added in java.time package

The java.time API includes various interfaces with default / static methods. ProGuard can backport the use of these methods can be backported, but in case you implement these interfaces yourself (which would be unusual), the resulting class can not be fully backported (due to missing default methods). This includes the following interfaces:

java.time.chrono.ChronoLocalDate
java.time.chrono.ChronoLocalDateTime
java.time.chrono.Chronology
java.time.chrono.ChronoPeriod
java.time.chrono.ChronoZonedDateTime
java.time.chrono.Era
java.time.temporal.Temporal
java.time.temporal.TemporalAccessor
java.time.temporal.TemporalField
java.time.temporal.TemporalUnit

In order to use the Java 8 time API and enable backporting in ProGuard, a minimum compileSdkVersion of 26 has to be used:

android {
    .....
    compileSdkVersion 26 // or a higher version
}

Furthermore, you need to add the threetenbp library as dependency to your project, e.g. like this:

repositories {
    mavenCentral()
}

dependencies {
    ...
    api group: 'org.threeten', name: 'threetenbp', version: '1.3.6'
}

It is highly recommended to use the threetenabp library from Jake Wharton instead, which is just as a wrapper around the threetenbp library and provides an efficient way to initialize the timezone database:

dependencies {
    ...
    compile 'com.jakewharton.threetenabp:threetenabp:1.0.5'
}

Additionally, you have to initialize the timezone database in your application class like this:

public void onCreate()
{
    super.onCreate();
    ...
    com.jakewharton.threetenabp.AndroidThreeTen.init(this);
}

If you use the Java 8 stream API in your project, but ProGuard can not backport to the included threetenbp library (e.g. due to an incompatible version), ProGuard issues warnings in the console log.