Android: AbstractMethodError for Function+lambda when proguard is used

There is some mess with proguard optimizations for android when Function is instantiated with lambda

com.google.common.base.Function<String, String> function = (u) -> u + "+1";
String s1 = function.apply("aaaa");

gives me

Process: by.stari4ek.tvirl.debug, PID: 21741
  java.lang.AbstractMethodError: abstract method "java.lang.Object com.google.common.base.Function.a(java.lang.Object)"

But changing it to

Function<String, String> function = new Function<String, String>() {
    @Override
    public String apply(@NonNull String input) {
        return input + "+1";
    }
};
String s1 = function.apply("aaaa");

works well

Proguard, mapping

-keepnames class com.google.common.**

com.google.common.base.Function -> com.google.common.base.Function:
    java.lang.Object apply(java.lang.Object) -> a
    boolean equals(java.lang.Object) -> equals

build.gradle

buildToolsVersion = '28.0.1'
compileSdkVersion = 27
minSdkVersion = 21
targetSdkVersion = 27

compileOptions {
  sourceCompatibility JavaVersion.VERSION_1_8
  targetCompatibility JavaVersion.VERSION_1_8
}
//
api 'com.google.guava:guava:26.0-android'

Devices

Reproduced on NVidia Shield (API 26) and Emulator (API 27)

[rickytribbia]

I fixed this one with:

-keepnames class com.google.common.** { *; }

This is needed because Proguard has some difficulty to manage obfuscating/renaming method name and keeping lambda functionality. I said “has some difficulty” because this error doesn’t appear always, but only in certain cases.

[ronshapiro]

I’m going to close this - if you gain any new information feel free to comment here and we’ll revisit.