Proguard removes overridden methods in abstract class as unused

Question

Proguard is removing the overridden methods in an abstract class as unused, even though they are used in the parent class. Here is a reference implementation showing the behaviour

public abstract class Animal {
    Animal() {
        born();
    }

    abstract void born();
}

public abstract class Human extends Animal {
    @Override
    void born() {
        System.out.println("Human is born.");
    }
}

The keep attributes are defined as:

-keep public class test.Human {
    public *;
}

Proguard removes the overridden born() methods in Human class as unused, even though it is used from the constructor of Animal class. The resulting mapping file is

test.Animal -> test.a:
    void born() -> a
test.Human -> test.Human:

This problem does not exist if the class Human is not abstract. If the class Human is non-abstract, the resulting mapping file is

test.Animal -> test.a:
    void born() -> a
test.Human -> test.Human:
    void born() -> a

As we can see, the method born() is retained in this case.

Is this a bug in proguard? Or is there any optimization setting that can provide the desired behaviour?

I am using proguard with android studio.

Answer 1

You configure ProGuard to keep public methods, while born() is package private. Your configuration should rather be like this.

-keep public class test.Human {
    <methods>;
}

It will keep all package private (default) methods of Human class.

If you want to keep methods but still allow obfuscation for them, you can use something like this:

-keep, allowobfuscation public class test.Human {
    <methods>;
}

Answer 2

Since the problem only manifests when these classes are part of a library (and without any concrete implementation within the library), I took the easy way out and added -dontshrink to retain (with obfuscation) all the classes and methods.

In most cases; class and method shrinking need not be done when releasing a library.

Still I believe it is a bug in usage graph to ignore the overridden method implementations. I have filed a bug in the proguard bug tracker. https://sourceforge.net/p/proguard/bugs/574/