Slicing Crashes on Interface Default Method Dispatch

[bystc]

Description

When a subclass combines superclass overrides with interface default methods, JavaSlicer fails to build the System Dependence Graph and crashes while computing interprocedural actions. Programs that call super.greet() and rely on hello() supplied by an implemented interface trigger a NullPointerException, so no slice is produced.

Steps to Reproduce

1. Save the following program as SuperDefaultMetamorph.java:

interface Friendly {
    default String hello(String name) {
        return "Hello " + name;
    }
}

class RootMetaGreeter {
    public String greet(String name) {
        return "Greetings " + name;
    }
}

class HybridMetaGreeter extends RootMetaGreeter implements Friendly {
    @Override
    public String greet(String name) {
        return hello(name) + " & " + super.greet(name);
    }
}

public class SuperDefaultMetamorph {
    public static void main(String[] args) {
        RootMetaGreeter greeter = new HybridMetaGreeter();
        String message = greeter.greet("World"); // slicing criterion
        System.out.println(message);
    }
}

2. Run JavaSlicer:

 java -jar sdg-cli-1.3.0-jar-with-dependencies.jar SuperDefaultMetamorph.java#24:message

Actual Behavior

JavaSlicer throws a NullPointerException during SDG construction:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "es.upv.mist.slicing.graphs.cfg.CFG.vertexSet()" because "cfg" is null
        at es.upv.mist.slicing.graphs.sdg.InterproceduralActionFinder.initialValue(InterproceduralActionFinder.java:127)
        at es.upv.mist.slicing.graphs.sdg.InterproceduralActionFinder.initialValue(InterproceduralActionFinder.java:25)
        at es.upv.mist.slicing.graphs.BackwardDataFlowAnalysis.lambda$analyze$0(BackwardDataFlowAnalysis.java:33)
        at java.base/java.util.LinkedHashMap$LinkedKeySet.forEach(LinkedHashMap.java:589)
        at java.base/java.util.Collections$UnmodifiableCollection.forEach(Collections.java:1092)
        at es.upv.mist.slicing.graphs.BackwardDataFlowAnalysis.analyze(BackwardDataFlowAnalysis.java:33)
        at es.upv.mist.slicing.graphs.sdg.InterproceduralActionFinder.save(InterproceduralActionFinder.java:41)
        at es.upv.mist.slicing.graphs.sdg.SDG$Builder.dataFlowAnalysis(SDG.java:182)
        at es.upv.mist.slicing.graphs.sdg.SDG$Builder.build(SDG.java:128)
        at es.upv.mist.slicing.graphs.jsysdg.JSysDG$Builder.build(JSysDG.java:44)
        at es.upv.mist.slicing.graphs.sdg.SDG.build(SDG.java:73)
        at es.upv.mist.slicing.cli.Slicer.slice(Slicer.java:208)
        at es.upv.mist.slicing.cli.Slicer.main(Slicer.java:286)

Expected Behavior

The slicer should construct CFG/SDG nodes for interface default methods and produce a valid slice that includes:

• HybridMetaGreeter.greet
• Friendly.hello default implementation
• RootMetaGreeter.greet

This ensures the slice compiles and retains the polymorphic behaviour of the original program.

Root Cause

• ClassGraph (see line ~136) contains a TODO noting that default method implementations in interfaces are ignored.
• Because the CFG for Friendly.hello is never built, InterproceduralActionFinder receives a null CFG when it tries to analyse the call from HybridMetaGreeter.greet to the default method, leading to the NPE.
• Baseline code that avoids interface defaults (only class inheritance) slices successfully, confirming the failure is specific to default-method dispatch.

Suggested Fix

Ensure that interface default methods are incorporated into the class graph and receive CFGs during SDG construction. Once their CFGs exist, interprocedural analysis will no longer attempt to dereference a null cfg, and slices involving default-method dispatch will succeed.