Java: ArrayList.clear removes elements from ArrayList passed to Map

Question

This is my first time writing Java code in years and I'm working on a Ghidra script that Maps system call symbols to their calling functions.

private HashMap<Symbol, Reference[]> symbolRefs = new HashMap<Symbol, Reference[]>();
private HashMap<Symbol, List<Function>> callerFuncs = new HashMap<Symbol, List<Function>>();

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.
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private void mapSysCallToCallerFunctions(FunctionManager funcMan) throws Exception {
    List<Function> funcs = new ArrayList<Function>();
    for(HashMap.Entry<Symbol, Reference[]> entry: this.symbolRefs.entrySet()) {
        for(Reference ref : entry.getValue()) {
            Function caller = funcMan.getFunctionContaining(ref.getFromAddress());
            if(caller != null) {
                funcs.add(caller);
            }
        }
        this.callerFuncs.put(entry.getKey(), funcs);
        funcs.clear();
    }
}

My problem is that I want to clear the "funcs" list, so that I can use the empty list again for the next iteration. This causes the Function List in my HashMap to be empty, too, for some unknown reason. If I print my HashMap here:

private void printCallerSymbolMap() throws Exception {
    for(HashMap.Entry<Symbol, List<Function>> entry: this.callerFuncs.entrySet()) {
        printf("Symbol %s:\n", entry.getKey().toString());
        for(Function func : entry.getValue()) {
            printf("Called by function %s\n", func.getName());
        }
    }
}

I just get the output:

INFO  Symbol system: (GhidraScript)  
INFO  Symbol system: (GhidraScript)

However, when I remove funcs.clear(), I get:

INFO  Symbol system: (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Called by function main (GhidraScript)  
INFO  Symbol system: (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Called by function main (GhidraScript)

It should be like that though:

INFO  Symbol system: (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Called by function system (GhidraScript)  
INFO  Symbol system: (GhidraScript) 
INFO  Called by function main (GhidraScript)

I have two system symbols as it is thunked.

"Use the empty list again"—you seem to understand perfectly well that _it's the same list_. Create a new one if you want a different list. — chrylis -cautiouslyoptimistic-, Jun 08 '20 at 15:46
I thought that putting the list inside the HashMap would create a new list in there. — Melvin, Jun 08 '20 at 15:56
Why? Java doesn't randomly create new objects; it always passes references to them. — chrylis -cautiouslyoptimistic-, Jun 08 '20 at 15:56
I'm just used to saving the value of variables somewhere to modify the variable without its original value getting lost. Thank you for clarifying that funcs actually still references the value in the Map. — Melvin, Jun 08 '20 at 16:07

score 1 · Accepted Answer · answered Jun 08 '20 at 15:47

Insted of clearing list, initialize the list every time.

private void mapSysCallToCallerFunctions(FunctionManager funcMan) throws Exception {
    List<Function> funcs;
    for(HashMap.Entry<Symbol, Reference[]> entry: this.symbolRefs.entrySet()) {
        funcs = new ArrayList<Function>();
        for(Reference ref : entry.getValue()) {
            Function caller = funcMan.getFunctionContaining(ref.getFromAddress());
            if(caller != null) {
                funcs.add(caller);
            }
        }
        this.callerFuncs.put(entry.getKey(), funcs);
    }
}

As an alternative you can also copy the list you're putting in the map: `callerFuncs.put(entry.getKey(), new ArrayList<>(funcs))` to make `clear()` work as you expected. — Emmanuel Chebbi, Jun 08 '20 at 16:09

Java: ArrayList.clear removes elements from ArrayList passed to Map

1 Answers1