Updating dictionary value by reference when passing it through another function in Java

Question

i am new to Java and was wondering if it is possible to update dictionary's values outside of the dictionary itself. I think i loose a connection to my 'nodesIncluded' set by creating a new instance in the 'process' method in ID2. Is there any neat way to always keep track of the object underneath the value in a dictionary.

Sorry guys, i initially pasted the wrong bit of code :)

public class Dictionary {

    @AllArgsConstructor
    private class Info {
        private Double average;
        private Set nodesIncluded;
    }

    private void initialize(){
        Map<String, Info> testDict = new HashMap<String, Info>(){{
        put("ID001", new Info(1.0d, new HashSet<>(Arrays.asList("a", "b"))));
        put("ID002", null);
        }};

        Info takenOut01 = testDict.get("ID001");
        Info takenOut02 = testDict.get("ID002");

        process(takenOut01, "c");
        process(takenOut02, "c");

        System.out.print(testDict.get("ID001").nodesIncluded.contains("c")); // true
        System.out.print(testDict.get("ID002").nodesIncluded.contains("c")); // NullPointerException
        System.out.print(testDict.get("ID002").nodesIncluded.contains("default")); // NullPointerException
    }

    private void process (Info takenOut, String newValue ) {
        if (takenOut == null) {
            takenOut = new Info(0.0d, new HashSet<>(Arrays.asList("default")));
        }
        takenOut.nodesIncluded.add(newValue);
    }

    public static void main(String[] args) {
        Dictionary test = new Dictionary();
        test.initialize();
    }
}

Answer 1

Java passes by value, but when it comes to objects, it's a little confusing. When objects are passed through a method, its reference value gets passed. This means that operations performed on the object within the method scope affects it outside of its scope.

public static class Bar {
    private int num = 0;
    public int getNum() { return num; }
    public void setNum(int num) { this.num = num; }
}

public static void setBar5(Bar bar) {
    bar.setNum(5);
    System.out.println(bar.getNum()); // outputs 5
}

public static void main(String[] args) {
    Bar bar = new Bar();
    setBar5(bar);

    System.out.println(bar.getNum());
}

In short, if you hold an object reference to an object that is also inside a Collection, any operations on it would look like you're modifying it.

Bar bar = new Bar();
List<Bar> bars = new ArrayList<>();
bars.add(bar);
bar.setNum(5);

for(Bar b : bars) {
    System.out.println(b.getNum());
}
// outputs 5

If you get the object from the collection and modify it, it will also affect any variables holding onto that object.

Bar bar2 = bar;

// modifying the element in the collection
bars.get(0).setNum(4);

// Even when we modify bar, bar2 gets affected.
System.out.println(bar2.getNum()); // outputs 4