Finding unique strings in two arraylists of objects

Question

I have two arraylists of objects, I want to know which strings are unique to arraylist 1, and which strings are unique to arraylist 2. What I have come up with is the forloop below, which I have to implement twice, reversing the positions of the arraylists. I'm hopeful someone can suggest a more elegant way to do this.

Per request, a bunch more stuff I guess I wrongfully assumed was implied in the code-snippet itself. And the output this produces is:

grape doesn't exist in second arrayList

pineapple doesn't exist in first arrayList

Works great, everything is great, but, per above, I'm hopeful someone with more knowledge of streams/java in general can provide a better solution than just running my stream twice, with the inputs reversed.

import java.util.ArrayList;

public class CompareTwoArrays {

    ArrayList<MyCustomObject> firstArrayListOfObjects = new ArrayList<>();
    ArrayList<MyCustomObject> secondArrayListOfObjects = new ArrayList<>();

    public void superSpecificExampleMethod() {
        firstArrayListOfObjects.add(new MyCustomObject(1, 1, "apple"));
        firstArrayListOfObjects.add(new MyCustomObject(1, 1, "orange"));
        firstArrayListOfObjects.add(new MyCustomObject(1, 1, "banana"));
        firstArrayListOfObjects.add(new MyCustomObject(1, 1, "grape"));
        secondArrayListOfObjects.add(new MyCustomObject(1, 1, "apple"));
        secondArrayListOfObjects.add(new MyCustomObject(1, 1, "pineapple"));
        secondArrayListOfObjects.add(new MyCustomObject(1, 1, "orange"));
        secondArrayListOfObjects.add(new MyCustomObject(1, 1, "banana"));

        for (MyCustomObject object : firstArrayListOfObjects) {
            if (!secondArrayListOfObjects.stream().map(MyCustomObject::getString).filter(object.getString()::equals).findFirst().isPresent()) {
                System.out.println(object.getString() + " doesn't exist in second arrayList");
            }
        }

        for (MyCustomObject object : secondArrayListOfObjects) {
            if (!firstArrayListOfObjects.stream().map(MyCustomObject::getString).filter(object.getString()::equals).findFirst().isPresent()) {
                System.out.println(object.getString() + " doesn't exist in first arrayList");
            }
        }
    }
}

class MyCustomObject {

    private int randomIntOne;
    private int randomIntTwo;
    private String string;

    public MyCustomObject(int randomIntOne, int randomIntTwo, String string) {
        this.randomIntOne = randomIntOne;
        this.randomIntTwo = randomIntTwo;
        this.string = string;
    }

    public String getString() {
        return string;
    }
}

Sorry, I'm a novice, and am not sure what you're asking for. By test case, do you mean an example of the arraylist of objects? That's the only thing I can think of that isn't present in the snippet. — Apple
– Apple, Commented Jul 16, 2021 at 19:57
Java Object class does not have method getString. Do you mean you have your own POJO with this method? — Nowhere Man
– Nowhere Man, Commented Jul 16, 2021 at 20:16
Yes, correct. Maybe that is the root cause of oleg.cherednik's request, my seemingly poor choice for a name of my generic object. — Apple
– Apple, Commented Jul 16, 2021 at 20:36

Nowhere Man · Accepted Answer · 2021-07-16 20:54:36Z

1

Assuming there are two array lists of objects MyObject containing strings:

List<MyObject> listOne = new ArrayList<>(Arrays.asList(
    new MyObject("aaa"), new MyObject("bbb"), new MyObject("ccc"), new MyObject("ddd")
));

List<MyObject> listTwo = new ArrayList<>(Arrays.asList(
    new MyObject("fff"), new MyObject("bbb"), new MyObject("ggg"), new MyObject("ddd")
));

To find "unique" objects in listOne that is those which are not available in listTwo there are several ways:

Use List::removeAll providing that the methods equals and hashCode are properly implemented in this class

removeAll should be applied to a copy of listOne

List<MyObject> diffOneMinusTwo = new ArrayList<>(listOne); // copy
diffOneMinusTwo.removeAll(listTwo); // ["aaa", "ccc"]

Use List::removeIf accepting a predicate and using a set of the strings contained in the objects of listTwo:

Set<String> listTwoStrings = listTwo
    .stream()
    .map(MyObject::getString)
    .collect(Collectors.toSet);
List<MyObject> diffOneMinusTwo = new ArrayList<>(listOne); // copy
diffOneMinusTwo.removeIf(x -> listTwoStrings.contains(x.getString()));

Use Stream API filter and collect - no copy is needed here but a temporary set of strings is used

List<MyObject> diffOneMinusTwo = listOne
        .stream()
        .filter(x -> !listTwoStrings.contains(x.getString()))
        .collect(Collectors.toList());

In Java 11 there is static Predicate::not method so the stream version may look like this (if hashCode and equals are implemented properly):

List<MyObject> diffOneMinusTwo = listOne
        .stream()
        .filter(Predicate.not(listTwo::contains)) // using method reference
        .collect(Collectors.toList());

The difference between listTwo and listOne can be created vice versa.

edited Jul 16, 2021 at 20:54

answered Jul 16, 2021 at 20:31

Nowhere Man

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5 Comments

Apple Over a year ago

Thanks, but unfortunately I don't have two lists of strings, I have two arraylists of objects which contain strings.

g00se Over a year ago

Don;t use the class name Object. That's the origin of all objects in Java itself. You might get way with it, but best not to try ;) Call it MyObject and check the API docs for overriding key methods of all objects: hashCode and equals. Those are essential to how collections behave w.r.t. your custom class. Use methods like contains() simply will NOT work unless you override those methods

Apple Over a year ago

@g00se Yes, I see that now. Out of curiosity, would my original code snippet, which was just one of the for loops, make any sense/even function if 'Object' was anything other than a custom object?

Apple Over a year ago

@AlexRudenko So, looking at your answer, it seems that the best option is what I already have, just running the stream twice, first with listA vs listB, and then with listB vs listA. Thanks. Not sure what alternative I was hoping for, just seems kind of privative.

Nowhere Man Over a year ago

Not exactly, in your code you run the stream operations on listTwo for each element of listOne that is an overkill. removeAll is the shortest way of finding a difference (what is implied in your task) between two collections providing that hashCode and equals are implemented properly.

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