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Removing custom objects from arraylist and returning the clean arrayList

stackoverflow.com › questions › 62470851 › removing-custom-objects-from-arraylist-and-returning-the-clean-arraylist

The problem with your code is not the removeDuplicates method. Instead is probably the equals method in the Road class. List.contains(...) uses this equals method to compare if the list contains the given element. So to make your code work as expected you should override the equals method in the Road class to perform the needed comparison. Depending on the Road class implementation this could look like this for example:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Road road = (Road) o;
    return Objects.equals(road.x, x) &&
            Objects.equals(road.y, y) &&
            Objects.equals(imgPath, road.imgPath) &&
            Objects.equals(color, road.color);
}

@Override
public int hashCode() {
    return Objects.hash(x, y, imgPath, color);
}

As already pointed out in the comments these methods are usually generated using your IDE and only adjusted when needed. When adjusting them you have to take into consideration that there is a contract (which can be found in the docs) between equals and hashcode which has to be considered for various reasons (for example using hashsets).

Answer from Jakob Em on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 62470851 › removing-custom-objects-from-arraylist-and-returning-the-clean-arraylist

java - Removing custom objects from arraylist and returning the clean arrayList - Stack Overflow

Top answer

1 of 2

3

The problem with your code is not the removeDuplicates method. Instead is probably the equals method in the Road class. List.contains(...) uses this equals method to compare if the list contains the given element. So to make your code work as expected you should override the equals method in the Road class to perform the needed comparison. Depending on the Road class implementation this could look like this for example:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Road road = (Road) o;
    return Objects.equals(road.x, x) &&
            Objects.equals(road.y, y) &&
            Objects.equals(imgPath, road.imgPath) &&
            Objects.equals(color, road.color);
}

@Override
public int hashCode() {
    return Objects.hash(x, y, imgPath, color);
}

As already pointed out in the comments these methods are usually generated using your IDE and only adjusted when needed. When adjusting them you have to take into consideration that there is a contract (which can be found in the docs) between equals and hashcode which has to be considered for various reasons (for example using hashsets).

2 of 2

3

Make sure you have overridden equals and hashCode methods in the class, Road something like:

@Override
public boolean equals(Object obj) {
    Road other = (Road) obj;
    return Objects.equals(imgPath, other.imgPath) && Objects.equals(color, other.color)
            && Objects.equals(x, other.x) && Objects.equals(y, other.y);
}

@Override
public int hashCode() {
    return Objects.hash(imgPath, color, x, y);
}

Also, instead of writing so many lines in your method, you can write just one line using Stream API as follows:

private List<Road> removeDuplicates(List<Road> array) {
    return array.stream().distinct().collect(Collectors.toList());
}

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More results from stackoverflow.com

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stackoverflow.com › questions › 13311700 › liste-containsobject-why

java - List<E>.contains(Object), why? - Stack Overflow

Top answer

1 of 7

7

if it's just for backwards compatibility, why not deprecate contains(Object) and create a contains(E)?

Because contains(Object) and contains(E) have the same type erasure (as you can see in this code sample) and hence would cause compilation errors. Also, deprecating methods was not an option, the top priority back then was to make legacy code work.

2 of 7

2

Because there is no need to have a template here : this would only prevent some tests and if an object isn't of the required class the method would answer false in any case.

It's much simpler to have in your code a simple test checking if the return of the function is a boolean than a test and a try/catch. The few cases where having a type checked at compile time would allow to find a bug aren't worth the overhead.

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