I define a Triple class with equals and hashcode method. According the docs,HashSet.contains() method of Java HashSet must returns the same value of Objects.equals(). But contains() method of Java HashSet method return true, when it suppose to be false.

class Triple{

    private List<Integer> triple;

    public Triple(int one, int two, int three){
        this.triple = new ArrayList<>();
        this.triple.add(one);
        this.triple.add(two);
        this.triple.add(three);
    }

    public int hashCode(){
        return triple.get(0).hashCode() + triple.get(1).hashCode() + triple.get(2).hashCode();
    }

    public boolean equals(Object t){
        Triple triple = (Triple)t;
        return triple.triple.contains(this.triple.get(0))
                && triple.triple.contains(this.triple.get(1))
                && triple.triple.contains(this.triple.get(2));
    }
}

public class Main {
    public static void main(String[] args) {
        HashSet<Triple> answersSet = new HashSet<>();
        Triple triple1 = new Triple(-4,0,4);
        Triple triple2 = new Triple(0,0,0);
        answersSet.add(triple1);
        boolean b = answersSet.contains(triple2); //returns true
        b = Objects.equals(triple1,triple2); //but it returns false, 
    }
}

>Solution :

Your equals is broken. You only test if the value is contained in the List, not that the three values are equal. I think you wanted

@Override
public boolean equals(Object o) {
    Triple t = (Triple) o;
    return triple.get(0).equals(t.triple.get(0))
            && triple.get(1).equals(t.triple.get(1))
            && triple.get(2).equals(t.triple.get(2));
}

Which (obviously) returns false with your example data. Also, you can simplify your constructor

public Triple(int one, int two, int three) {
    this.triple = List.of(one, two, three);
}