Merging two List of objects in java 8

Question

If you want to implement equals and hashCode, the place to do it is inside the class Parent. Within that class add the methods like

    @Override
    public int hashCode() {
        return Objects.hash(getAttrib1(), getAttrib2(), getAttrib3(),
            // …
                            getAttrib19(), getAttrib20());
    }

    @Override
    public boolean equals(Object obj) {
        if(this==obj) return true;
        if(!(obj instanceof Parent)) return false;
        Parent p=(Parent) obj;
        return Objects.equals(getAttrib1(), p.getAttrib1())
            && Objects.equals(getAttrib2(), p.getAttrib2())
            && Objects.equals(getAttrib3(), p.getAttrib3())
            // …
            && Objects.equals(getAttrib19(), p.getAttrib19())
            && Objects.equals(getAttrib20(), p.getAttrib20());
    }

If you did this, distinct() invoked on a Stream<Parent> will automatically do the right thing.

---

If you don’t want (or can’t) change the class Parent, there is no delegation mechanism for equality, but you may resort to ordering as that has a delegation mechanism:

Comparator<Parent> c=Comparator.comparing(Parent::getAttrib1)
        .thenComparing(Parent::getAttrib2)
        .thenComparing(Parent::getAttrib3)
        // …
        .thenComparing(Parent::getAttrib19)
        .thenComparing(Parent::getAttrib20);

This defines an order based on the properties. It requires that the types of the attributes itself are comparable. If you have such a definition, you can use it to implement the equivalent of a distinct(), based on that Comparator:

List<Parent> result = Stream.concat(list1.stream(), list2.stream())
        .filter(new TreeSet<>(c)::add)
        .collect(Collectors.toList());

There is also a thread-safe variant, in case you want to use it with parallel streams:

List<Parent> result = Stream.concat(list1.stream(), list2.stream())
        .filter(new ConcurrentSkipListSet<>(c)::add)
        .collect(Collectors.toList());

For example:

public class Parent {

    public int no;
    public String name;

    @Override
    public int hashCode() {
        return (no << 4) ^ name.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Parent))
            return false;
        Parent o = (Parent)obj;
        return this.no == o.no && this.name.equals(o.name);
    }
}

Override the equals and hashCode methods in Parent class to avoid duplicates from the lists. This will give you the exact result what you want.