Java中2个对象列表之间的区别

Question

I have a obj type MyObj as follow:

  class MyObj{
    String id;
    String username;
    String fullName;
    String age;

    //getters & setters
}

Suppose we have 2 lists containing different number of elements like so:

List<MyObj> listA
List<MyObj> listB

I have a generic method that detects elements from listA that are missing in listB :

 public static <T> List<T> getListDifference(List<T> list1, List<T> list2) {
        Collection<T> first = new HashSet<T>(list1);
        Collection<T> second = new HashSet<T>(list2);
        first.removeAll(second);
        return new ArrayList<T>(first);
    }

If objects from listA and listB has the same fields for all items, everything works just fine.

The problem is that some items has only id and username but others can have fullName or age too, and as result this method doesn't work anymore. I'd like to keep the same logic, considering only id field because it's present in all objects.

One obvious method is to copy only object's id field in other List<String> and work with obtained lists to detect elements, then just search for obj from both lists by id . This method has a big complexity, because of multiple iterations. Is there a short way to achieve this?

Answer 1

You have to implement equals and hashCode to work with sets. Sets needs a way of recognising if one object is equal to another, and default behaviour is not gonna work here.

By the way, I suggest using retainAll method on a list, check this out.

Answer 2

override MyObjs equals method:

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    MyObj myObj = (MyObj) o;

    return (id != null ? !id.equals(myObj.id) : myObj.id != null);
}

IntelliJ/Android Studio have helpers for automatically generating equals and hashCode where you can choose the necessary properties.

Answer 3

As @ Kevin Esche mentioned, you have to implement equals() and hashCode() in your MyObj POJO, and according to your situation, they should be:

   @Override
public int hashCode() {
    return Objects.hash(id);
}

@Override
public boolean equals(Object obj) {
    if (obj == this) return true;
    if (!(obj instanceof MyObj)) return false;

    MyObj myObj= (MyObj) obj;
    return Objects.equals(id, myObj.id);
}

That will work even if ages are different and the ids are the same for example.