如何比较两个包含Java中同一类对象的Arraylists？

Question

I have 2 lists of different sizes which belong to the same class Officer which has attributes:

Integer id;
Integer level; 
String role;

I want to compare these two lists list1 and list2 so that I can generate two new lists insertLst and deleteList.

Both list 1 and list2 contain unique elements.

insertLst : This list contains items in list1 and not in list2.

deleteList : This list contains items in list2 and not in list1.

I have no idea how this should be done , might be using The org.apache.commons.collections.CollectionUtils API .

Could you suggest me a java code for generating these two lists insertLst and deleteList?

Answer 1

If you want to use org.apache.commons.collections.CollectionUtils then you should use subtract method.

Collection<Officer> insertList= CollectionUtils.subtract(list1, list2);
Collection<Officer> deleteList = CollectionUtils.subtract(list2, list1);

or you could write it like this (passing empty lists that are filled in method) not to add another library to your dependencies:

static <T> void process(List<T> list1, List<T> list2, List<T> insertList, List<T> deleteList) {
    for (T t: list1) {
        if (!list2.contains(t)) {
            insertList.add(t);
        }
    }
    for (T t: list2) {
        if (!list1.contains(t)) {
            deleteList.add(t);
        }
    }
}

Don't forget to override equals and hashcode methods for you class Officer in both cases. Please refer here for explanation

Answer 2

Make sure your Officer class overrides the equals() (and hashCode() ) method and it will be really easy:

List<Officer> list1 = ... // your list1
List<Officer> list2 = ... // your list2
List<Officer> insertLst = new ArrayList<>(list1);
insertLst.removeAll(list2);
List<Officer> deleteList= new ArrayList<>(list2);
deleteList.removeAll(list1);

Answer 3

final List<Officer> one = new ArrayList<Officer>();
...
final List<Officer> two = new ArrayList<Officer>();
...
final List<Officer> insertList = new ArrayList<Officer>(one);
insertList.removeAll(two);
final List<Officer> removeList = new ArrayList<Officer>(two);
removeList.removeAll(one);

Edit: As jlordo said, make sure you override hashCode and Equals

Answer 4

You have to loop through the two lists ....

One way to do this could be this one:

ArrayList <Officer> list1= ....
ArrayList <Officer> list2= ....
ArrayList <Officer> insertList= new  ArrayList <Officer>();
ArrayList <Officer> deleteList= new  ArrayList <Officer>();

for(Officer x:list1){
  for(Officer y:list2){
     //business logic here
     if(x.id==y.id){
       insertList.add(x); //example code to add an Object in a List
     }
  }
}

The business logic part is up to you... you can compare data between the two lists and decide what goes where...

Hope this helps