How to find non unique elements in an array of class objects in Java

Question

I'm not sure how to say what I need so here's an example:

class Foo{
int x;
int y;
int z;
}
ArrayList<Foo> a;

The array has objects <a,b,c>,<a,c,d>,<b,c,e>,<b,e,f>,<c,e,f>,<g,h,i> etc

How can I retrieve all objects that have a common x value. In the above example I would like to retrieve <a,b,c>,<a,c,d> and <b,c,e>,<b,e,f> only.

I'm looking for an efficient way to do it.

Answer 1

您可以遍历所有Foo ，将它们添加到Map<Integer, Collection<Foo>> ，其中键是Foo的x值，而值是Foos的集合。