如何计算地图中复杂对象的唯一值

Question

I have the following sample structure:

class MyObject {
    private String type;
    private String level;
    
}

Map<String,List<MyObject>> map = new HashMap<>();

MyObject myObject1 = new MyObject();
myObject1.setType("x");
myObject1.setLevel("5");

MyObject myObject2 = new MyObject();
myObject2.setType("y");
myObject2.setLevel("5");

List<MyObject> list1 = new ArrayList<>();
list1.add(myObject1);
list1.add(myObject2);

map.put("1",list1);

MyObject myObject3 = new MyObject();
myObject3.setType("x");
myObject3.setLevel("4");

List<MyObject> list2 = new ArrayList<>();
list2.add(myObject3);

map.put("2",list2);

MyObject myObject4 = new MyObject();
myObject4.setType("x");
myObject4.setLevel("5");

MyObject myObject5 = new MyObject();
myObject5.setType("y");
myObject5.setLevel("5");

List<MyObject> list3 = new ArrayList<>();
list3.add(myObject4);
list3.add(myObject5);

map.put("3",list3);

...

Based on this map, I need to create an intermediate object or some structure where I will store information about the unique values ​​of the map. In the example above, key 1 and key 3 are the same value so I need to store the information that the combination x = 5, y = 5 occurred twice in the map. The combination x = 4 appeared once in the map. There can be many combinations.

Any suggestions on how to do it the easiest way?

Answer 1

Since this looks like a homework question asking how to generally do the task I will not include code.

Think through what you have to do, write methods you'll need, implement them when you think you have all the pieces you need. Start with a stub for the method that does what you want.

The thing you can have duplicates of in the map (why are they in the map, no idea) are lists. Write a method that compares lists and returns whether they are the same.

To write that method you need a method that can compare MyObject. Best way would be to override equals() method.

Next, it'll be a question if order in the lists matters. If yes, than List equals method will work for you (read the javadoc to see exactly what it does). If not you'll need to write custom code to handle that, or sort the lists before comparison (which would involve writing a comparator for MyObject), or use a library that has that functionality (there should be something in Apache Commons).

Now that we have all that all we come back to the main method, use the ones we wrote appropriately, and all we need is do something with the results. Generally anything will do, a map with the list as key and amount of occurences as value will be simplest unless you have some more constraints or operations to do on the results.