Question

I have a list of objects in this format :

class myObj {

    private String identifier;

    public myObj(String identifier){
        this.identifier = identifier;
    }

}

List<myObj> allobjects = new ArrayList<myObj>();
allobjects.add(new myObj("123"));
allobjects.add(new myObj("123"));
allobjects.add(new myObj("123"));
allobjects.add(new myObj("123"));
allobjects.add(new myObj("1234"));
allobjects.add(new myObj("12345"));
allobjects.add(new myObj("12"));
allobjects.add(new myObj("12"));

What is an elegant method of extracting the duplicate objects into seperate Lists ?
So in above example a new List is returned containing two lists. The first lists
contains :

new myObj("123");
new myObj("123");
new myObj("123");
new myObj("123"); 

The second list contains :

new myObj("12");
new myObj("12");

A possible solution is to create a new object :

List<List<myObj>> newList = new ArrayList<List<myObj>>

And then for each element in the list ‘allobjects’ iterate over each element and for each element that is contained more than once add it to the list. Then at the
end of the iteration for the current element add the newly created list to ‘newList’

Is this acceptable or is there another solution ?

Answer 1

Add equals and hashCode methods to the myObj class, so that you can use them as Map keys:

class myObj {

    private String identifier;

    public myObj(String identifier){
        this.identifier = identifier;
    }

    public int hashCode(){
        return identifier.hashCode();
    }

    public boolean equals(Object o){
        return identifier.equals(((myObj)o).identifier);
    }
}

Then declare a Map:

Map<myObj, List<myObj>> map = new HashMap<myObj, List<MyObj>>()

and iterate through the original list. Use the myObjs as a map key, retrieving each time a list that corresponds to that myObj. If you encounter a certain myObj for the first time, don’t forget to create the list:

for(myObj obj : allobjects){
    List<myObj> list = map.get(obj);
    if(list == null){
        list = new ArrayList<myObj>();
        map.put(obj, list);
    }
    list.add(obj);
}