Java extended collection filtering

问题

I have to filter and to sort a ArrayList wíth objects.
- Every object has 2 integer pairs => 4 ints per Object.
- Every value of Column_1 < Column_2 and
- Every value of Column_3 < Column_4.
... so each pair represents a distance.

1.) Distance in 1st(Column_1,Column_2) pair and 2nd(Column_3, Column_4,) pair have to be equal.

2.) if there exists in the list a Obj_1 , whose Column_2 value is equal to Column_1 value+1 of Obj_2 and

3.) if there exists in the list a Obj_1 , whose Column_4 value is equal to Column_3 value+1 of Obj_2

then this objects should be merged to one Object respecting values in each pair. ...minimal values in(Column_1,Column_3) and maximal values(Column_2,Column_4)

Example:

Column_1  Column_2  Column_3  Column_4

----------- before filtering --------------

1. 506       520     771       785
2. 106       110     210       214
3. 502       505     181       184
4. 714       717     270       273
5. 106       110     310       314
6. 111       115     215       219
7. 521       524     767       770
8. 502       505     350       353
9. 100       105     204       209

-----------after filtering----------

1. 100        115    204       219
2. 106        110    310       314
3. 502        505    181       184
4. 714        717    270       273
5. 502        520    767       785

How can this kind of filtering be done in Java?

回答1:

~~My first approach would be to~~

implement Comparable (or write a Comparator) based on the logic described above,
write an equals method implementing the same logic,
fill the contents of the ArrayList into a TreeSet,
iterate over the set to merge adjacent elements when applicable.

The TreeSet will order its elements by natural ordering (or using a provided Comparator).

Refined approach

Unfortunately I failed to notice that distinct objects may contain overlapping intervals, thus items to be merged may not be adjacent. An improvement to the above could be:

implement Comparable based on Columns 1 and 3,
(write an equals method comparing all columns),
sort the contents of the ArrayList,
for each element in the list, search for elements fulfilling conditions 2) and 3) above, which can then be merged to the original element. Since the list is ordered, you can use binary search over the sublist after the current item. If the current element is merged with another, Columns 2 and 4 are updated, but 1 and 3 not, so the ordering does not change. The other element can then be removed from the list, and the search repeated with the new values of columns 2 & 4, for the sublist after the removed element.

回答2:

Google collections gives you the option to specify a filter predicate that will operate on all entries and decide which to preserve:

Collections2.filter(yourCollection, new Predicate<YourType>() {
    @Override
    public boolean apply(YourType param) {
    // return whether to retain or remove
    }
});

来源：https://stackoverflow.com/questions/3712669/java-extended-collection-filtering

标签

java

sorting

collections

filtering