Sorting by enum and integer in an object in a list in Java?

Question

I have this class and enum in my program (names changed and simplified):

public enum Types {
    FIRST, SECOND, THIRD
}
public class TestClass {
    Types types;
    int num;
    public TestClass(Types types, int num) {
        this.types = types;
        this.num = num;
}

Now let's say I have TestClass objects in an ArrayList like this:

ArrayList<TestClass> list = new ArrayList<>();
list.add(new TestClass(Types.THIRD, 2);
list.add(new TestClass(Types.THIRD, 1);
list.add(new TestClass(Types.FIRST, 3);
list.add(new TestClass(Types.FIRST, 1);
list.add(new TestClass(Types.FIRST, 2);

I would like to sort the list first based on the enums and then based on the num, so that the end result would be this:

[(Types.FIRST, 1), (Types.FIRST, 2), (Types.FIRST, 3), (Types.THIRD, 1), (Types.THIRD, 2)]

I see that I could use Comparator, but I'm unsure exactly how I could sort by enums and then by numbers inside the enums. Two comparators? Nested comparators? What would be the optimal solution for this?

A lot will come down to how re-usable a solution you want (ie, if you have a Comparator for the Types enum already or not), but basically, if the comparison between Types is 0, then you compare the int, but you might need to apply some additional "weighting" to make it work just right — MadProgrammer
– MadProgrammer, Commented Jan 25, 2016 at 22:17

Louis Wasserman · Accepted Answer · 2016-01-25 22:19:38Z

3

Are you using Java 8? If so, you can write

Comparator<TestClass> comparator = 
   comparing(tc -> tc.types)
      .thenComparingInt(tc -> tc.num);

...and then you could write

list.sort(comparator);

answered Jan 25, 2016 at 22:19

Louis Wasserman

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1 Comment

Stephen Rosenthal Over a year ago

You might have to add some extra type parameters like this: Comparator<TestClass> comparator = Comparator.<TestClass, Types>comparing(x -> x.types).thenComparing(x -> x.num)

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