Sorting using Comparator Interface and java 8 Streams

Question

Parent is a class which is inherited by Child. which is inherited by GrandChild. Each class contains List of the child class(i.e Parent contains List of Child and Child contains List of GrandChild). Each class contains 50 attributes(attrib1-atrib50). getChildList() returns the arrayList of objects of type Child getGrandChildList() returns the arrayList of objects of type GrandChild

Let resultSet be List of Parent

List<Parent> resultSet

Now I want to sort the list based on some attributes. For example if I want to sort resultSet based on two parent attributes(say Attribute 1 and attribute 2, I use this code.

Comparator<Parent> byFirst = (e1, e2) -> e2.getAttrib1().compareTo(e1.getAttrib1());
Comparator<Parent> bySecond = (e1, e2) -> e1.getAttrib2().compareTo(e2.getAttrib2());

Comparator<Parent> byThird = byFirst.thenComparing(bySecond);


List<Parent> sortedList = resultSet.stream().sorted(byThird).collect(Collectors.toList());

Now I want to sort the parentlist based on attribute 1 of Child class and attribute 1 of GrandChild class. How should I sort this.

Solution

Use Comparator.comparing to make the comparators. Just figure out what you want to compare. It will looks something like this, except you will write whatever logic you want to use to extract the values to compare:

Comparator<Parent> byAttr1ofFirstChild = Comparator.comparing(
    parent -> parent.getChildren().get(0).getAttr1()
);

Comparator<Parent> byAttr1ofFirstGrandChild = Comparator.comparing(
    parent -> parent.getChildren().get(0).getGrandChildren().get(0).getAttr1()
);


List<Parent> sortedList = parents.stream()
    .sorted(byAttr1ofFirstChild.thenComparing(byAttr1ofFirstGrandChild))
    .collect(toList());

Comparator.comparing would also make the examples in your question much nicer (using static imports) :

Comparator<Parent> byFirst = comparing(Parent::getAttrib1, reverseOrder());
Comparator<Parent> bySecond = comparing(Parent::getAttrib2);