How to sort with condition in JAVA8 lambda

Question

I'm using Java-8 lambda to sort a list.

Here is list of

[(t1, tester1), (t4, tester4), (t3, tester3), (t2, tester2)]

after sort

[(t2, tester2), (t1, tester1), (t3, tester3), (t4, tester4)]

I want to get result as above

String specialId = 't2'

List<Test> list = new ArrayList<>();

Test test1 = new Test();
test1.setId = "t1"
test1.setName = "tester1"
list.add(test1)

Test test2 = new Test();
test2.setId = "t4"
test2.setName = "tester4"
list.add(test2)

Test test3 = new Test();
test3.setId = "t3"
test3.setName = "tester3"
list.add(test3)

Test test4 = new Test();
test4.setId = "t2"
test4.setName = "tester2"
list.add(test4)

Stream<Test> s1 = list.stream.filter(t -> t.getId.equals(specialId));
Stream<Test> s2 = list.stream.filter(t -> !t.getId.equals(specialId))
                    .sorted(Comparator.comparing(Test::getName));

Stream<Test> s = Stream.concat(s1, s2);

List<Test> result = s.collect(Collectors.toList());

I am getting the expected result. But I want use stream only once. How can I sort it?

Answer 1

You could put the special ID treatment into the comparator. The two comparisons can be chained using thenComparing.

1. First check if the ID ≠ special ID. Since false < true, this will bring the special ID before everything.
2. Then sort by name.

Stream<Test> s = list.stream().sorted(
    Comparator.comparing((Test t) -> !t.getId().equals(specialId))
              .thenComparing(Test::getName)
);

Answer 2

you can do in one sorting.

public static void main(String[] args) {
    String specialId = "t2";

    List<Test> list = new ArrayList<>();

    Test test1 = new Test();
    test1.setId("t1");
    test1.setName("tester1");
    list.add(test1);

    Test test2 = new Test();
    test2.setId("t2");
    test2.setName("tester2");
    list.add(test2);

    Test test3 = new Test();
    test3.setId("t3");
    test3.setName("tester3");
    list.add(test3);

    Test test4 = new Test();
    test4.setId("t4");
    test4.setName("tester4");
    list.add(test4);

    List<Test> sorted = list
            .stream()
            .sorted((t1, t2) -> {
                if (t1.getId().equals(specialId)) {
                    return -1;
                }

                if (t2.getId().equals(specialId)) {
                    return 1;
                }

                return t1.getId().compareTo(t2.getId());
            })
            .collect(Collectors.toList());

    sorted.forEach(System.out::println);
}

static class Test {
    String id;
    String name;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Test{" +
                "id='" + id + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}