对一个arraylist对象的值进行排序，并在一个lambda中打印它们的多个

Question

My goal is to print out two values out of four - name (string) and hair colour (enum) - that describe a certain object in an arraylist. The problem lies with sorting the printed list alphabetically by name. It all has to be in one lambda.

I'm stuck with just printing it out, without sorting. I've already set up the overrides but since I'm still learning, I can't seem to get it work.

public void printAllNamesColours() {
        people.stream()
            .forEach(n->System.out.println(n.getName() + " " + n.getColour()));
    }

@Override
public int compareTo(Person person) {
        int ret = name.compareToIgnoreCase(person.name);
        if (ret == 0) {
            ret = colour.compareTo(person.colour);
        }
        if (ret == 0) {
            ret = age - person.age;
        }
        return ret;
    }

Answer 1

You can sort it using Stream.sorted as:

people.stream()
      .sorted() // considering your `compareTo` implementation
      .forEach(n -> System.out.println(n.getName() + " " + n.getColour()));

Answer 2

As your Person objects are Comparable , you can directly call the sorted method and print the result.

people.stream()
      .sorted()
      .forEachOrdered(n->System.out.println(n.getName() + " " + n.getColour()));

note that I've used forEachOrdered as opposed to forEach to guarantee the elements are printed in the sorted order.

---

You may also decide to sort my name only or by colour only without changing your current compareTo implementation, in which case you can use a custom comparator:

people.stream()
      .sorted(Comparator.comparing(Person::getName, String.CASE_INSENSITIVE_ORDER))
      .forEachOrdered(n->System.out.println(n.getName() + " " + n.getColour()));

and by colour:

people.stream()
      .sorted(Comparator.comparing(Person::getColour, String.CASE_INSENSITIVE_ORDER))
      .forEachOrdered(n->System.out.println(n.getName() + " " + n.getColour()));

Answer 3

As the other answer tells you, you need to add the .sorted() operation in the stream

---

Next to that I'd like to suggest you a different way for comparing your Person :

static Comparator<Person> comparator = 
        Comparator.comparing(Person::getName, String.CASE_INSENSITIVE_ORDER)
                  .thenComparing(Person::getColour)
                  .thenComparing(Person::getAge);

@Override
public int compareTo(Person person) {
    return comparator.compare(this, person);
}

// with the different getters

Answer 4

listPersonVO.stream().sorted((o1, o2)->o1.getName().trim().compareTo(o2.getName().trim())).collect(Collectors.toList());

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