Sorting multiple attribute with Lambda Expressions

Question

This is my list:

Name: Ben     || Age: 5 || Group: 1
Name: Andy    || Age: 6 || Group: 2
Name: Charlie || Age: 6 || Group: 2
Name: Ben     || Age: 5 || Group: 1
Name: Andy    || Age: 5 || Group: 2
Name: Charlie || Age: 5 || Group: 1

I want to sort the list by Group, if Group is equal then by Age, and if Age is equal then by Name. But so far I can only sort by one attribute, using Lambda Expressions:

list.sort((Object o1, Object o2) -> o1.getGroup().compareTo(o2.getGroup()));

If I try

o1.getGroup().compareTo(o2.getGroup()) && o1.getAge().compareTo(o2.getAge())

it's turned out error...

Answer 1

Change lambda expression to lambda {block}, and you don't have to specify the parameter types:

list.sort((o1, o2) -> {
    int cmp = o1.getGroup().compareTo(o2.getGroup());
    if (cmp == 0)
        cmp = Integer.compare(o1.getAge(), o2.getAge());
    if (cmp == 0)
        cmp = o1.getName().compareTo(o2.getName());
    return cmp;
});

Answer 2

You can use the static method Comparator.comparing to create a comparator based on a function that returns a comparable value. Such a comparator can be chained with others.

Assuming your type is called Person, you would have:

Comparator<Person> c = Comparator
        .comparing(p -> p.getGroup())
        .thenComparing(p -> p.getAge())
        .thenComparing(p -> p.getName())

If any of the getters return a primitive type, you have to use - for example - comparingInt and thenComparingInt, respectively. You can also use method references:

Comparator<Person> c = Comparator
        .comparing(Person::getGroup)
        .thenComparing(Person::getAge)
        .thenComparing(Person::getName)

But ... if your class has a natural ordering according to these values, you better let it implement the interface Comparable and write the compare logic in there:

class Person implements Comparable<Person> {
    ...
    @Override
    public int compareTo(Person other) {
        int compare = Integer.compare(getGroup(), other.getGroup());
        if (compare == 0) {
            compare = Integer.compare(getAge(), other.getAge());
        }
        if (compare == 0) {
            compare = getName.compareTo(other.getName());
        }
        return compare;
    }
}

This code snippet can also be used in a lambda expression:

list.sort((o1, o2) -> {
    int compare = Integer.compare(o1.getGroup(), o2.getGroup());
    if (compare == 0) {
        compare = Integer.compare(o1.getAge(), o2.getAge());
    }
    if (compare == 0) {
        compare = o1.getName.compareTo(o2.getName());
    }
    return compare;
});