按内部列表的第一个元素对列表列表进行排序（Java或groovy）

Question

I have a list which contains a list of strings like so...

 ["Running Shoes", "Men's Shoes", "Men's Walking Shoes"]
 ["Team Sports", "Ice Hockey", "Recreational Ice Skates"]
 ["Pro Sports", "Baseball", "Baseball Gloves", "Adult Gloves"]

I want to sort this list by the first element in the sublist, meaning the element at index 0. So in this example after the sort, the 'Running Shoes' element would be first, followed by 'Pro Sports' element and last would 'Team Sports'.

Could you someone give some pointers or share some code? In Java or Groovy.

Answer 1

In groovy:

list.sort { it[0][0] }

If you don't want to mutate the original list, and return the sorted list, you can do

list.sort(false) { it[0][0] }

Answer 2

Well, P comes before R, so if I'm understanding the problem correctly, the order would be 'Pro Sports, Baseball', 'Running Shoes', and 'Team Sports'. You can sort this easily with Groovy like this:

[
     ["Running Shoes", "Men's Shoes", "Men's Walking Shoes"],
     ["Team Sports", "Ice Hockey", "Recreational Ice Skates"],
     ["Pro Sports", "Baseball", "Baseball Gloves", "Adult Gloves"]
].toSorted { a, b -> a[0] <=> b[0] }

The output looks like this:

[
    ['Pro Sports', 'Baseball', 'Baseball Gloves', 'Adult Gloves'],
    ['Running Shoes', "Men's Shoes", "Men's Walking Shoes"],
    ['Team Sports', 'Ice Hockey', 'Recreational Ice Skates']
]

You can also go the long way with a Comparator:

class ListComparator implements Comparator<List> {
    int compare(List obj1, List obj2) {
        obj1[0].compareTo(obj2[0])
    }

    boolean equals(Object obj) {
        this == obj
    }
}

[
     ["Running Shoes", "Men's Shoes", "Men's Walking Shoes"],
     ["Team Sports", "Ice Hockey", "Recreational Ice Skates"],
     ["Pro Sports", "Baseball", "Baseball Gloves", "Adult Gloves"]
].toSorted( new ListComparator()) == [
    ['Pro Sports', 'Baseball', 'Baseball Gloves', 'Adult Gloves'],
    ['Running Shoes', "Men's Shoes", "Men's Walking Shoes"],
    ['Team Sports', 'Ice Hockey', 'Recreational Ice Skates']
]

Hint

You're getting down votes because you can learn this by reading Groovy's documentation.