I’ve came across that kata from @brjavaman and @yanaga to teach lambdas, one of the new features of JDK 8.

There are some unit tests to validate the solution. I’ve found it a good opportunity to exercise the use of lambdas so I decided to solve it. Below is my solution to this kata.

The first method should take the String list and sort all the String elements in ascending (ASCII) order:

    /**
     * This method should take the String List and sort all the String elements in ascending (ASCII) order.
     *
     * @return The sorted values in ascending ASCII order.
     */
    public List getSortedStrings() {
        return values
                .stream()
                .sorted()
                .collect(Collectors.toList());
    }

The other method should take the String list and:

1. filter the elements that contains one or more digits
2. transform (map) the remaining Strings into Integers
3. sort the Integers in ascending order

        
    /**
     * This method should take the String List and:
     * 

     * 
filter the elements that contains one or more digits;
     * 
transform (map) the remaining Strings into Integers;
     * 
sort the Integers in ascending order.
     * 
     *
     * @return
     */
    public List getSortedIntegers() {
        return values
                .stream()
                .filter(s -> s.matches("\\d+"))
                .map(Integer::valueOf)
                .sorted()
                .collect(Collectors.toList());
    }

The last method should take the String list and:

1. filter the elements that contains one or more digits
2. transform (map) the remaining Strings into Integers
3. sort the Integers in descending order

    
    /**
     * This method should take the String List and:
     * 

     * 
filter the elements that contains one or more digits;
     * 
transform (map) the remaining Strings into Integers;
     * 
sort the Integers in descending order.
     * 
     *
     * @return
     */
    public List getSortedDescendingIntegers() {
        return values
                .stream()
                .filter(s -> s.matches("\\d+"))
                .map(Integer::valueOf)
                .sorted(Comparator.reverseOrder())
                .collect(Collectors.toList());
    }

Note that the steps filter the elements that contains one or more digits and transform (map) the remaining Strings into Integers are identical. So I decided to extract the partial Stream into a method with the Extract Method refactoring support on IntelliJ IDEA:

    private Stream integersWithOneOrMoreDigits() {
        return values
                .stream()
                .filter(s -> s.matches("\\d+"))
                .map(Integer::valueOf);
    }

Then I refactored the the solution to use the new extracted method:

    
    public List getSortedIntegers() {
        return integersWithOneOrMoreDigits()
                .sorted()
                .collect(Collectors.toList());
    }

    public List getSortedDescendingIntegers() {
        return integersWithOneOrMoreDigits()
                .sorted(Comparator.reverseOrder())
                .collect(Collectors.toList());
    }

I re-run the tests and they all passed. What do you think about this solution? Do you suggest other ones?