Lambda expression to change string - Java

Question

String[] planets = new String[] { "Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" }; 

I have a simple array of planet names, and I want to undo every name that has been added to the array (rather than string order).
I keep struggling with lambda syntax:

 Arrays.sort(planets, (first, second) -> new StringBuilder(first).reverse().toString()); System.out.println(Arrays.toString(planets)); 

I am currently getting:

Several markers on this line

• The reverse() method in type StringBuilder not applicable to arguments (String)
• Type mismatch: cannot convert from String to int

Answer:

Thanks everyone for the suggestions ... asList made me close. I was still sorting wrong.

So, I need to find a way to make array.sort work because it was the destination (take array.sort and lambda it).

I pinged an instructor to ask for clarification here, but I'm running out of time.

Here is my work trying to save array.sort

  // lambda expression , long to short System.out.println("5) Sorted by length (descending):"); Arrays.sort(planets, (first, second) -> second.length() - first.length()); System.out.println(Arrays.toString(planets)); // lambda expression , reverse the name , then sort in ascending order // hint use new StringBuilder(first).reverse().toString() to reverse the string System.out.println("6) Sorted in dictionary order of the reversed name (ascending)"); Arrays.sort(planets, (first, second) -> new StringBuilder(first).reverse().toString().compareTo(second)); System.out.println(Arrays.toString(planets)); // lambda expression , reverse the name , then sort in descending order System.out.println("7) Sorted in dictionary order of the reversed name (descending)"); Arrays.sort(planets, (first, second) -> new StringBuilder(second).reverse().toString().compareTo(first)); System.out.println(Arrays.toString(planets)); 

What I can’t figure out for me here for life is that all 3 expressions give me exactly the same result ... each of them gives me the original array, sorted the longest.

This is my result:

5) Sort by length (descending): [Neptune, Mercury, Jupiter, Uranus, Saturn, Venus, Earth, Mars]

6) Sorting in the dictionary the reverse name order (ascending) [Neptune, Mercury, Jupiter, Uranus, Saturn, Venus, Earth, Mars]

7) Sorting in the dictionary the reverse name order (descending) [Neptune, Mercury, Jupiter, Uranus, Saturn, Venus, Earth, Mars]

Answer 1

An alternative solution that does not use the Stream API and updates the lines in place:

 Arrays.asList(planets).replaceAll(s -> new StringBuilder(s).reverse().toString()); 

Demo:

 String[] planets = new String[] { "Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune" }; Arrays.asList(planets).replaceAll(s -> new StringBuilder(s).reverse().toString()); System.out.println(Arrays.toString(planets)); // [yrucreM, suneV, htraE, sraM, retipuJ, nrutaS, sunarU, enutpeN] 

Answer 2

This will do it for you:

 public static void main(String[] args) { String[] arr = {"earth", "mars", "mercury"}; List<String> ll = Arrays.stream(arr).map(s -> new StringBuilder(s).reverse().toString()).collect(Collectors.toList()); System.out.println(ll); } 

O / P:

 [htrae, sram, yrucrem] 

Or, if you just want to print, use this:

 String[] arr = {"earth", "mars", "mercury"}; Arrays.stream(arr).map(s -> new StringBuilder(s).reverse().toString()).forEach(System.out::println); 

O / P:

 htrae sram yrucrem 

Answer 3

Arrays.sort requires comparison as a second argument. It can be implemented using a lambda expression that takes two strings as parameters (in your case) and returns an integer (this is a SAM requirement defined in the Comparator interface).

 (first, second) -> new StringBuilder(first).reverse().toString() 

is a lambda expression that takes two strings (the type is output by the compiler), but it returns a string, not an integer, so the error is:

 java: incompatible types: cannot infer type-variable(s) T (argument mismatch; bad return type in lambda expression java.lang.String cannot be converted to int) 

I have a simple array of planet names, and I want to undo every name that has been added to the array

In this case, you can simply use Arrays.setAll to modify the array:

 Arrays.setAll(planets, i -> new StringBuilder(planets[i]).reverse().toString());