Add checkstyle linter

ale_linters/java/checkstyle.vim

@@ -0,0 +1,46 @@
+" Author: Devon Meunier <devon.meunier@gmail.com>
+" Description: checkstyle for Java files
+
+function! ale_linters#java#checkstyle#Handle(buffer, lines) abort
+    let l:patterns = [
+        \ '\v\[(WARN|ERROR)\] .*:(\d+):(\d+): (.*)',
+        \ '\v\[(WARN|ERROR)\] .*:(\d+): (.*)',
+        \]
+    let l:output = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:patterns)
+        let l:args = {
+        \   'lnum': l:match[2] + 0,
+        \   'type': l:match[1] =~? 'WARN' ? 'W' : 'E'
+        \ }
+
+        let l:col = l:match[3] + 0
+        if l:col > 0
+            let l:args['col'] = l:col
+            let l:args['text'] = l:match[4]
+        else
+            let l:args['text'] = l:match[3]
+        endif
+
+        call add(l:output, l:args)
+    endfor
+
+    return l:output
+endfunction
+
+function! ale_linters#java#checkstyle#GetCommand(buffer) abort
+    return 'checkstyle '
+    \ . ale#Var(a:buffer, 'java_checkstyle_options')
+    \ . ' %t'
+endfunction
+
+if !exists('g:ale_java_checkstyle_options')
+    let g:ale_java_checkstyle_options = '-c /google_checks.xml'
+endif
+
+call ale#linter#Define('java', {
+\   'name': 'checkstyle',
+\   'executable': 'checkstyle',
+\   'command_callback': 'ale_linters#java#checkstyle#GetCommand',
+\   'callback': 'ale_linters#java#checkstyle#Handle',
+\})