pairs challenge improvement

http://codingbat.com/prob/p126332

public Map<String, String> pairs(String[] strings) {
  Map map=new HashMap();
  for(String str:strings){
    int len=str.length();
    map.put(str.substring(0,1), str.substring(len-1,len));
  }
  return map;
}

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xpected      Run            
pairs(["code", "bug"]) → {"b": "g", "c": "e"}      {"b": "g", "c": "e"}      OK      
pairs(["man", "moon", "main"]) → {"m": "n"}      {"m": "n"}      OK      
pairs(["man", "moon", "good", "night"]) → {"g": "d", "m": "n", "n": "t"}      {"g": "d", "m": "n", "n": "t"}      OK      
pairs([]) → {}      {}      OK      
pairs(["a", "b"]) → {"a": "a", "b": "b"}      {"a": "a", "b": "b"}      OK      
pairs(["are", "codes", "and", "cods"]) → {"a": "d", "c": "s"}      {"a": "d", "c": "s"}      OK      
pairs(["apple", "banana", "tea", "coffee"]) → {"a": "e", "b": "a", "c": "e", "t": "a"}      {"a": "e", "b": "a", "c": "e", "t": "a"}      OK      
other tests
OK      

I wrote as above and passing all steps. Any improvements to my code. please advise