如何巧用HashMap一行代码统计单词出现次数详解

public void countBefore8(){
  Map<String,Integer> wordCount= new HashMap<>();
  String[] wordArray= new String[]{"we","are","the","world","we"};
  for(String word: wordArray){
   //如果存在则加1，否则将值设置为1
   if(wordCount.containsKey(word)) {
    wordCount.put(word, wordCount.get(word) + 1);
   }else{
    wordCount.put(word, 1);
   }
  }
 }

default V compute(K key,
   BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
  Objects.requireNonNull(remappingFunction);
  V oldValue = get(key);

  V newValue = remappingFunction.apply(key, oldValue);
  if (newValue == null) {
   // delete mapping
   if (oldValue != null || containsKey(key)) {
    // something to remove
    remove(key);
    return null;
   } else {
    // nothing to do. Leave things as they were.
    return null;
   }
  } else {
   // add or replace old mapping
   put(key, newValue);
   return newValue;
  }
 }

public void countAfter8WithCompute(){
  Map<String,Integer> wordCount= new HashMap<>();
  String[] wordArray= new String[]{"we","are","the","world","we"};
  Arrays.asList(wordArray).forEach(word ->{
   wordCount.putIfAbsent(word,0);
   wordCount.compute(word,(w,count)->count+1);
  });
 }

public void countAfter8WithCompute2(){
  Map<String,Integer> wordCount= new HashMap<>();
  String[] wordArray= new String[]{"we","are","the","world","we"};
  Arrays.asList(wordArray).forEach(word -> wordCount.compute(word,(w, count)->count == null ? 1 : count + 1));
 }

default V merge(K key, V value,
   BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
  Objects.requireNonNull(remappingFunction);
  Objects.requireNonNull(value);
  V oldValue = get(key);
  V newValue = (oldValue == null) ? value :
     remappingFunction.apply(oldValue, value);
  if (newValue == null) {
   remove(key);
  } else {
   put(key, newValue);
  }
  return newValue;
 }

public void countAfter8WithMerge(){
  Map<String,Integer> wordCount= new HashMap<>();
  String[] wordArray= new String[]{"we","are","the","world","we"};
  Arrays.asList(wordArray).forEach(word->wordCount.merge(word, 1, (oldCount, one) -> oldCount + one));
 }

public void countAfter8WithMerge(){
  Map<String,Integer> wordCount= new HashMap<>();
  String[] wordArray= new String[]{"we","are","the","world","we"};
  Arrays.asList(wordArray).forEach(word->wordCount.merge(word, 1, Integer::sum));
 }