java8 streamList转换使用详解

Map<String, Object>; resultMap = lists
    .stream()
    .flatMap(map ->map.entrySet().stream())
    .collect(Collectors.toMap(e ->e.getKey(), e->e.getValue(),(a,b)->a)));

Map<String, Object> map = maps.stream()
    .map(Map::entrySet)
    .flatMap(Set::stream)
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,(a,b)->a)));

Map<String, Object>; resultMap = lists
    .stream()
    .collect(Collectors.toMap(Entry::getProtity, Entry::getProtity,(a,b)->a)));

//方式1
  public static void letterCount1(String s) {
   s=s.replaceAll(" +", "");
   //1,转换成字符数组
   char c[]=s.toCharArray();
  
   Map<Character, Integer> tree=new TreeMap<Character, Integer>();
   for (int i = 0; i < c.length; i++) {
 //第一次：a,1
 //第二次：a,2 
   //2,获取键所对应的值
 Integer value=tree.get(c[i]);
   //3,存储判断
 tree.put(c[i], value==null? 1:value+1);
   }
   System.out.println(tree);
 }
   
  //方式2 使用流
  //这个在测试特殊字符,比如\  \n时,他的顺序会不对,这个是Map造成的
  //解决办法使用TreeMap
  public static void letterCount2(String s) {
   s=s.replaceAll(" +", "");
   TreeMap<String, Long> result = Arrays.stream(s.split(""))
             .sorted()
//      .collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
       .collect(Collectors.groupingBy(Function.identity(),TreeMap::new,Collectors.counting()));
    System.out.println(result);
   
  }
  //或者
 public static void letterCount2_1(String s) throws Exception {
 s=s.replaceAll(" +", "");
 Stream<String> words = Arrays.stream(s.split(""));
 Map<String, Integer> wordsCount = words.collect(Collectors.toMap(k -> k, v -> 1,
                            (i, j) -> i + j));
 System.out.println(wordsCount);
 }
  
  //方式3 使用Collections.frequency
  //其实就是字符串变成集合存每个字串,把每个字串循环跟集合比较
  public static void letterCount3(String s) {
   s=s.replaceAll(" +", "");
   List<String> list=Arrays.asList(s.split(""));
   Map<String,Integer> map=new TreeMap<String, Integer>();
   for (String str : list) {
   map.put(str, Collections.frequency(list, str));
 }
   System.out.println(map);
  }
  
  //方式4
  public static void letterCount4(String s) {
   s=s.replaceAll(" +", "");
   String[] strs = s.split("");
   Map<String,Integer> map=new TreeMap<String, Integer>();
   for (String str : strs) {
   map.put(str, stringCount(s, str));
 }
   System.out.println(map);
  }
  
  //方式5
  public static void letterCount5(String s) {
   s=s.replaceAll(" +", "");
   String[] strs = s.split("");
   Map<String,Integer> map=new TreeMap<String, Integer>();
   for (String str : strs) {
   map.put(str, stringCount2(s, str));
 }
   System.out.println(map);
  }
  
  //巧用split
  public static int stringCount(String maxstr, String substr) {
 // 注意
 // 1.比如qqqq,没有找到,则直接返回这个字符串
 // 2.比如qqqjava,末尾没有其他字符,这时也不会分割,所以可以添加一个空格
 // 3.java11开头没有字符，没有关系，自动空填充
 // 4.对于特殊字符,要注意使用转义符
 int count = (maxstr + " ").split(substr).length - 1;
 // System.out.println("\"" + minstr + "\"" + "字符串出现次数：" + count);
 return count;
 }
 
  //如果要不区分大小写，则compile(minstr,CASE_INSENSITIVE)
 public static int stringCount2(String maxstr, String substr) {
 int count = 0;
 Matcher m = Pattern.compile(substr).matcher(maxstr);
 while (m.find()) {
  count++;
 }
    return count;
 }

 public static void wordStringCount(String s) {
   //这里开始是字符串,分割后变成字符串流
    Map<String, Long> result = Arrays.stream(s.split("\\s+"))
                  .map(word -> word.replaceAll("[^a-zA-Z]", ""))
                        .collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
    System.out.println(result);   
  }

 //统计一个文本中单词的个数
  public static void wordFileCount(String path) throws IOException{
   //这里一开始字符串流
   //先分割
   //在变成字符流
   //在筛选
   Map<String, Long> result = Files.lines(Paths.get(path),Charset.defaultCharset())
            .parallel()
    //字符串流--分割--字符串流
   .flatMap(str->Arrays.stream(str.split(" +"))) 
   .map(word -> word.replaceAll("[^a-zA-Z]", ""))
   //去掉空
   .filter(word->word.length()>0) 
   .collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
   System.out.println(result);
  }
 //优化:更精确的是根据非单词来分组
  public static void wordFileCount0(String path) throws IOException{
   Map<String, Long> result = Files.lines(Paths.get(path),Charset.defaultCharset())
    .parallel()
    //字符串流--分割--字符串流
    .flatMap(str->Arrays.stream(str.split("[^a-zA-Z]+"))) 
    //去掉\n
    .filter(word->word.length()>0) 
    .collect(Collectors.groupingBy(Function.identity(),Collectors.counting()));
   System.out.println(result);
  }