java8 统计字符串字母个数的几种方法总结(推荐)

//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]);
//		   反编译：Integer value = (Integer)tree.get(Character.valueOf(c[i]));
	   //3,存储判断
		tree.put(c[i],value==null? 1:value+1);
	  }
	  
	  //如果要求结果格式：a(5)b(4)c(3)d(2)e(1)
	  StringBuilder sbu=new StringBuilder();
	  for(Character key:tree.keySet()){
		Integer count=tree.get(key);
		sbu.append(key).append("(").append(count).append(")");
	  }
	  //将sbu转换为字符串
	  System.out.println(sbu.toString());
	}
   
  //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);
  	
  }
  
  //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>();
  	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(" +",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);
  }

public void testName() {
      System.out.println("a");
   }

    public void testName(String ... s) {
        //不传参数,s会默认初始化一个对象
     System.out.println("b");
  }