TreeMap的使用实例：统计字符出现次数

import java.util.Map;import java.util.Set;import java.util.TreeMap;/** * TreeMap使用实例 * 将字符串内字符出现的次数进行统计输出 * @author franky * */public class DemoForCalcStr {public static void main(String[] args) {String str = "jkasdljglgfgfgfgjhjhhkjasdjkljowoiiuxncv";printCalcStr(str);//String str2 = "";//printCalcStr(str2);//抛出异常}private static <K, V> void printCalcStr(String str) {//1.判断是否为空字符串，如果为空则抛出空字符串异常if(str.isEmpty()){throw new NullStringException("字符串不能为空");}//2.将字符串分解为字符数组char[] chars = str.toCharArray();//3.new一个TreeMap，遍历字符数组，字符为key,取出value，判断值是否为null//为空则存入初始化value 1，如果不为空则自增1，实现自然排序Map<Character,Integer> map = new TreeMap<Character,Integer>();for(Character c:chars){Integer i = map.get(c);int count = 0;if(i!=null){count = i;}++count;map.put(c, count);}//4.利用StringBuilder输出统计的字符串Set<Character> set = map.keySet();StringBuilder sb = new StringBuilder();for (Character character : set) {Integer integer = map.get(character);sb.append(character+"("+integer+")");}System.out.println(sb.toString());}}/** * 自定义空字符串异常 * @author franky *  */class NullStringException extends RuntimeException{/** *  */private static final long serialVersionUID = 1L;public NullStringException() {super();}public NullStringException(String message, Throwable cause,boolean enableSuppression, boolean writableStackTrace) {super(message, cause, enableSuppression, writableStackTrace);}public NullStringException(String message, Throwable cause) {super(message, cause);}public NullStringException(String message) {super(message);}public NullStringException(Throwable cause) {super(cause);}}