方法一：String.toCharArray( ) ; 返回的是char[ ]陣列；

方法二：String.split("") ; 返回的是String [ ] 陣列；

演示的程式碼如下：

import java.util.*;

public class DemoMap {
	public static void main(String[] args) {
		test1();
		System.out.println("===============================");
		test2();
	}

	public static void test1() {
		String str = "abcdabcdab";
		//通過String.split("")方法
		String[] s = str.split("");
 

		System.out.println("s.length==" + s.length);

		Map<String, Integer> map = new TreeMap<String, Integer>();
		for (int i = 1; i < s.length; i++) {
			Integer value = map.get(s[i]);
			if (value != null) {
				map.put(s[i], ++value);
			} else {
				map.put(s[i], 1);
			}
		}
		System.out.println("map==" + map);
	}

	public static void test2() {
		String str = "abcdabcdab";
		//通過String.toCharArray()方法
		char[] c = str.toCharArray();
 

		System.out.println("c.length==" + c.length);

		Map<Character, Integer> map = new TreeMap<Character, Integer>();
		for (int i = 0; i < c.length; i++) {
			Integer value = map.get(c[i]);
			if (value != null) {
				map.put(c[i], ++value);
			} else {
				map.put(c[i], 1);
			}
		}
		System.out.println("map==" + map);

	}
}