菜鸟求解答 ，用数组或者HashMap

选择题答案abcd的分布情况，需要统计abcd出现的次数。现从键盘输入一行由abcd构成的答案字符串，
统计并输出abcd出现的次数。(如果未出现，则次数输出0)
输入输出示例：
请输入：
aacdbcc
a2 b1 c3 d1

 /* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner  sc = new Scanner(System.in);
        String s = sc.nextLine();
        char[] s1 = s.toLowerCase().toCharArray();
        int[] a = new int[4];
        for (int i = 0; i < s1.length; i++)
        {
            a[s1[i] - 'a']++;
        }
        for (int i = 0; i < 4; i++)
        {
            System.out.print(String.valueOf((char)('a' + i)) + a[i]  + " ");
        }
    }
}

 用数组就可以了
canner  sc = new Scanner(System.in);
String s = sc.nextLine();
char[] s1 = s.toLower().toCharArray();
int[] a = new int[4];
for (int i = 0; i < s1.length; i++)
{
a[s1[i] - 'a']++;
}
for (i = 0; i < 4; i++)
{
System.out.print((char)(i + 'a') + a[i]  + " ");
}

a2 b1 c3 d1
http://ideone.com/DXljjY