The first non-repeating character in the character stream (Java)

The first non-repeating character in the character stream (Java)

Topic: Please implement a function to find the first character in the character stream that appears only once. For example, when only the first two characters "go" are read from the character stream, the first character that appears only once is "g". When reading the first six characters "google" from the character stream, the first character that appears only once is "l".
Code:

import java.util.HashMap;
import java.util.Map;

/**
 * @desc 字符流中第一个不重复的字符
 * @author zhaoliang
 * @date 20200415
 */
public class Main50 {

    private Map<Character,Integer> map = new HashMap<>();//保存每个字符出现的次数

    private StringBuilder str = new StringBuilder();//保存字符流

    private int index = 0;//保存字符只出现一次的第一个位置

    //Insert one char from stringstream
    public void Insert(char ch)
    {
        str.append(ch);
        map.put(ch,map.getOrDefault(ch,0)+1);//无取0，有+1
    }
    //return the first appearence once char in current stringstream
    public char FirstAppearingOnce()
    {
        while (index <str.length()){
            if(map.get(str.charAt(index))==1){
                return str.charAt(index);
            }
            index++;
        }
        return '#';
    }
}