一道从字符串中提取字符

public class GoodCode{
	//假如有字符串“6sabcsssfsfs33” ，
	//用最有快速的方法去掉字符“ab3”，不能用java内置字符串方法（indeOf,substring,replaceAll等）？ 
	
	public void isGood(){
		String str = "6sabcsssfsfs33";
		char c1 ='a';
		char c2 ='b';
		char c3 ='3';
		
		char[] c = str.toCharArray();
		StringBuilder sl = new StringBuilder();
		for(char temp:c){
			if(temp != c1 && temp !=c2 && temp !=c3)
				sl.append(temp);
		}
		
		System.out.println(sl.toString());
		
	}
	
	public static void main(String[] args){
		GoodCode g = new GoodCode();
		g.isGood();
	}
}