JAVA实现字符串问题

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class DeleteChars {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String s1=null,s2=null;
		try {
			s1 = br.readLine();
			s2 = br.readLine();
		} catch (IOException e) {
			System.out.println("error!");
			e.printStackTrace();
		}
		if(s1==null||s2==null){
			System.out.println("error!");
			return;
		}
		int len1 = s1.length();
		int len2 = s2.length();
		boolean[] flag = new boolean[len1];
		for(int i = 0;i<len1;i++){
			flag[i] = false;
		}
		for(int i=0;i<len2;i++){
			for(int j=0;j<len1;j++){
				if(s2.charAt(i)==s1.charAt(j)){
					flag[j] = true;
				}
			}
		}
		for(int i=0;i<len1;i++){
			if(!flag[i])System.out.print(s1.charAt(i));
		}
	}
}