字符串转成整数

public class test {

public static void main(String[] args) {
    int a = String2Int("+-123");
    System.out.println(a);
}

public static int String2Int(String str){
    if(str==null || str.length()==0){
        return 0;
    }
    boolean flag = true;
    char[] chars = str.toCharArray();
    int index = 0;
    if(chars[0]=='+'){
        index++;
    }
    if(chars[0]=='-'){
        flag = false;
        index++;
    }
    int res = 0;
    for(int i=index; i<str.length(); i++){
        if(chars[i]<'0' || chars[i]>'9'){
            return 0;
        }
        res *= 10;
        res += chars[i]-'0';
    }
    if(!flag){
        res *=-1;
    }
    return res;
}

}