public static int getFirstNum(String s) {
    if (s == null) return -1;
    for (char c : s.toCharArray()) {
        if (c>='0' && c<='9') return (int)(c-'0');
    }
    return -1;
}
//调用
String s = "abc24us43";
int n = getFirstNum(s);

//递归
public static int getFirstNum(String s, int index) {
    if (s==null) return -1;
    if (index == s.length) return -1; //递归结束条件
    char c = s.charAt(index);
    if (c>='0' && c<='9') return (int)(c-'0');
    return (s, index+1);
}
//调用
String s = "abc24us43";
int n = getFirstNum(s, 0);

------解决方案--------------------
public class CSDNTest {
	public static String getNum(String str){
		for(int i=0;i<str.length()-1;i++){
			char  ch = str.charAt(i);
			try {
				int j = Integer.parseInt(ch+"");
				return j+"";
			} catch (NumberFormatException e) {
				
			}
		}
		return "-1";
	}
	public static void main(String[] args) {
		System.out.println(CSDNTest.getNum("abd3c"));
	}
}