public class Demo {
    public static void main(String[] args) {
        print("hello",10);
    }
    public static void print(String str,int length){
        String result ="";
        if(str!=null) {
            if(str.length() >= length) {
                result = str.substring(0, length);
            } else {
                StringBuffer sb = new StringBuffer(str);
                for (int i = 0;i<length -str.length(); i++) {
                    sb.append("~");//后面用空格填补你能看到吗？
                }
                result = sb.toString();
            }
            System.out.println("结果是："+result);
        } else {
            System.out.println("请输入字符串");
        }
        
    }
}