title: NumberFormatException when attempting to parse string as an integer
body: <pre><code>Exception in thread "main" java.lang.NumberFormatException: For input string: " 400"
          at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
          at java.lang.Integer.parseInt(Integer.java:470)
          at java.lang.Integer.parseInt(Integer.java:514)
          at Library.loadBooks(Library.java:191)
          at UseLibrary.main(UseLibrary.java:102)
  </code></pre>
  
  <p>what's the problem?</p>