Question

I have the following code:

HashMap<String, String> hm = new HashMap<String, String>();
hm.put("title", "testTitle");
hm.put("year", "testYear");

String s = "<title> - <year>";

String result = s.replaceAll("<([^>]*)>", hm.get("$1"));

The problem is when I execute it, it returns me this exception:

Exception in thread “main” java.lang.NullPointerException at
java.util.regex.Matcher.appendReplacement(Unknown Source) at
java.util.regex.Matcher.replaceAll(Unknown Source) at
java.lang.String.replaceAll(Unknown Source)

I don’t understand why, because when executed alone, hm.get(“title”) does work, so why with replaceAll it is not?

Thanks in advance.

Answer 1

Consider the following line:

String result = s.replaceAll("<([^>]*)>", hm.get("$1"));

1. First, hm.get("$1") gets executed (and returns null since presumably there’s no entry in hm with the key equal to "$1").
2. Second, s.replaceAll("<([^>]*)>", null) is called and triggers the NPE.

One way to do what you’re trying to do is to use a Matcher and a loop.