Java multikeymap put-method

I want to implement the put and get-methods for a multikeymap in Java with two keys pointing to a va开发者_开发问答lue. I have written the put-method like this:

public ValueType put(KeyTypeA key1, KeyTypeB key2, ValueType value) {

 HashMap<KeyTypeB, ValueType> mappi = outerMap.get(key1);
 ValueType oldvalue;
 if (mappi.containsKey(key2)) {
  oldvalue = mappi.get(key2);
 } else {
  oldvalue = null;
 }

However, when I test this method (I have initialized mappi properly as an instance of my own multikeymap-type) with

mappi.put("xxxxx", 13, "xxxxx");

I get a NullPointerException at

if (mappi.containsKey(key2)) {

What could cause the exception? In my class MultiKeyMap I initialize the multikeymap object in the constructor, so that should be ok.

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If it's not homework, you could use MultiKeyMap from Apache Commons Collections, instead of redoing it from scratch.

To answer your question, I suspect the fault lays in HashMap<KeyTypeB, ValueType> mappi = outerMap.get(key1);. If key1 has no value in outerMap, then mappi will be null, causing the NullPointerException