遍历HashMap的HashMap

Question

Why is this wrong?

for (Entry<String, HashMap> temp : wordCountforFile.entrySet()) {
        System.out.println(temp.getKey());
        for(Entry<String, Integer> temp1: temp.getValue()){
            //Wrong?
        }
    }

where: HashMap wordCountforFile = new HashMap();

HashMap<String, Integer> wordCount = new HashMap<String, Integer>();

and

wordCountforFile.put("Any String", wordCount);

Answer 1

This is wrong because you didn't specify the generics for the HashMap temp . Also, you should iterate over the entrySet() in the innermost loop if you're expecting a Map.Entry . You should use:

for (Entry<String, HashMap<String, Integer>> temp : wordCountforFile.entrySet()) {
    System.out.println(temp.getKey());
    for(Entry<String, Integer> temp1 : temp.entrySet()){
        // ...
    }
}

---

As a side note, I typically find that if you have a Map of a Map, it means that one of those maps logically represents some kind of entity that can be simplified if you were to move it to its own class. Just a thought.

Answer 2

    HashMap<String, HashMap<String, Integer>> wordCountforFile = new HashMap<String, HashMap<String, Integer>>();

    HashMap<String, Integer> wordCount = new HashMap<String, Integer>();

    wordCountforFile.put("Any String", wordCount);

    for (Map.Entry<String, HashMap<String, Integer>> temp : wordCountforFile.entrySet()) {
        System.out.println(temp.getKey());
        for(Map.Entry<String, Integer> temp1: temp.getValue().entrySet()){

        }
    }
 }

wordCountForFile is not defined with generics. without generics an explicit type casting should be done.

   for(Map.Entry<String, HashMap<String, Integer>> temp : ((Set<Map.Entry<String, HashMap<String, Integer>>>) wordCountforFile.entrySet()))

But this is not always safe because it can cause ClassCastException if an entry is not of type String & HashMap. It is always better to use generics if the type of key and value in a map is known to avoid ClassCastException.

temp.getValue() returns a map of String & Integer.If you want to iterate through each entry in this map use temp.getValue().entrySet()