Is there a way to implement an array of LinkedHashMap?

问题

I am trying to implement an array of LinkedHashMap but I don't know if it's possible...

For the moment my code looks like as follows :

public class LHMap {

    public static void main(String[] args) {

     LinkedHashMap<String, Integer>[] map = null;

  for (int i = 0; i < 5; i++) {
         map[i] = new LinkedHashMap<String, Integer>();
  }

  map[0].put("a", 0);
  System.out.println(map[0].get("a"));

 }
}


System.out.println(extracted(map)[0].get("a"));

returns a "NullPointerException"...

Have you got any idea how to implement this?

EDIT : 1. erase extracted(), 2. table->array

回答1:

I don't know what your extracted() method is trying to do. You're getting a NullPointerException because you're passing map into extracted and then you're returning it immediately. In your example, you're passing in null and then returning it. You can't find the i^th subscript (or any subscript) on a null array.

Instead of an array of LinkedHashMap, would a List work?

List<Map<String, Integer>> listOfMaps = new ArrayList<Map<String, Integer>>();

for(int i = 0; i < 5; i++) {
   listOfMaps.add(new LinkedHashMap<String, Integer>());
}

listOfMaps.get(0).put("a", 0);
System.out.println(listOfMaps.get(0).get("a"));

EDIT

You cannot instantiate an array with generics, by the way. I'd suggest going with the list if you want the type-safety. Otherwise you'd have to make do with:

LinkedHashMap<String, Integer>[] map = new LinkedHashMap[5];

This will give you warnings, however.

回答2:

map[i] = new LinkedHashMap<String, Integer>();

That will cause an NPE because you never initialize map, or rather, you explicitly initialize it to NULL:

LinkedHashMap<String, Integer>[] map = null;

EDIT:

You need to instantiate the array, e.g.:

map = (LinkedHashMap<String, Integer>[]) new LinkedHashMap[5];

Although you'll get an "unchecked conversion" warning with that code. Also check this for the generic array creation issue (thanks for those who pointed it out).

来源：https://stackoverflow.com/questions/4128923/is-there-a-way-to-implement-an-array-of-linkedhashmap

标签

java

nullpointerexception

linkedhashmap