数组列表 <Integer> 抛出NullPointerException

Question

I have a method that returns an ArrayList <Integer> . When i return it and if its null i get a nullPointerException . Now i want to have some sort of validation and prevent the null pointer exception hapenning.

These are what i tried, but i still get the nullpointerexception

Map<Integer,ArrayList<Integer>> personMap = new HashMap<Integer,ArrayList<Integer>>();

public ArrayList<Integer> method(int departmentID){
  if (personMap.get(new Integer(departmentID)) !=null)
     return personMap.get(new Integer(departmentID));
  else 
     return null;


}

I still get the nullPointerException .

approach 2.

public ArrayList<Integer> method(int departmentID){
  if (personMap.get(new Integer(departmentID)).isEmpty()) return null;
  else return personMap.get(new Integer(departmentID));


}

I still get the nullpointerexception . Can someone help me solve this ?

From a Testing class i will call the method as follows;

Class1 cl = new Class1();

if (c1.method(112121)) // do something

Answer 1

First of all, your Map declaration won't compile: -

Map<Integer,ArrayList<Integer>> personMap = new ArrayList<Integer,ArrayList<Integer>>();

You should have new HashMap<Integer,ArrayList<Integer>>(); on the RHS.

Secondly, your if statement, will again not compile: -

if (c1.method(112121))

Your method() returns an ArrayList , whereas in Java , you can only have boolean value as a condition in if .

---

Modification Needed: -

You should change your if to: -

if (c1.method(112121) != null)

and use first method.

In fact, you don't need an if-else there. You can simply return the value from the map: -

public ArrayList<Integer> method(int departmentID){ 
     return personMap.get(new Integer(departmentID));
}

Now, this will return null , if the key is not in the HashMap , else it will return the corresponding ArrayList .

Answer 2

If a map doesn't have an entry for a key, get() returns null .
If you then call .isEmpty() on that you get an NPE.

Instead test for map.get(key) == null or better, !map.contains(key)

Answer 3

Your first approach is the same as:

public ArrayList<Integer> method(int departmentID){
    return personMap.get(new Integer(departmentID));
}

and the second one is clearly able to throw a NPE:

public ArrayList<Integer> method(int departmentID){
  if (personMap.get(new Integer(departmentID)).isEmpty()) return null;
  else return personMap.get(new Integer(departmentID));
}

If personMap.get(new Integer(departmentID)) returns null (which it does, when the map has no entry for this key) then calling a method on that throws an NPE:

if (personMap.get(new Integer(departmentID)).isEmpty())

Answer 4

First of all I would NOT recommend returning null from a method. You should return empty ArrayList.

Map<Integer,ArrayList<Integer>> personMap = new HashMap<Integer, ArrayList<Integer>> ();

public ArrayList<Integer> someMethod(int departmentID){

    if (personMap.get(departmentID) !=null)
        return personMap.get(departmentID);
    else
        return new ArrayList<Integer>();

}