Problem in iterating HashMap

[Keerthi Kumar]

Dear All,

I am facing an issue while iterating Hash Map. Through a query I am fetching list of users and list of emails of those users and i am adding those to a hash map as shown below:

HashMap<ArrayList<String>,ArrayList<String>> valueMap = new HashMap<ArrayList<String>,ArrayList<String>>();
ArrayList<String> userIdList = new ArrayList<String>();
ArrayList<String> emailList = new ArrayList<String>();

valueMap = utilDao.getUserEmailIds(con,details);

userIdList = valueMap.get("userList"); //Here it is throwing NullPointer Exception

But the valueMap object contains the required data from the database.
Request to please help me to resolve this issue

[Martin Vajsar]

Why are you using an ArrayList as both a key and a value? The Map can hold multiple key-value pair on its own, you don't need to use lists to be able to put more keys/values into it. (Using a list as a key is very weird. And even if it had a merit in your case I don't see, it would still be dangerous, as the list is mutable; keys whose value can change cannot be used with a Map.)

The exception you're encountering is most likely caused because the utilDao.getUserEmailIds method is returning a null.

I'd suggest to read the Collections tutorial.

[Campbell Ritchie]

Martin Vajsar wrote: . . . keys whose value can change cannot be used with a Map. . . .

Oh, yes they can!

As long as you don’t mind all your references vanishing mysteriously whenever its hash code changes

/**
 * A correctly created mutable class, but there might be some spellling errors.
 * Sorry, you will have to correct them yourself.
 */
public final class MyNumber
{
   private int i;

   public MyNumber(int i)
   {
      this.i = i;
   }

   public int getI()
   {
      return i;
   }

   public void setI(int i)
   {
      this.i = i;
   }

   @Override public int hashCode()
   {
      return i;
   }

   @Override public boolean equals(Object o)
   {
      return this == o ||
             o instanceof MyNumber && //instanceof works for final classes
             ((MyNumber)o).i == this.i;
   }

   @Override public String toSring()
   {
      return "MyNumber is " + i;
   }
}

MyNumber num = new NyNumber(1);
/* New Map, capacity 16 */
Map<MyNumber, String> names = new HashMap<MyNumber, String>(16);
names.put(num, "Campbell");
System.out.println(names.get(num));
num.setI(2);
System.out.println(names.get(num)+ " eek!");