Java HashMap<String, ArrayList<Object[]>> how to add an empty array?

Question

Im using HashMap in my application and sometimes I need to add a key (String) with a null value (emty array list of objects). But Netbeans 7.4 says:

Exception in thread "main" java.lang.NullPointerException
    at test.Version.main(Version.java:35)
Java Result: 1

to this code:

        HashMap<String, ArrayList<Object[]>> d = null;

        ArrayList<Object[]> a;
        a = new ArrayList<>();

        d.put("key1", a);

I dont want to use a MultiMap. Is there any other way how to solve it easily?

You haven't initialized d to an instance of HashMap. You're trying to call put on a null reference. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Oct 18, 2013 at 13:21
d is null! Please add row d = new Hashmap<String, ArrayList<Object[]>> — Joe Taras
– Joe Taras, Commented Oct 18, 2013 at 13:22
Just initialize d, for example d = new HasMap<String, ArrayList<Object>>(); — Tafari
– Tafari, Commented Oct 18, 2013 at 13:24

Bucket · Accepted Answer · 2013-10-18 13:28:42Z

5

You're getting a NullPointerException because d is null, and you try to de-reference it with your call to d.put("key1", a).

You can fix this by changing your initialization of d to

HashMap<String, ArrayList<Object[]>> d = new HashMap<String, ArrayList<Object[]>>();

Now that d isn't null, you can use the methods native to HashMap, like d.put("key1", a).

edited Oct 18, 2013 at 13:28

answered Oct 18, 2013 at 13:22

Bucket

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Pignic · Accepted Answer · 2013-10-18 13:23:13Z

4

Map<String, List<Object[]>> d = new HashMap<String, List<Object[]>>;
List<Object[]> a = new ArrayList<Object[]>();
d.put("key1", a);

If d is null, then -> NullPointerException ;)

answered Oct 18, 2013 at 13:23

Pignic

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anubhava · Accepted Answer · 2013-10-18 13:23:08Z

2

You can't call map.put without creating an instance of it at first, your map is still null.

You need to instantiate it first:

Map<String, ArrayList<Object[]>> d = new HashMap<String, ArrayList<Object[]>>();

and then:

d.put("key1", a);

answered Oct 18, 2013 at 13:23

anubhava

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Ilya Neborako · Accepted Answer · 2013-10-18 13:26:28Z

2

You wrote:

HashMap<String, ArrayList<Object[]>> d = null;

and then you try to put element to a null:

d.put("key1", a);

You must first declare instance of HashMap:

 HashMap<String, ArrayList<Object[]>> d = new HashMap<String, ArrayList<Object[]>>();

answered Oct 18, 2013 at 13:26

Ilya Neborako

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