Storing HashMap inside HashMap

Question

I have tried to store HashMap inside another HashMap but first inserted values changed as second time inserted value.

Here is my code.

   HashMap<String ,HashMap<Integer ,Integer>> map1=new HashMap<>();
   HashMap<Integer ,Integer> map2=new HashMap<>(); 
   map2.put(1,1);
   map2.put(2,1);
   map2.put(3,1); 
   map1.put("1", map2); 
   System.out.println("After Inserting first value   "+map1.entrySet());
   /* OutPut:  After Inserting first value  [1={1=1, 2=1, 3=1}]*/

   map2.clear(); //i cleared map 2 values

   map2.put(4,2); 
   map2.put(5,2); 
   map2.put(6,2); 
   map1.put("2", map2); 
   System.out.println("After Inserting second value   "+map1.entrySet()); 
   /*output :  After Inserting second value    [2={4=2, 5=2, 6=2}, 1={4=2, 5=2, 6=2}]*/

The first time I got output as 1={1=1, 2=1, 3=1}] after inserting second "key, value" [2={4=2, 5=2, 6=2}, 1={4=2, 5=2, 6=2}] I got key "1" values changed to key "2".

clear affect the object that you stored. put does not make a copy, it keep a direct reference to that object. You need to create a new instance. — njzk2
– njzk2, Commented Oct 24, 2013 at 13:59

Ravi K Thapliyal · Accepted Answer · 2013-10-24 14:07:47Z

5

You need to create a new instance of HashMap before the second put() call

// map2.clear();
map2 = new HashMap<Integer, Integer>();

Map#clear() does not give you a new Map instance. Hence, both map1 keys 1 and 2 end up reusing the same instance of map2 and hence you see all the values repeat themselves.

Try printing your Map container after Map#clear() and again after adding new values

map2.clear(); //i cleared map 2 values
System.out.println("After clearing   "+map1.entrySet()); 

map2.put(4,2); 
map2.put(5,2); 
map2.put(6,2); 
System.out.println("After adding new values   "+map1.entrySet());

You can clearly see it affecting key 1 as well.

Output :

After Inserting first value   [1={1=1, 2=1, 3=1}]
After clearing   [1={}]
After adding new values   [1={4=2, 5=2, 6=2}]
After Inserting second value   [2={4=2, 5=2, 6=2}, 1={4=2, 5=2, 6=2}]

edited Oct 24, 2013 at 14:07

answered Oct 24, 2013 at 14:00

Ravi K Thapliyal

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Ivaylo Strandjev · Accepted Answer · 2013-10-24 14:01:39Z

0

You store a reference to the HashMap map2 in map1, not a copy. That is why all subsequent changes to map2 also affect the first map inserted into map1.

answered Oct 24, 2013 at 14:01

Ivaylo Strandjev

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libik · Accepted Answer · 2013-10-24 14:02:00Z

0

You SHOULD NOT clear the map. Note that you adding HashMap map2 which is created with this :

HashMap<Integer ,Integer> map2=new HashMap<>();

It means there is object created with adress memory value. And this address memory value is put into the "bigger" HashMap.

If you clear/change map2, you also clear it in your bigger HashMap, because it is just pointing to the same object!

You have to create new instance, so instead of

map2.clear();

You have to do this:

map2=new HashMap<>();

answered Oct 24, 2013 at 14:02

libik

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Charu Khurana · Accepted Answer · 2013-10-24 14:02:37Z

0

Your first inserted values are changed because first reference is referring to map2 and so does second reference. map2 object is same and it is referred in both places.

I guess what you want is create new object for each map2

answered Oct 24, 2013 at 14:02

Charu Khurana

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Vithushan · Accepted Answer · 2014-10-01 09:12:07Z

0

Try clearing both map. It works.

map1.clear();
map2.clear();

answered Oct 1, 2014 at 9:12

Vithushan

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