Question

I tried to create a list of maps. In the following code, I’m expecting to get

[{start=1,text=ye}, {start=2,text=no}]

however, I only got

[{start=2,text=no}, {start=2,text=no}]

How to avoid overriding the first map? Here is my code:

HashMap mMap = new HashMap();
ArrayList list = new ArrayList();
list.add(new HashMap());
mMap.put("start",1);
mMap.put("text","yes");
list.add(mMap); 
mMap.put("start",2);
mMap.put("text","no");
list.add(mMap); 
System.out.println("Final result: " + list );

thanks!

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As a learner of Java who came from a procedure language background (SAS), I spent quite a few hours learning and experimenting ArrayList, LinkedList, Map, LinkedMap, etc— I coldn’t get it to work. And I don’t understand why with my limited knowledge. Now, these following answers are all excellent! They explained very important data structure in Java, at least for me.

THANK YOU ALL!!!!

Answer 1

You need to create a new HashMap for every entry, instead of reusing the existing one. This would work:

HashMap mMap = new HashMap();
mMap.put("start",1);
mMap.put("text","yes");
list.add(mMap); 
mMap = new HashMap(); // create a new one!
mMap.put("start",2);
mMap.put("text","no");
list.add(mMap); 

also, you can remove the list.add(new HashMap()); as that adds an empty map to your list that is never populated.