Jun 11

Reducing the size of List in Java

I had a List that had around 70000 objects in it and I needed only the first 15000. I was about to create a new empty array list and iterate 15000 times over the current one in order to get a smaller list. That would have been a mess.

I searched a bit and realized there was an easier (and I’m hoping it’s more efficient) way to reduce the size of a list.

//templist has 70000 objects in it

tempList.subList(15000 – 1, tempList.size()-1).clear();

//templist now has the first 15000 objects in it

//15000 – 1 because the positions start from 0


Jun 11

Fetch Multiple Objects by key in JDO using getObjectsById

On Google Appengine I needed to fetch multiple objects, the key to which is present with me. I tried using PersistenceManager#getObjectsById(Collection) method and passed a list of keys to it. But that threw an exception. On further probing I realized this is not how #getObjectsById works.

There might be other ways of doing it but this is how I’ve done it :

List list = new ArrayList();

for(Key key : <iterate-over-list-of-keys>){

list.add(persistenceManager.newObjectIdInstance(<class-name-here>, key));


Collection objects = persistenceManager.getObjectsById(list);

Hopefully this helps you use the getObjectsById method in JDO on google appengine.

Apr 11

Convert json string to Map in Java using Gson

I’ve been using Gson from quite some time now. It’s a Java library developed by Google that is used to convert JSON to Java objects and vice versa. You can read more about Gson here.

I came across this json string which I had to convert into a Map. Here’s how you do it:

Type type =
 new TypeToken<Map<String, String>>(){}.getType();
Map<String, String> map =
 gson.fromJson("{'key1':'123','key2':'456'}", type);

Hope that helps you convert a json string into a java Map.