ArrayList of objects vs Arraylist of HashMaps

Question

I have a file containing approximately 10,000 json dumps. Each json has about 20 fields, out of which only 5 are of use to me. I need to iterate over the file, parse each json and store the relevant elements for further processing.

In Java what will be an efficient data structure to store the relevant json fields. I am confused between an ArrayList of Objects (for which I will create a bean to hold the various fields) and an ArrayList of HashMaps (where each of the relevant json fields will be stored as key value pairs).

Which of the two is better in regards to memory usage and computation?

Utsav Gupta · Accepted Answer · 2014-11-22 04:01:25Z

2

It depends on your use case. If you are going to use all the 5 fields as it is . Like putting it in a database, or displaying in UI, then the first approach (array of beans). If you are going to use the fields selectively, (1 out 5 fields here, and another of 5 fields there) then the sceond approach is better (array of hash maps).

answered Nov 22, 2014 at 4:01

Utsav Gupta

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MeBigFatGuy · Accepted Answer · 2014-11-22 04:01:27Z

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a List of Beans has better type safety and readability. Use that until you can prove that there is a problem with that approach.

answered Nov 22, 2014 at 4:01

MeBigFatGuy

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b4hand · Accepted Answer · 2014-11-22 04:10:32Z

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If you have a fixed set of fields an Object will be smaller than a HashMap.

The HashMap has to store the keys as Strings for each instance. Also accessing the fields in an Object will be much faster. Accessing a field in an Object is a single byte code operation. Accessing a HashMap requires computing the hash for the given field and then access an element in an array.

Regardless, performance is probably not a dominant factor for this particular problem and using an Object will probably be more readable.

edited Nov 22, 2014 at 4:10

answered Nov 22, 2014 at 4:02

b4hand

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