Which is efficient in terms of memory: char[] or String?

Question

I am developing an Android application. The primary requirement of the application is that it should be efficient in terms of memory. So, which of these should I proceed with?

String batterylevel;
batterylevel = Float.toString(batteryPct);

or

char batterylevel[];
batterylevel = Float.toString(batteryPct).toCharArray();

Answer 1

In Oracle's JDK a String has four instance-level fields:

• A character array
• An integral offset
• An integral character count
• An integral hash value

That means that each String introduces an extra object reference (the String itself), and three integers in addition to the character array itself. (The offset and character count are there to allow sharing of the character array among String instances produced through the String#substring() methods, a design choice that some other Java library implementers have eschewed.) Beyond the extra storage cost, there's also one more level of access indirection, not to mention the bounds checking with which the String guards its character array.

Strings are immutable. That means once you have created the string, if another process can dump memory, there's no way (aside from reflection) you can get rid of the data before GC kicks in which means waste of memory.

With an array, you can explicitly wipe the data after you're done with it: you can overwrite the array with anything you like.

So as far as I can conclude is char[] is better in terms of memory for your case.