Why does appending “” to a String save memory?

Question

I used a variable with a lot of data in it, say String data. I wanted to use a small part of this string in the following way:

this.smallpart =          


        

Answer 1

When you use substring, it doesn't actually create a new string. It still refers to your original string, with an offset and size constraint.

So, to allow your original string to be collected, you need to create a new string (using new String, or what you've got).

Answer 2

Just to sum up, if you create lots of substrings from a small number of big strings, then use

   String subtring = string.substring(5,23)

Since you only use the space to store the big strings, but if you are extracting a just handful of small strings, from losts of big strings, then

   String substring = new String(string.substring(5,23));

Will keep your memory use down, since the big strings can be reclaimed when no longer needed.

That you call new String is a helpful reminder that you really are getting a new string, rather than a reference to the original one.

Answer 3

As documented by jwz in 1997:

If you have a huge string, pull out a substring() of it, hold on to the substring and allow the longer string to become garbage (in other words, the substring has a longer lifetime) the underlying bytes of the huge string never go away.