Why doesn't Ruby have a real StringBuffer or StringIO?

I looked at the ruby documentation for StringIO, and it looks like what you want is StringIO#string, not StringIO#to_s

Thus, change your code to:

s = StringIO.news << 'foo's << 'bar's.string

Like other IO-type objects in Ruby, when you write to an IO, the character pointer advances.

>> s = StringIO.new=> #<StringIO:0x3659d4>>> s << 'foo'=> #<StringIO:0x3659d4>>> s << 'bar'=> #<StringIO:0x3659d4>>> s.pos=> 6>> s.rewind=> 0>> s.read=> "foobar"

I did some benchmarks and the fastest approach is using the String#<< method. Using StringIO is a little bit slower.

s = ""; Benchmark.measure{5000000.times{s << "some string"}}=>   3.620000   0.100000   3.720000 (  3.970463)>> s = StringIO.new; Benchmark.measure{5000000.times{s << "some string"}}=>   4.730000   0.120000   4.850000 (  5.329215)

Concatenating strings using the String#+ method is the slowest approach by many orders of magnitude:

s = ""; Benchmark.measure{10000.times{s = s + "some string"}}=>   0.700000   0.560000   1.260000 (  1.420272)s = ""; Benchmark.measure{10000.times{s << "some string"}}=>   0.000000   0.000000   0.000000 (  0.005639)

So I think the right answer is that the equivalent to Java's StringBuffer is simply using String#<< in Ruby.