Strings are immutable - that means I should never use += and only StringBuffer?

Question

Strings are immutable, meaning, once they have been created they cannot be changed.

So, does this mean that it would take more memory if you append things with += th

Answer 1

Exactly. You should use a StringBuilder though if thread-safety isn't an issue.

As a side note: There might be several String objects using the same backing char[] - for instance whenever you use substring(), no new char[] will be created which makes using it quite efficient.

Additionally, compilers may do some optimization for you. For instance if you do

static final String FOO = "foo";
static final String BAR = "bar"; 

String getFoobar() {
  return FOO + BAR; // no string concatenation at runtime
}

I wouldn't be surprised if the compiler would use StringBuilder internally to optimize String concatenation where possible - if not already maybe in the future.