About string concatenation behaviours

Question

I understand that, given the immutability of strings, something like

String a="";
for(int i=0;i++<9;)
    a+=i;

is highly inefficient, because initially a string is instantiated and put in the string pool, then with a+=i a new string is created (0 in the first loop), referenced by a and the previous one is now eligible for garbage collection. And this happens nine times.

A better way is using StringBuilder:

StringBuilder a=new StringBuilder("");
for(int i=0;i++<9;)
    a.append(i);

But what happens behind the scenes when I instantiate the string with the new keyword?

String a=new String("");
for(int i=0;i++<9;)
    a+=i;

I know that in this case a doesn't get interned (it's not in the string pool), but is it still immutable? What does the a+=i instruction do in this time? Is the behaviour identical to my first example?

Answer 1

Only String literals, or Strings you call the intern() method on are put in the String pool. Concatenation does not automagically intern a String, so your examples will be identical with regards to the String pool.

String abc = new String("abc"); //"abc" is put on the pool
abc += "def"; //"def" is put on the pool, but "abcdef" is not
String xyz = "abcdefghi".substring(0, 6).intern(); //"abcdef" is now added to the pool and returned by the intern() function
String xyz = "test"; //test is put on the pool
xyz += "ing"; //ing is put on the pool, but "testing" is not

And to expand on this, note that the String constructor does not intern (or not intern) a String automatically. The use of String literals (Strings in quotation marks in your code) is what causes the String to be on the String pool.

String abc = "abc"; //"abc" is in the pool
String def = "def"; //"def" is in the pool
String str1 = new String(abc + def); //"abcdef" is not in the pool yet
String str2 = new String("abcdef"); //"abcdef" is on the pool now

Also note that the String copy constructor is almost never used, since Strings are immutable anyway.

For more info, read the answers here, here, and here.