Shouldn't I do `String s = new String("a new string");` in Java, even with automatic string interning?

Question

Ok, this question is an extension of this question

Java Strings: "String s = new String("silly");"

The above question asked the same question as this one, but I have a new doubting point.

According to Effective Java and the answers of above question, we should not do String s = new String("a new string");, because that will create unnecessary object.

I am not sure about this conclusion, because I think Java is doing automatic string interning, which means for a string, anyway there is only one copy of it in the memory.

So let's see String s = new String("a new string");.

"a new string" is already a string which has been created in the memory.

When I do String s = new String("a new string");, then the s is also "a new string". So according to automatic string interning, s should be pointed to the same memory address of "a new string", right?

Then how can we say we create unnecessary objects?

British

There is only one copy of the String unless you do new String(...). Don't do it unless you have to. – Toxoid

+1 for interesting question, never thought about it. However, most of the time you can check this stuff by looking into the differences (or lack of differences) in bytecode between the 2 versions. – Orpington

interning is only automatically performed on String literals. – Goodtempered

Because there's another string, that's what new does. – Bock

What makes you think that new String("a new string") is better than "a new string"? I can't understand why you want to use the former. – Attainment

@DavidHeffernan no I am not saying i prefer and I am just wondering whether the statement we should NOT do that is true or not. Also, if new string("a string") is so bad, why doesn't Java simply remove this constructor? – British

You can do it if you want. Nobody dies if you do. You just create extra objects that aren't needed. – Attainment

@DavidHeffernan: I disagree. A puppy is killed every time this is done. I know this for a fact. – Toxoid

@HovercraftFullOfEels I guess that is meant to be funny. But really, do you believe that it ever really matters that much? Presumably you have evidence. I mean, I wouldn't use new to create strings but I think it would be hard to come up with a scenario where it actually mattered. – Attainment

@DavidHeffernan: nah, it doesn't matter that much, unless the code is creating a lot of String objects in a tight loop, and then object creation can be somewhat expensive. It's just a good habit to avoid creating new Strings so as to not run into this problem inadvertently. – Toxoid

Answer 1

String a = "foo"; // this string will be interned
String b = "foo"; // interned to the same string as a
boolean c = a == b; //this will be true
String d = new String(a); // this creates a new non-interned String
boolean e = a == d; // this will be false
String f = "f";
String g = "oo";
String h = f + g; //this creates a new non-interned string
boolean i = h == a // this will be false
File fi = ...;
BufferedReader br = ...;
String j = br.readLine();
boolean k = a == j; // this will always be false. Data that you've read it is not automatically interned

Cila

Answer 2

You may want to read more about String literal pool in JVMs. Quick googling pointed me to this article: http://www.xyzws.com/Javafaq/what-is-string-literal-pool/3 which seems quite valid.

Also you may be interested in Integer literal pooling as well as other literals pooling in Java.

Coplin

Answer 3

using "=" instead of "=new String" is better since it might result in a single instance instead of multiple instances.

Viewless