If you have two instances of a String, and they are equal, in Java they will share the same memory. How is this implemented under the hood?
EDIT: My application uses a large number of String objects, many of which are identical. What is the best way to make use of Java String constant pool, as to avoid creating custom flyweight implementation?
If you have two instances of a String, and they are equal, in Java they will share the same memory
This is actually not 100% true.
This blog post is a decent explanation of why this is so, and what the String constant pool is.
Look at the source code of java.lang.String (the source for entire java api is part of the JDK).
To summarize: A String wraps a subsequence of a char[]. That backing char[] is never modified. This is accomplished by neither leaking nor capturing this char[] outside the String class. However, several Strings can share the same char[] (see Implementation of String.substring).
There is also the mechanism of interning, as explained in the other answers.
String literals are interned in Java, so there's really only one String object with multiple references (when they are equal, which is not always the case). See the java.net article All about intern() for more details.
There's also a good example/explanation in section 3.10.5 String Literals of the JLS that talks about when Strings are interned and when they'll be distinct.
That's not necessary true. Example:
String s1 = "hello";
String s2 = "hello";
System.out.println(s1 == s2); // true
but:
String s1 = new String("hello");
String s2 = new String("hello");
System.out.println(s1 == s2); // false
Now the second form is discouraged. Some (including me) think that String shouldn't even have a public constructor. A better version of the above would be:
String s1 = new String("hello").intern();
String s2 = new String("hello").intern();
System.out.println(s1 == s2); // true
Obviously you don't need to do this for a constant String. It's illustrative.
The important point about this is that if you're passed a String or get one from a function you can't rely on the String being canonical. A canonical Object satisfies this equality:
a.equals(b) == b.equals(a) == (a == b)
for non-null instances a, b, of a given Class.
To answer your edited question, Sun JVMs have a -XX:+StringCache option, which in my observation can reduce the memory footprint of a String heavy application significantly.
Otherwise, you have the option of interning your Strings, but I would be careful about that. Strings that are very large and no longer referenced will still use memory for the life of the JVM.
Edit (in response to comment): I first found out about the StringCache option from here:
-XX:+StringCache Enables caching of commonly allocated strings.
Tom Hawtin describes some type of caching to improve some benchmarks. My observation when I put it on IDEA was that the memory footprint (after a full garbage collection) went way down over not having it. It is not a documented parameter, and may indeed just be about optimizing for some benchmarks. My observation is that it helped, but I wouldn't build an important system based on it.
Two things to be careful about:
new String("abc") constructor, just use the literal "abc".intern() returns always strings that are pooled.
If your identical Strings come from a fixed set of possible values, then a Type-Safe Enumeration is what you want here. Not only will it reduce your String count, but it will make for a more solid application. Your whole app will know this String has semantics attached to it, maybe even some convenience methods.
My favorite optimizations are always the ones that can be defended as making the code better, not just faster. And 9 times out of 10, replacing a String with a concrete type leads to more correct and self-documenting code.
