Java 7-String.intern（）行为

Question

I've read this answer about how to check if a string is interned in Java, but I don't understand the following results:

String x = args[0]; // args[0] = "abc";
String a = "a";
String y = a + "bc";
System.out.println(y.intern() == y); // true

But if I declare a string literal:

String x = "abc";
String a = "a";
String y = a + "bc";
System.out.println(y.intern() == y); // false

Besides, without any string literal, the args[0] seems to be directly interned:

// String x = "abc";
String y = args[0];
System.out.println(y.intern() == y); // true (???)
// false if the first line is uncommented

Why does y.intern() == y change depending on whether x is a literal or not, even for the example when the command-line argument is used?

I know literal strings are interned at compile time , but I don't get why it affects in the previous examples. I have also read several questions about string interning, like String Pool behavior , Questions about Java's String pool and Java String pool - When does the pool change? . However, none of them gives a possible explanation to this behaviour.

Edit:

I wrongly wrote that in third example the result doesn't change if String x = "abc"; is declared, but it does.

Answer 1

It is because y.intern() gives back y if the string was not interned before. If the string already existed, the call will give back the already existing instance which is most likely different from y .

However, all this is highly implementation dependent so may be different on different versions of the JVM and the compiler.

Answer 2

Implementation details might differ. But this is exactly the behavior I would expect. Your first case means that commandline arguments are not interned by default. Hence y.intern() returns the reference to y after interning it.

The second case is where the VM automatically interns the literal, so that y.intern() returns the reference to x , which is different from y .

And the last case again happens because nothing is interned by default, so the call to intern() returns the reference to y . I believe it is legal to intern String more aggressively, but this is the minimal behavior required by the spec as I understand it.