StringBuffer and String pool

java

Solution

Why does the string "I don't get it." get in the string pool.

The `"I don't get it."` string does not get into the interning pool.

One way to verify it is as follows:

StringBuffer str2 = new StringBuffer(" I don't");
StringBuffer str3 = str2.append(" get it.");
String str = new String(str3.toString());
if (str == str.intern()) {
    System.out.println("It was not interned before"); // <<== This is printed
} else {
    System.out.println("It was interned before");
}

If `String`'s content is interned, the call of `intern()` will return a different ("canonical") object. As you can see, the above returns the same object, meaning that the object on which you call `intern()` just became the "canonical" one (i.e. has been interned).

On the other hand, if you remove the `append`, you'd get a different result:

StringBuffer str2 = new StringBuffer(" I don't");
StringBuffer str3 = str2;
String str = new String(str3.toString());
if (str == str.intern()) {
    System.out.println("It was not interned before"); // <<== This is printed
} else {
    System.out.println("It was interned before");
}

Now the string inside `str3` is `" I don't"`. Its copy is already interned, because it's the same as the string constant used in creation of the `str2`.

You can run the first and the second programs side by side to see the difference for yourself.

The reason why `str2 == str3` is `true` has nothing to do with string pools (the slang word is "string interning"). The two are equal because `StringBuffer.append` returns the object on which the `append` is invoked, i.e. `str2`. You do not have a second object - there's only one `StringBuffer` with two references to it. The content of that `StringBuffer` is the concatenation of the `" I don't"` and `" get it."` strings.

Problem

Given the following code: ``` StringBuffer str2 = new StringBuffer(" I don't"); StringBuffer str3 = str2.append(" get it."); if (str2 == str3) { System.out.println("Equal"); } ``` My lecturer says , that in this case both `str2` and `str3` will refer to the same object and the string "I don't get it" will be inserted into the "String pool". I think that I get why `str2` and `str3` will now refer to the same object, but Why does the string "I don't get it." get in the string pool when the `str3` assignment occurs? For instance, if I do : ``` String s = "abcd"; ``` then I know that now the string "abcd" will be inserted into the "String pool" IF its not already there. I would love to get an explanation.

Original source