Underlying mechanism of String pooling in Java?

Question

I was curious as to why Strings can be created without a call to new String(), as the API mentions it is an Object of class java.lang.String

So how are we able to use String s="hi" rather than String s=new String("hi")?

This post clarified the use of == operator and absence of new and says this is due to String literals being interned or taken from a literal pool by the JVM, hence Strings are immutable.

On seeing a statement such as

String s="hi" 

for the first time what really takes place ?

1. Does the JVM replace it like this String s=new String("hi") , wherein an Object is created and "hi" is added to the String literal pool and so subsequent calls such as String s1="hi" are taken from the pool?

2. Is this how the underlying mechanism operates? If so, then is

   String s=new String("Test"); String s1="Test"; 

   the same as

   String s="Test"; String s1="Test"; 

   in terms of memory utilization and efficiency?

3. Also, is there any way by which we can access the String Pool to check how many String literals are present in it, how much space is occupied, etc.?

Answer 1

The Java compiler has special support for string literals. Suppose it did not, then it would be really cumbersome to create strings in your source code, you'd have to write something like:

// Suppose that we would not have string literals like "hi" String s = new String(new char[]{ 'h', 'i' }); 

To answer your questions:

1. More or less, and if you really want to know the details, you'd have to study the source code of the JVM, which you can find at OpenJDK, but be warned that it's huge and complicated.

2. No, those two are not equivalent. In the first case you are explicitly creating a new String object:

   String s=new String("Test"); 

   which will contain a copy of the String object represented by the literal "Test". Note that it is never a good idea to write new String("some literal") in Java - strings are immutable, and it is never necessary to make a copy of a string literal.

3. There's no way I know of to check what's in the string pool.