intern() method

[Prakash Mahto]

Hi
can anyone please explain intern() method by giving examples here?
I am getting confused in it also.

Thanks

[Sunny Bhandari]

intern() method is a part of String class so you can call intern on String objects

The result of calling this method is that the strings are moved to the literal pool.

So next time you create a string literal object, JVM will find it in the heap.

class Test{
   public static void main(String args[]) {
      String str1="abc";
      String str2="def";
      String str3 = str1 + str2;
      str3 = str3.intern();
  
      String str4 = "abcdef";
      System.out.println("str3 == str4 is " + (str3 == str4));
   }
}

Javadoc for intern() method will make it more clear to you.

String java.lang.String.intern()

intern
public String intern()
Returns a canonical representation for the string object.
A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification


Returns:
a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.

[Bert Bates]

It's an interesting topic, but it's not on the exam.

[Sunny Bhandari]

Yes, Bert moved it to proper forum...

[Andreas Svenkson]

Err, just out of curiousity, how can you be sure this isn't on the exam? The method was atleast mentioned in my course-book whichs purpose is solely to prepare for the SCJP and SCJD.

I'm also curious about the following...

If you declare a String like this:

String s = new String("hi");

This will create a new String object in the program space (I believe). Now, if we assume the string "hi" is already in the string pool, and you issue the following:

String s = new String("hi).intern()

Will this in fact make 's' point to the string in the literal pool, and also avoid the creation of the extra String object in the program space?

// Andreas

[Sunny Bhandari]

hi,

extra string will still be created but yes s will point to string in pool.

[Rafal Sojka]

Hi,
I found really interesting webpages that explain many nuances of the String literal pools and the intern() method:
- What is String literal pool?
- String is special (Look at the "String Literal vs. String Object" subsection.)

I hope it helps you a little.
Best regards,
foxrafi

[Jesper de Jong]

Andreas Svenkson wrote:Err, just out of curiousity, how can you be sure this isn't on the exam? The method was atleast mentioned in my course-book whichs purpose is solely to prepare for the SCJP and SCJD.

Bert is one of the authors of the SCJP Study Guide and also worked with Sun on the actual exam itself. You can trust him to know exactly what is and is not on the SCJP exam.

[Prakash Mahto]

Thanks everyone for explaining intern() and making me clear that it is not on the exam

[Andreas Svenkson]

Jesper de Jong wrote:

Andreas Svenkson wrote:Err, just out of curiousity, how can you be sure this isn't on the exam? The method was atleast mentioned in my course-book whichs purpose is solely to prepare for the SCJP and SCJD.

Bert is one of the authors of the SCJP Study Guide and also worked with Sun on the actual exam itself. You can trust him to know exactly what is and is not on the SCJP exam.

Good to know ^^

// Andreas