String.intern()

There have been lots of posts regarding String literals and String objects.Yet another one to reveal some more depth.

We knew the String literals first lookup to String constant pool if it exists, before actually creating a new instance whereas string object creation with "new" always creates a new String and add that to the string constant pool.

So, how can we force the String objects created with "new" to lookup to Constant pool before actually creating new instance?

There comes the native method intern

The source comment says...


/**
* Searches an internal table of strings for a string equal to this String.
* If the string is not in the table, it is added. Answers the string contained
* in the table which is equal to this String. The same string object is always
* answered for strings which are equal.
*
* @return the interned string equal to this String
*/

public native String intern();



This can be illustrated via following example,

String s="TEST";
String t="TEST";
String k=new String("TEST").intern();
String p=new String("TEST");
if(s==t)
out.println("s=t");
if(s==k && k==t)
out.println("s=k & k=t");
if(s==p || t==p)
out.println("s=p Or t=p");

if(p==k)
out.println("p=k");

Output:
s=t
s=k & k=t


This shows, the String objects created with explicit "new" does not lookup the String constant pool unless specified as intern!