Re: String.intern() (Was: create a string of <n> equal chars <c>)

Daniel Pitts <>
Thu, 15 Jul 2010 11:23:18 -0700
On 7/15/2010 12:43 AM, Kevin McMurtrie wrote:

In article<axp%n.70738$Lj2.7209@newsfe05.iad>,
  Daniel Pitts<> wrote:

On 7/14/2010 9:42 AM, Kevin McMurtrie wrote:

In article<>,
   Roedy Green<> wrote:

On 13 Jul 2010 15:01:35 GMT, Andreas Leitgeb
<> wrote, quoted or indirectly quoted
someone who said :

It seems so basic that I can't believe such a feature wasn't in
the standard library:

it is part of the common11 tools for JDK 1.1+

The method is called StringTools.rep

       * Produce a String of a given repeating character.
       * @param c the character to repeat
       * @param count the number of times to repeat
       * @return String, e.g. rep('*',4) returns "****"
       * @noinspection WeakerAccess,SameParameterValue
      public static String rep( char c, int count )
          if ( c == ' '&& count<= SOMESPACES.length() )
              return SOMESPACES.substring( 0, count );
          char[] s = new char[count];
          for ( int i = 0; i< count; i++ )
              s[ i ] = c;
          return new String( s ).intern();

       * used to efficiently generate Strings of spaces of varying
      private static final String SOMESPACES = " ";

Why use intern() on the second case? It's has always been undocumented
where the pool storage is and what the cost of using it is. The only
time I use that method is when generating keys for a Properties class.

Why even use it there? I don't think I've ever seen a legitimate case
for using intern(). The *closest* I've seen to a valid use is someone
wanted to use it for synchronization based on a String key.

Sample code:

import java.util.Map;
import java.util.Properties;

public class Foo
   private static void runLookupTest (final Properties p)
     int c= 0;
     for (int i= 0; i< 10000000; ++i)
       c+= (p.get("") != null) ? 1 : 0;
       c+= (p.get("") != null) ? 1 : 0;
       c+= (p.get("") != null) ? 1 : 0;
     if (c != 30000000)
       throw new RuntimeException ("Bad test: " + c);

   private static void runTestCycle (boolean warmup) throws IOException
     //Create Properties with string keys that are not internalized
     final Properties p= new Properties();
     p.load(new StringReader( "\n"
                 + "\n"
                 + "\n"));

     //Internalized lookup of non-internalized keys
     final long start1= System.nanoTime();
     final long end1= System.nanoTime();

     //Run keys through String.intern()
     final Properties temp= (Properties)p.clone();
     p.clear(); //put() doesn't overwrite an existing key
     for (Map.Entry<Object, Object> e : temp.entrySet())
       p.put(String.valueOf(e.getKey()).intern(), e.getValue());

     //Internalized lookup of internalized keys
     final long start2= System.nanoTime();
     final long end2= System.nanoTime();

     if (warmup)
       System.out.println("Not internalized: "
           + (end1 - start1)/1000000f + " ms");
       System.out.println("Internalized: "
           + (end2 - start2)/1000000f + " ms");

   public static void main (final String args[]) throws Exception


Not internalized: 1725.81 ms
Internalized: 794.556 ms

Not internalized: 1725.5449 ms
Internalized: 795.296 ms

Not internalized: 1725.703 ms
Internalized: 794.618 ms

So you save 1 second reading 10 million properties, when a program
probably reads fewer than 1000 properties. So, during the executions of
the program, you have saved some nanoseconds. Good for you.

Daniel Pitts' Tech Blog: <>

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