Fastest way to pad a number in Java to a certain number of digits
- by Martin
Am trying to create a well-optimised bit of code to create number of X-digits in length (where X is read from a runtime properties file), based on a DB-generated sequence number (Y), which is then used a folder-name when saving a file.
I've come up with three ideas so far, the fastest of which is the last one, but I'd appreciate any advice people may have on this...
1) Instantiate a StringBuilder with initial capacity X. Append Y. While length < X, insert a zero at pos zero.
2) Instantiate a StringBuilder with initial capacity X. While length < X, append a zero. Create a DecimalFormat based on StringBuilder value, and then format the number when it's needed.
3) Create a new int of Math.pow( 10, X ) and add Y. Use String.valueOf() on the new number and then substring(1) it.
The second one can obviously be split into outside-loop and inside-loop sections.
So, any tips? Using a for-loop of 10,000 iterations, I'm getting similar timings from the first two, and the third method is approximately ten-times faster. Does this seem correct?
Full test-method code below...
// Setup test variables
int numDigits = 9;
int testNumber = 724;
int numIterations = 10000;
String folderHolder = null;
DecimalFormat outputFormat = new DecimalFormat( "#,##0" );
// StringBuilder test
long before = System.nanoTime();
for ( int i = 0; i < numIterations; i++ )
{
StringBuilder sb = new StringBuilder( numDigits );
sb.append( testNumber );
while ( sb.length() < numDigits )
{
sb.insert( 0, 0 );
}
folderHolder = sb.toString();
}
long after = System.nanoTime();
System.out.println( "01: " + outputFormat.format( after - before ) + " nanoseconds" );
System.out.println( "Sanity check: Folder = \"" + folderHolder + "\"" );
// DecimalFormat test
before = System.nanoTime();
StringBuilder sb = new StringBuilder( numDigits );
while ( sb.length() < numDigits )
{
sb.append( 0 );
}
DecimalFormat formatter = new DecimalFormat( sb.toString() );
for ( int i = 0; i < numIterations; i++ )
{
folderHolder = formatter.format( testNumber );
}
after = System.nanoTime();
System.out.println( "02: " + outputFormat.format( after - before ) + " nanoseconds" );
System.out.println( "Sanity check: Folder = \"" + folderHolder + "\"" );
// Substring test
before = System.nanoTime();
int baseNum = (int)Math.pow( 10, numDigits );
for ( int i = 0; i < numIterations; i++ )
{
int newNum = baseNum + testNumber;
folderHolder = String.valueOf( newNum ).substring( 1 );
}
after = System.nanoTime();
System.out.println( "03: " + outputFormat.format( after - before ) + " nanoseconds" );
System.out.println( "Sanity check: Folder = \"" + folderHolder + "\"" );
