Example:

//Instantiating a Workbook object
Workbook workbook = new Workbook();

Worksheet sheet = workbook.getWorksheets().get(0);

//Get Conditional Formatting
ConditionalFormattingCollection cformattings = sheet.getConditionalFormattings();

//Adds an empty conditional formatting
int index = cformattings.add();

//Get newly added Conditional formatting
FormatConditionCollection fcs = cformattings.get(index);

//Sets the conditional format range.
CellArea ca = new CellArea();

ca.StartRow = 0;

ca.EndRow = 0;

ca.StartColumn = 0;

ca.EndColumn = 0;

fcs.addArea(ca);

ca = new CellArea();

ca.StartRow = 1;

ca.EndRow = 1;

ca.StartColumn = 1;

ca.EndColumn = 1;

fcs.addArea(ca);

//Sets condition
 int idx = fcs.addCondition(FormatConditionType.ICON_SET);
 
 FormatCondition cond = fcs.get(idx);
   
 //Sets condition's type
 cond.getIconSet().setType(IconSetType.ARROWS_GRAY_3);

//Add custom iconset condition.
 ConditionalFormattingIcon cfIcon = cond.getIconSet().getCfIcons().get(0);
 
 cfIcon.setType(IconSetType.ARROWS_3);
 
 cfIcon.setIndex(0);
 
 ConditionalFormattingIcon cfIcon1 = cond.getIconSet().getCfIcons().get(1);
 
  cfIcon1.setType(IconSetType.ARROWS_GRAY_3);
  
  cfIcon1.setIndex(1);
  
  ConditionalFormattingIcon cfIcon2 = cond.getIconSet().getCfIcons().get(2);
  
  cfIcon2.setType(IconSetType.BOXES_5);
  
  cfIcon2.setIndex(2);

//Saving the Excel file
workbook.save("C:\\output.xls");