Try using following code:
WebElement dropdown = driver.findElement(By.id("id_dropdown"));
Select list = new Select(dropdown);
List<WebElement> dropdown_list = list.getOptions();
for (int i = 0; i < dropdown_list.size(); i++) {
String dropdown_value = dropdown_list.get(i).getText();
System.out.println("dropdown values are " + dropdown_value);
}