String myframe;
List<WebElement> frames = driver.findElements(By.tagName("iframe"));
System.out.println("No. of frames = " + frames.size());
for(int i=0;i<=frames.size();i++){
myframe = frames.get(0).getAttribute("id");
System.out.println(myframe);
//myframe = frames.get(0).getAttribute("name");
//System.out.println(myframe);
driver.switchTo().frame(myframe);
driver.findElement(By.xpath("//*[@id='document.documentHeader.documentDescription']")).sendKeys("test");
}
//driver.findElement(By.cssSelector("input#document.documentHeader.documentDescription")).sendKeys("test");
//driver.findElement(By.xpath("//*[@id='document.documentHeader.documentDescription']")).sendKeys("test");
<input type="text" title="* A free-form text field that describes the purpose or function of the document." style=" " id="document.documentHeader.documentDescription" onblur="" onchange="" value="" tabindex="1" size="45" maxlength="255" name="document.documentHeader.documentDescription">