I used like this,but not found.
WebElement element = androidDriver.findElementByClassName(android.widget.Toast);
So I search in every major forum and portal,they solved it by blow:
WebDriverWait wait = new WebDriverWait(androidDriver, 3);
WebElement toastView = wait.until(ExpectedConditions.presenceOfElementLocated(
By.xpath(".//*[contains(@text,'" + toast + "')]")));
But it works well in the condition of you has known the toast text.