import org.openqa.selenium.By;
import org.openqa.selenium.SearchContext;
import org.openqa.selenium.WebElement;

class ByTestId extends By {

    private final String testId;

    public ByPlaceholder(String testId) {
        this.testId = testId;
    }

    @Override
    public List<WebElement> findElements(SearchContext context) {
        return context.findElements(By
                .xpath("//*[@testid = '"+testId+"']"));
    }

    @Override
    public WebElement findElement(SearchContext context) {
        return context.findElement(By
                .xpath("//*[@testid = '"+testId+"']"));
    }
}

//for single webelement
WebElement el = driver.findElement(new ByTestId("addbutton"));

//for list of webelements
WebElement el = driver.findElements(new ByTestId("addbutton"));