FEST-Swing

FEST-Swing is a Java library designed specifically for functional Swing GUI testing with a fluent interface which makes the creation and maintenance of GUI tests effortless. The significance of GUI testing cannot be overemphasized. Even the basic capabilities of a GUI interface have some level of complexity, thus the need to test to ensure the entire system is safe and robust. Software code without comprehensive testing is bound to have bugs, hence the need to test applications.

A properly tested application has a greater chance of success. GUI testing is also essential during application maintenance. During this phase, code is frequently refactored to improve design, which often includes great portions of the user interface. A comprehensive test suite that covers GUI code provides the confidence needed to guarantee bugs aren't inadvertently introduced.

To illustrate this, consider an example test which verifies that an error message is displayed if the user forgets to enter their password while trying to log in to an application. FEST-Swing provides a simple-to-use API that makes such tests incredibly easy to execute. The following code demonstrates how to conduct this test using the FEST-Swing library:

```java
dialog.comboBox("domain").select("Users");
dialog.textBox("username").enterText("leia.organa");
dialog.button("login").click();
dialog.optionPane().requireErrorMessage()
.requireMessage("Please enter your password");.
```
Overall, FEST-Swing is an excellent library that delivers reliable functionality for functional Swing GUI testing with a simple-to-use API, thus making the creation and maintenance of GUI tests stress-free.