This means that you are using a Java version as sources in this project, which is different from the Java Platform of your NetBean.

To remove the warning:

1. Right-click on the project in the Porjects window and select Properties.

2. Select Libraries from the Categories in the popup window, look for what is set as your Java Platform on the top of the right-side panel.

3. Select Sources under the Categories.

4. In the right-side panel, set the Source/Binary Format to the Java version used by your Java Platform.

5. Click the OK button and recompile your project.