NetBeans contains files with errors - resolved

If you have a class X in project B extends a class Y in project A, error badges may occur in project B after you modified class Y and rebuilt project A. When you point the cursor at the error badge, it says "Contains files with errors". When you open the java file of class X and point the cursor at the error line, it says "Class X cannot convert to class Y".

To fix this problem

1. Right-click on project B and select "Porperties".
2. Click "Libraries" on the left under Categories.
3. Select the library or the jar of project A in the right pane and click the "Remove" button to remove it.
4. Click "OK" to close the window.
5. Restart NetBeans.
6. Repeat steps 1 and 2.
7. Click the "Add Library" or "Add JAR/Folder" button to add the library or the jar of project A back to project B.         

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