@tuan010,
Adding GSON jar file to your project should fix this issue. If added, check if the checkbox next to the lib (gson-2.x.jar) in: ‘Project Properties’->’Java Build Path’->’Order and Export’ tab checked or not, if not select/check the checkbox. Clean and Build the project after changing the mentioned setting and it should work fine. Please refer this article on Importing GSON into Eclipse for more information.
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Vishwas R
Team CanvasJS
