You have to import it first. For instance:
// This file was generated by Mendix Studio Pro.
//
// WARNING: Only the following code will be retained when actions are regenerated:
// - the import list
// - the code between BEGIN USER CODE and END USER CODE
// - the code between BEGIN EXTRA CODE and END EXTRA CODE
// Other code you write will be lost the next time you deploy the project.
// Special characters, e.g., é, ö, à, etc. are supported in comments.
package templaterformendix.actions;
import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import com.mendix.core.Core;
import com.mendix.systemwideinterfaces.core.IContext;
import com.mendix.systemwideinterfaces.core.IMendixObject;
import com.mendix.webui.CustomJavaAction;
import hr.ngs.templater.Configuration;
import hr.ngs.templater.IDocumentFactory;
import templaterformendix.implementation.TemplaterJson;
You are then able to use these in your code.
Regards,
Ronald