String filePath = ""; // the path of a kt file
String fileContent = "" // the string content of a kt file
KotlinCoreEnvironment environment = KotlinCoreEnvironment.createForProduction(
  	Disposer.newDisposable(),
  	new CompilerConfiguration(),
  	EnvironmentConfigFiles.JVM_CONFIG_FILES
);
PsiFileFactoryImpl factory = (PsiFileFactoryImpl) PsiFileFactory.getInstance(environment.getProject());
PsiFile psiFile = factory.createFileFromText(filePath, KotlinLanguage.INSTANCE, fileContent);
KtFile ktFile = (KtFile)psiFile;