PsiJavaFile file is not found inside org.jetbrains.intellij

Hello,

I am new in developing plugins for intellij. I follow the doc tutorial with gradle implemantation :

plugins {
id 'java'
id "org.jetbrains.intellij" version "0.4.10"
}
I want to use PsiJavaFile but i cant find it inside plugin.

PsiFile psiFile = PsiManager.getInstance(project).findFile(file);
PsiJavaFile psiJavaFile = (PsiJavaFile)psiFile;

Thanks