Get PsiClass from PsiFile

Just typecast PsiFile to PsiJavaFile. Then call getClasses() method.

PsiJavaFile psiJavaFile = (PsiJavaFile)psiFile;
final PsiClass[] classes = psiJavaFile.getClasses();

To support both Java and Kotlin, you will have to type cast to "PsiClassOwner"

val psiClassOwner = psiFile as PsiClassOwner
psiClassOwner.classes.forEach { ... }