Intellij showing nonstandard annotations on standard Java classes?

Question

Whilst using Intellij Idea quick documentation feature ( CTRL + Q ) sometimes I find standard JDK classes to be annotated by nonstandard annotations. Those annotations are not there when one CTRL -clicks the annotated feature to view its source code. For example, the following picture shows argument of java.util.Collections.unmodifiableList to be annotated with @org.jetbrains.annotations.NotNull :

Why would a standard class be decorated with third party annotation?

This is not consistent, as for example, checkedNavigableSet from java.util.collections is annotated differently, no package for @NotNull is given:

The phenomenon does not preclude development, I am just curious as to the cause of QuickDoc behaving that way. Maybe those annotations put there by Intellij team for convenience?

Answer 1

According to doc from here that annotations are used by Idea's inspection mechanism, so statement Collections.unmodifiableList(null) is marked with warning of non-null contract violation. As you noticed this annotations are not being present in original code, so I guess they were added somewhere in a middle between JDK and Editor. It's still unclear why it appears with package or without it.