What is the distinction between an uninitialized variable and a variable explicitly set to null, and how do both relate to null errors?

An **uninitialized variable** is a reference variable that has been declared but has not yet been assigned any object instance or `null`. Its value is undefined, and attempting to use it without initialization will often result in a compilation error or a runtime error depending on the language and scope. A variable **explicitly set to null** means it has been assigned the `null` literal, indicating that it intentionally refers to no object. Both scenarios can lead to null errors if the program attempts to dereference (i.e., access methods or fields of) the variable. An uninitialized variable might implicitly hold a `null` value in some contexts (e.g., instance variables in Java), leading to a `NullPointerException` when used, while a variable explicitly set to `null` will directly cause an NPE upon dereference.