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To refer to a static member of another class, we typically *qualify* that member
name by prepending the name of the class it's in, and a dot:
`TheClass.theMethod()`.
But the Java language also permits you to qualify this call using any
*expression* (typically, a variable) whose static type is the class that
contains the method: `instanceOfTheClass.theMethod()`.
Doing this creates the appearance of an ordinary polymorphic method call, but it
behaves very differently. For example:
```
public class Main {
static class TheClass {
public static int theMethod() {
return 1;
}
}
static class TheSubclass extends TheClass {
public static int theMethod() {
return 2;
}
}
public static void main(String[] args) {
TheClass instanceOfTheClass = new TheSubclass();
System.out.println(instanceOfTheClass.theMethod());
}
}
```
`TheSubclass` appears to "override" `theMethod`, so we might expect this code to
print the number `2`.
The code, however, prints the number `1`. The runtime type of
`instanceOfTheClass`, `TheSubclass`, is ignored; only the static type of the
reference, as seen by `javac`, matters.
In fact, the instance that `instanceOfTheClass` points to is *entirely*
irrelevant. To prove this, set the variable to `null` and run again. The program
will *still* print `1`, not throw a `NullPointerException`!
Qualifying a static reference in this way creates an unnecessarily confusing
situation. To prevent it, only qualify static method calls using a class name,
never an expression (that is, `TheClass.theMethod()` or `theMethod()`, but not
`anInstanceOfTheClass.theMethod()`).
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