StaticQualifiedUsingExpression
A static variable or method should be qualified with a class name, not expression

Category
Severity
WARNING
Maturity

Alternate names: static, static-access, StaticAccessedFromInstance

The problem

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, always qualify static method calls using a class name, never an expression.

Suppression

Suppress false positives by adding an @SuppressWarnings("StaticQualifiedUsingExpression") annotation to the enclosing element.