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When a class declares a static method m, the declaration of m hides any method m', where the signature of m is a subsignature of the signature of m' and the declaration of m' is both in the superclasses and superinterfaces of the declaring class and also would otherwise be accessible to code in the declaring class (The Java Language Specification, §8.4.8.2 "Hiding (by Class Methods)" [JLS 20052015]).

An instance method defined in a subclass overrides another instance method in the superclass when both have the same name, number and type of parameters, and return type.

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Noncompliant Code Example

In this noncompliant code example, the programmer hides the static method rather than overriding it. Consequently, the code invokes the displayAccountStatus() method of the superclass at two different call sites instead of invoking the superclass method at one call site and the subclass method at the other, causing it to print  Account details for admin despite being instructed to choose user rather than admin.

class GrantAccess {
  public static void displayAccountStatus() {
    System.out.println("Account details for admin: XX");
  }
}

class GrantUserAccess extends GrantAccess {
  public static void displayAccountStatus() {
    System.out.println("Account details for user: XX");
  }
}

public class StatMethod {
  public static void choose(String username) {
    GrantAccess admin = new GrantAccess();
    GrantAccess user = new GrantUserAccess();
    if (username.equals("admin")) {
      admin.displayAccountStatus();
    } else {
      user.displayAccountStatus();
    }
  }

  public static void main(String[] args) {
    choose("user");
  }
}

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The methods inherited from the superclass can also be overloaded in a subclass. Overloaded methods are new methods unique to the subclass and neither hide nor override the superclass method [Java Tutorials 2008].

Technically, a private method cannot be hidden or overridden. There is no requirement that private methods with the same signature in the subclass and the superclass bear any relationship in terms of having the same return type or throws clause, the necessary conditions for hiding [JLS 20052015]. Consequently, hiding cannot occur when private methods have different return types or throws clauses.

Exceptions

MET07-J-EX0: Occasionally, an API provides hidden methods. Invoking those methods is not a violation of this rule provided that all invocations of hidden methods use qualified names or method invocation expressions that explicitly indicate which specific method is invoked. If the displayAccountStatus() were a hidden method, for example, the following implementation of the choose() method would be an acceptable alternative:

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Confusing overriding and hiding can produce unexpected results.

Automated Detection

Automated detection of violations of this rule is straightforward. Automated determination of cases in which method hiding is unavoidable is infeasible. However, determining whether all invocations of hiding or hidden methods explicitly indicate which specific method is invoked is straightforward.

Bibliography

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