MET00-A. Understand the difference between overriding and hiding

An instance method defined in a subclass can override another instance method in the superclass when:

• Both have the same name
• Number and type of parameters is same
• Return type is same

The Hiding term is used in context of a class method that has the same signature as the corresponding class method in the superclass.

The difference between these two is that the overridden method gets invoked from the subclass while the hidden method may get invoked from either the superclass or the subclass, depending on how it is invoked.

Non-Compliant Code Example

To differentiate between overriding and hiding, a common nomenclature is used. The method to be invoked is decided at either compile time (if the base method is static, as in this non-compliant example) according to the type of the qualifier or at run time otherwise (for non-static methods). A qualifier is a part of the invocation expression before the dot (for example, admin and user here).

This non-compliant example attempts to override a static method but fails to consider it as a hiding case. Thus the displayAccountStatus method of the superclass gets invoked on both the calls. Moreover, expressions that are normally used for dynamic dispatch while overriding have been used even though this is impossible with static methods.

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

class GrantUserAccess extends GrantAccess {
  public static void displayAccountStatus() {
    System.out.print("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");	
  }
}

Compliant Solution

This compliant solution correctly classified this case as hiding and uses absolute class names GrantAccess and GrantUserAccess to clearly state the intent. Refrain from qualifying a static method invocation with an expression meant for dynamic dispatch.

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

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

public class StatMethod {
  public static void choose(String username) {
  if(username.equals("admin"))
    GrantAccess.displayAccountStatus();
  else
    GrantUserAccess.displayAccountStatus();
  }
  
  public static void main(String[] args) {
    choose("user");	
  }
}

References

Sun Java Tutorial http://java.sun.com/docs/books/tutorial/java/IandI/override.html
Java Puzzlers 6.48