An instance method defined in a subclass can override overrides another instance method in the superclass when:
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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 noncompliant code 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, the variables admin and user here in this noncompliant code example).
This noncompliant example attempts to override a static method but fails to consider it as a hiding case. As a result 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 achieving this is impossible with static methods.
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|---|---|---|
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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");
}
}
|
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This compliant solution correctly classifies this case as hiding and uses absolute, fully qualified class names GrantAccess and GrantUserAccess to clearly state the intent. Refrain from qualifying a static method invocation with an expression meant for dynamic dispatch.
| Code Block | ||
|---|---|---|
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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");
}
}
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Note that "In a subclass, you can overload the methods inherited from the superclass. Such overloaded methods neither hide nor override the superclass methodsâthey are new methods, unique to the subclass." \[[Tutorials 08|AA. Java References#Tutorials 08]\]. |
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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_. \[[JLS 05|AA. Java References#JLS 05]\]. Consequently, there may be no _hiding_ when the methods have different return types or {{throws}} clauses. |
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