Method overriding:
Overriding is creating an instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass.
package com.candidjava.core;
public class User
{
public void salary()
{
System.out.println("common salary functionality");
}
}package com.candidjava.core;
public class Student extends User
{
public void salary()
{
System.out.println("logic specific to student class and its subclass");
}
}Rules for overriding
- Overriding method MUST have the same argument list (if not, it might be a case of overloading).
- Overriding method MUST have the same return type; the exception is co-variant return (used as of Java 5) which returns a type that is a subclass of what is returned by the overridden method.
- Overriding method MUST NOT have more restrictive access modifier, but MAY have less restrictive one.
- Overriding method MUST NOT throw new or broader checked exceptions, but MAY throw fewer or narrower checked exceptions or any unchecked exceptions.
- Abstract methods MUST be overridden in its first non-abstract subclass.
- Final methods CANNOT be overridden.
Is it possible to override Constructor?
No, since constructor cannot be inherited it cannot be overridden.
Is it possible to Override Static Method?
No. If a subclass defines a static method with the same signature as a static method in the superclass, then the method in the subclass hides the one in the superclass.
Legal and Illegal overriding
Legal: Overriding with less restrictive access modifiers
package com.candidjava.core;
public class User
{
protected void salary()
{
System.out.println("common salary functionality");
}
}
package com.candidjava.core;
public class Student extends User
{
public void salary() //legal to have less restrictive access modifiers
{
System.out.println("logic specific to student class and its subclass");
}
}Illegal:
Overriding with more restrictive access modifiers.
package com.candidjava.core;
public class User
{
protected void salary()
{
System.out.println("common salary functionality");
}
}
package com.candidjava.core;
public class Student extends User
{
private void salary() //Illegal to have more restrictive access modifiers
{
System.out.println("logic specific to student class and its subclass");
}
}Legal: Overriding with narrower checked exception
package com.candidjava.core;
public class User
{
public void salary() throws Exception
{
System.out.println("common salary functionality");
}
}
package com.candidjava.core;
public class Student extends User
{
public void salary() throws Exception
{
System.out.println("logic specific to student class and its subclass");
}
}Illegal: Overriding final method
package com.candidjava.core;
public class User {
final public void salary() throws Exception {
System.out.println("common salary functionality");
}
}
package com.candidjava.core;
public class Student extends User
{
public void salary()
{
System.out.println("logic specific to student class and its subclass");
}
}