Theory
In any object-oriented programming language, Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes. When a method in a subclass has the same name, same parameters or signature and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class.
Method overriding is one of the way by which java achieve Run Time Polymorphism.The version of a method that is executed will be determined by the object that is used to invoke it. If an object of a parent class is used to invoke the method, then the version in the parent class will be executed, but if an object of the subclass is used to invoke the method, then the version in the child class will be executed. In other words, it is the type of the object being referred to (not the type of the reference variable) that determines which version of an overridden method will be executed.
Rules for method overriding:
1. Overriding and Access-Modifiers : The access modifier for an overriding method can allow more, but not less, access than the overridden method. For example, a protected instance method in the super-class can be made public, but not private, in the subclass. Doing so, will generate compile-time error.
2. Final methods can not be overridden : If we don’t want a method to be overridden, we declare it as final. Please see Using final with Inheritance .
3. Static methods can not be overridden(Method Overriding vs Method Hiding) : When you defines a static method with same signature as a static method in base class, it is known as method hiding.
4. Private methods can not be overridden : Private methods cannot be overridden as they are bonded during compile time. Therefore we can’t even override private methods in a subclass.(See this for details).
5. The overriding method must have same return type (or subtype) : From Java 5.0 onwards it is possible to have different return type for a overriding method in child class, but child’s return type should be sub-type of parent’s return type. This phenomena is known as covariant return type.
6. Invoking overridden method from sub-class : We can call parent class method in overriding method using super keyword.
7. Overriding and abstract method: Abstract methods in an interface or abstract class are meant to be overridden in derived concrete classes otherwise a compile-time error will be thrown.
8. Overriding and synchronized/strictfp method : The presence of synchronized/strictfp modifier with method have no effect on the rules of overriding, i.e. it’s possible that a synchronized/strictfp method can override a non synchronized/strictfp one and vice-versa.
9. Overriding and Exception-Handling : Below are two rules to note when overriding methods related to exception-handling.
10. Overriding and constructor : We can not override constructor as parent and child class.