As the ear hit eardrums “overriding” we quickly get to know that it can be done either virtue of different datatypes or arguments passed to a function what a programmer learned initially while learning polymorphism in java. Before JDK 5.0, it was not possible to override a method by changing the return type. When we override a parent class method, the name, argument types, and return type of the overriding method in child class has to be exactly the same as that of the parent class method. The overriding method was said to be invariant with respect to return type.
Java version 5.0 onwards it is possible to have different return types for an overriding method in the child class, but the child’s return type should be a subtype of the parent’s return type. The overriding method becomes variant with respect to return type.
The co-variant return type is based on the Liskov substitution principle.
Now geeks you must be wondering about why to use for which we will be listing down the advantages as follows:
- It helps to avoid confusing type casts present in the class hierarchy and thus making the code readable, usable and maintainable.
- We get the liberty to have more specific return types when overriding methods.
- Help in preventing run-time ClassCastExceptions on returns
Note: If we swap return types of Base and Derived, then above program would not work. Please see this program for example.
Example Two classes used for return types
Java
// Java Program to Demonstrate Different Return Types// if Return Type in Overridden method is Sub-type// Class 1class A {}// Class 2class B extends A {}// Class 3// Helper class (Base class)class Base { // Method of this class of class1 return type A fun() { // Display message only System.out.println("Base fun()"); return new A(); }}// Class 4// Helper class extending above classclass Derived extends Base { // Method of this class of class1 return type B fun() { // Display message only System.out.println("Derived fun()"); return new B(); }}// Class 5// Main classpublic class GFG { // Main driver method public static void main(String args[]) { // Creating object of class3 type Base base = new Base(); // Calling method fun() over this object // inside main() method base.fun(); // Creating object of class4 type Derived derived = new Derived(); // Again calling method fun() over this object // inside main() method derived.fun(); }} |
Output:
Base fun() Derived fun()
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