Overriding and Overloading
■Overriding
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.
class SuperClass {
public void methodA(int i) {
// calculate a + b
}
}
class SubClass {
@Override
public void methodA(int i) {
// calculate a + b
// output the result of a + b
}
}
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.
class Parent
{
// private methods are not overridden
private void m1() { System.out.println("From parent m1()");}
protected void m2() { System.out.println("From parent m2()"); }
}
class Child extends Parent
{
// new m1() method
// unique to Child class
private void m1() { System.out.println("From child m1()");}
// overriding method
// with more accessibility
@Override
public void m2() { System.out.println("From child m2()");}
}
class Main
{
public static void main(String[] args)
{
Parent obj1 = new Parent();
obj1.m2();
Parent obj2 = new Child();
obj2.m2();
}
}
From parent m2()
From child m2()
２. Final methods can not be overridden If we don’t want a method to be overridden, we declare it as final.
// can't be overridden
final void show();
class Child extends Parent
{
// This would produce error
void show() { }
}
error: show() in Child cannot override show() in Parent
void show() { }
^
overridden method is final
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.
The following table summarizes what happens when you define a method with the same signature as a method in a super-class.
| SUPERCLASS INSTANCE METHOD | SUPERCLASS STATIC METHOD | |
|---|---|---|
| SUBCLASS INSTANCE METHOD | Overrides | Generates a compile-time error |
| SUBCLASS STATIC METHOD | Generates a compile-time error | Hides |
class Parent
{
// Static method in base class which will be hidden in subclass
static void m1() { System.out.println("From parent static m1()");}
// Non-static method which will be overridden in derived class
void m2() { System.out.println("From parent non-static(instance) m2()"); }
}
class Child extends Parent
{
// This method hides m1() in Parent
static void m1() { System.out.println("From child static m1()");}
// This method overrides m2() in Parent
@Override
public void m2() { System.out.println("From child non-static(instance) m2()");}
}
class Main
{
public static void main(String[] args)
{
Parent obj1 = new Child();
// As per overriding rules this should call to class Child static
// overridden method. Since static method can not be overridden, it
// calls Parent's m1()
obj1.m1();
// Here overriding works and Child's m2() is called
obj1.m2();
}
}
From parent static m1()
From child non-static(instance) m2()
４.Private methods can not be overridden
５.The overriding method must have same return type (or subtype)
６.Invoking overridden method from sub-class
We can call parent class method in overriding method using super keyword.
// base class
class Parent
{
void show() {
System.out.println("Parent's show()");
}
}
// inherited class
class Child extends Parent
{
// This method overrides show() of Parent
@Override
void show() {
super.show();
System.out.println("Child's show()");
}
}
class Main
{
public static void main(String[] args) {
Parent obj = new Child();
obj.show();
}
}
Parent's show()
Child's show()
７.**Overriding and constructor ** We can not override constructor as parent and child class can never have constructor with same name(Constructor name must always be same as Class name).
８.Overriding and Exception-Handling Below are two rules to note when overriding methods related to exception-handling.
If the super-class overridden method does not throws an exception, subclass overriding method can only throws the unchecked exception, throwing checked exception will lead to compile-time error.
class Parent
{
void m1() { System.out.println("From parent m1()");}
void m2() { System.out.println("From parent m2()");}
}
class Child extends Parent
{
@Override
// no issue while throwing unchecked exception
void m1() throws ArithmeticException {
System.out.println("From child m1()");
}
@Override
// compile-time error
// issue while throwin checked exception
void m2() throws Exception {
System.out.println("From child m2");
}
}
error: m2() in Child cannot override m2() in Parent
void m2() throws Exception{ System.out.println("From child m2");}
^
overridden method does not throw Exception
If the super-class overridden method does throws an exception, subclass overriding method can only throw same, subclass exception. Throwing parent exception in Exception hierarchy will lead to compile time error.Also there is no issue if subclass overridden method is not throwing any exception.
class Parent
{
void m1() throws RuntimeException {
System.out.println("From parent m1()");
}
}
class Child1 extends Parent
{
@Override
// no issue while throwing same exception
void m1() throws RuntimeException {
System.out.println("From child1 m1()");
}
class Child2 extends Parent
{
@Override
// no issue while throwing subclass exception
void m1() throws ArithmeticException {
System.out.println("From child2 m1()");
}
}
class Child3 extends Parent
{
@Override
// no issue while not throwing any exception
void m1() {
System.out.println("From child3 m1()");
}
}
class Child4 extends Parent
{
@Override
// compile-time error
// issue while throwing parent exception
void m1() throws Exception {
System.out.println("From child4 m1()");
}
}
error: m1() in Child4 cannot override m1() in Parent
void m1() throws Exception
^
overridden method does not throw Exception
９.Overriding and abstract method Abstract methods in an interface or abstract class are meant to be overridden in derived concrete classes otherwise compile-time error will be thrown.
１０.Overriding and synchronized/stricfp method The presence of synchronized/stricfp modifier with method have no effect on the rules of overriding, i.e. it’s possible that a synchronized/stricfp method can override a non synchronized/stricfp one and vice-versa.
■Overloading Overloading allows different methods to have same name, but different signatures where signature can differ by number of input parameters or type of input parameters or both. Overloading is related to compile time (or static) polymorphism.
public class Sum {
// Overloaded sum(). This sum takes two int parameters
public int sum(int x, int y) {
return (x + y);
}
// Overloaded sum(). This sum takes three int parameters
public int sum(int x, int y, int z) {
return (x + y + z);
}
// Overloaded sum(). This sum takes two double parameters
public double sum(double x, double y) {
return (x + y);
}
// Driver code
public static void main(String args[]) {
Sum s = new Sum();
System.out.println(s.sum(10, 20));
System.out.println(s.sum(10, 20, 30));
System.out.println(s.sum(10.5, 20.5));
}
}
30
60
31.0
what is the advantage? We don’t have to create and remember different names for functions doing the same thing. For example, in our code, if overloading was not supported by Java, we would have to create method names like sum1, sum2,… or sum2Int, sum3Int, … etc.
Reference