Overload Vs Overriding.

Method overriding is when a child class redefines the same method as a parent class, with the same parameters. For example, the standard Java class java.util.LinkedHashSet extends java.util.HashSet. The method add() is overridden in LinkedHashSet. If you have a variable that is of type HashSet, and you call its add() method, it will call the appropriate implementation of add(), based on whether it is a HashSet or a LinkedHashSet. This is called polymorphism.
Method overloading is defining several methods in the same class, that accept different numbers and types of parameters. In this case, the actual method called is decided at compile-time, based on the number and types of arguments. For instance, the method System.out.println() is overloaded, so that you can pass ints as well as Strings, and it will call a different version of the method.

Overloading is defining functions that have similar signatures, yet have different parameters.

Overriding is only pertinent to derived classes, where the parent class has defined a method and the derived class wishes to override that function.

Overriding	Overloading
Methods name and signatures must be same.	Having same method name with different Signatures.
Overriding is the concept of runtime polymorphism	Overloading is the concept of compile time polymorphism
When a function of base class is re-defined in the derived class called as Overriding	Two functions having same name and return type, but with different type and/or number of arguments is called as Overloading
It needs inheritance.	It doesn't need inheritance.
Method should have same data type.	Method can have different data types
Method should be public.	Method can be different access specifies

Example

Overriding

public class MyBaseClass

{

public virtual void MyMethod()

{

Console.Write("My BaseClass Method");

}

public class MyDerivedClass : MyBaseClass

{

public override void MyMethod()

{

Console.Write("My DerivedClass Method");

}

Overloading

int add(int a, int b)

int add(float a , float b)

Introduction

Q: Method Overloading ?
Ans:
Method overloading means having two or more methods with the same name but different signatures in the same scope. These two methods may exist in the same class or anoter one in base class and another in derived class.

class Person
{

private String firstName;
private String lastName;
Person()
{

this.firstName = "";
this.lastName = "";
}
Person(String FirstName)
{

this.firstName = FirstName;
this.lastName = "";
}
Person(String FirstName, String LastName)
{

this.firstName = FirstName;
this.lastName = LastName;
}

} Calling Overloaded Methods.

Person(); // as a constructor and call method without parameter
Person(userFirstName); // as a constructor and call method with one parameter(like User's first Name)
Person(userFirstName,userLastName); // as a constructor and call method with one parameter(like User's first Name)

When to use Method Overloading?
Generally, you should consider overloading a method when you have required same reason that take different signatures, but conceptually do the same thing.
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Q: Method Overriding?
Ans:
Method overriding means having a different implementation of the same method in the inherited class. These two methods would have the same signature, but different implementation. One of these would exist in the base class and another in the derived class. These cannot exist in the same class.

Overriding methods
Overriding method definitions
In a derived class, if you include a method definition that has the same name and exactly the same number and types of parameters as a method already defined in the base class, this new definition replaces the old definition of the method.
Explanation
A subclass inherits methods from a superclass. Sometimes, it is necessary for the subclass to modify the methods defined in the superclass. This is referred to as method overriding. The following example demonstrates method overriding.
Step 1 In this example we will define a base class called Circle
class Circle
{

//declaring the instance variable
protected double radius;

public Circle(double radius)
{
this.radius = radius;
}
// other method definitions here
public double getArea()
{
return Math.PI*radius*radius;
}//this method returns the area of the circle

}// end of class circle
When the getArea method is invoked from an instance of the Circle class, the method returns the area of the circle.
Step 2 The next step is to define a subclass to override the getArea() method in the Circle class. The derived class will be the Cylinder class. The getArea() method in the Circle class computes the area of a circle, while the getArea method in the Cylinder class computes the surface area of a cylinder.
The Cylinder class is defined below.
class Cylinder extends Circle
{

//declaring the instance variable
protected double length;
public Cylinder(double radius, double length)
{
     super(radius);
     this.length = length;
}
// other method definitions here

public double getArea()
{
     // method overriden here
     return 2*super.getArea()+2*Math.PI*radius*length;
}//this method returns the cylinder surface area

}// end of class Cylinder

When the overriden method (getArea) is invoked for an object of the Cylinder class, the new definition of the method is called and not the old definition from the superclass(Circle).

Example code This is the code to instantiate the above two classes
Circle myCircle;
myCircle = new Circle(1.20);
Cylinder myCylinder;
myCylinder = new Cylinder(1.20,2.50);

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In Java, what’s the difference between method overloading and method overriding?

The difference between overriding and overloading in Java is a common source of confusion – but it is fairly simple to understand. Let’s start the discussion by talking more about method overloading. Overloading in Java can occur when two or more methods in the same class share the same name or even if a child class shares a method with the same name as one of it’s parent classes. But, in order to actually have overloaded methods, the methods not only have to have the same name, but there are other conditions that must be satisfied – read below to see what those conditions are.

Suppose we have a class called TestClass which has 2 methods, and both methods have the same name – let’s say that name is “someMethod” – this would be considered to be method overloading if at least one of these 2 things is true:

1.) The number of parameters is different for the methods   
2.) The parameter types are different.

How to NOT overload methods:

It’s important to understand that method overloading is NOT something you can accomplish by doing these 2 things:

1.  Changing the return type of the method   
2.  Changing the name of the method parameters, but 
    not changing the parameter types.

Confused? Well, here are some very helpful examples of where overloading would be both valid and invalid:

Examples of Method Overloading in Java – both valid and invalid:

//compiler error - can't overload based on the   
//type returned -
//(one method returns int, the other returns a float):    

int changeDate(int Year) ;  
float changeDate (int Year);    

//compiler error - can't overload by changing just 
//the name of the parameter (from Month to Year):    

int changeDate(int Month) ;  
float changeDate(int Year);    

//valid case of overloading, since there is an   
//extra parameter in the first method:        

int changeDate(int Year, int Month) ;  
int changeDate(int Year);    

//also a valid case of overloading, since the   
//parameters are of different types:    

int changeDate(float Year) ;  
int changeDate(int Year);

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What about method overriding?

Overriding methods is completely different from overloading methods. If a derived class requires a different definition for an inherited method, then that method can be redefined in the derived class. This would be considered overriding. An overridden method would have the exact same method name, return type, number of parameters, and types of parameters as the method in the parent class, and the only difference would be the definition of the method.
Let’s summarize the differences between overloading and overriding. When overloading, one must change either the type or the number of parameters for a method that belongs to the same class. But, overriding a method means that a method inherited from a base class is what’s being changed.
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Difference Between Method Overloading and Method Overriding

	Method Overloading	Method Overriding
Definition	In Method Overloading, Methods of the same class shares the same name but each method must have different number of parameters or parameters having different types and order.	In Method Overriding, sub class have the same method with same name and exactly the same number and type of parameters and same return type as a super class.
Meaning	Method Overloading means more than one method shares the same name in the class but having different signature.	Method Overriding means method of base class is re-defined in the derived class having same signature.
Behavior	Method Overloading is to “add” or “extend” more to method’s behavior.	Method Overriding is to “Change” existing behavior of method.
	Overloading and Overriding is a kind of polymorphism.Polymorphism means “one name, many forms”.
Polymorphism	It is a compile time polymorphism.	It is a run time polymorphism.
Inheritance	It may or may not need inheritance in Method Overloading.	It always requires inheritance in Method Overriding.
Signature	In Method Overloading, methods must have different signature.	In Method Overriding, methods must have same signature.
Relationship of Methods	In Method Overloading, relationship is there between methods of same class.	In Method Overriding, relationship is there between methods of super class and sub class.
Criteria	In Method Overloading, methods have same name different signatures but in the same class.	In Method Overriding, methods have same name and same signature but in the different class.
No. of Classes	Method Overloading does not require more than one class for overloading.	Method Overriding requires at least two classes for overriding.
Example	Class Add { int sum(int a, int b) { return a + b; } int sum(int a) { return a + 10; } }	Class A // Super Class { void display(int num) { print num ; } } //Class B inherits Class A Class B //Sub Class { void display(int num) { print num ; } }

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What is Method Overloading and Overriding in Java - Example Differences Tutorial

What is method overloading and overriding in Java
Method overloading and method overriding in Java is two important concept in Java which allows Java programmer to declare method with same name but different behavior. Method overloading and method overriding is based on polymorphism in Java. In case of method overloading, method with same name co-exists in same class but they must have different method signature, while in case of method overriding, method with same name is declared in derived class or sub class.Method overloading is resolved using static binding in Java at compile time while method overriding is resolved using dynamic binding in Java at runtime. In short When you overload a method in Java its method signature got changed while in case of overriding method signature remains same but a method can only be overridden in sub class. Since Java supports polymorphism and resolve object at run-time it is capable to call overridden method in Java. By the way difference between method overloading and overriding is also one of the popular Java design question and appear in almost all levels of Java interviews.

What is method overloading and overriding in Java?

In this Java tutorial we will see how Java allows you to create two methods of same name by using method overloading and method overriding. We will also touch base on how methods are bonded or called by Compiler and Java Virtual Machine and finally we will answer of popular interview questions difference between method overloading and method overriding in Java. This article is in my series of Java article which discusses about Interview e.g. Difference between Synchronized Collection and Concurrent Collection or How to Stop Thread in Java. Please let me know if you have some other interview questions and you are looking answer or reason for that and here in Javarevisited we will try to find and discuss those interview questions.

How to overload a method in Java

If you have two methods with same name in one Java class with different method signature than its called overloaded method in Java. Generally overloaded method in Java has different set of arguments to perform something based on different number of input. You can also overload constructor in Java, which we will see in following example of method overloading in Java. Binding of overloading method occurs during compile time and overloaded calls resolved using static binding. To overload a Java method just changes its signature. Just remember in order to change signature you either need to change number of argument, type of argument or order of argument in Java if they are of different types. Since return type is not part of method signature simply changing return type will result in duplicate method and you will get compile time error in Java. In our example of Loan and PersonalLoan class, createLoan method is overloaded. Since you have two crateLoan() method with one takes one argument lender while other take two argument both lender and interestRate. Remember you can overload static method in Java, you can also overload private and final method in Java but you can not override them.

How to override a method in Java

In order to override a Java method, you need to create a child class which extends parent. Overridden method in Java also shares same name as original method in Java but can only be overridden in sub class. Original method has to be defined inside interface or base class, which can be abstract as well. When you override a method in Java its signature remains exactly same including return type. JVM resolves correct overridden method based upon object at run-time by using dynamic binding in Java. For example in our case when we call personalLoan.toString() method even though personalLoan object is of type Loan actual method called would be from PersonalLoan class because object referenced by personalLoan variable is of type PersonalLoan(). This is very useful technique to modify behavior of a function in Java based on different implementation. Equals, hashcode and compareTo methods are classic example of overridden methods in Java.

Another important point is that you can not override static method in Java because they are associated with Class rather than object and resolved and bonded during compile time and that’s the reason you cannot override main method in Java. Similar to static, private and final methods are also not overridden in Java. By the way, as part of overriding best practice, always use @Override annotation, while overriding method from an abstract class or interface.

Rules of Method Overriding in Java

Following are rules of method overriding in java which must be followed while overriding any method. as stated earlier private, static and final method can not be overridden.

Method signature must be same including return type, number of method parameters, type of parameters and order of parameters
Overriding method can not throw higher Exception than original or overridden method. means if original method throws IOException than overriding method can not throw super class of IOException e.g. Exception but it can throw any sub class of IOException or simply does not throw any Exception. This rule only applies to checked Exception in Java, overridden method is free to throw any unchecked Exception.
Overriding method can not reduce accessibility of overridden method , means if original or overridden method is public than overriding method can not make it protected.

Difference between method overloading vs overriding in Java

Overloading vs Overriding in Java is one of the popular java interview questions at many companies and asked at different levels of programmers. Here are some of the silent difference between overloading and overriding in Java. Though more important is to understand how to use both overloading and overriding, these difference are good from interview perspective and gives some basic idea as well:

1) First and most important difference between method overloading and overriding is that, In case of method overloading in Java, Signature of method changes while in case of method overriding it remain same.

2) Second major difference between method overloading vs overriding in Java is that You can overload method in one class but overriding can only be done on subclass.

3) You can not override static, final and private method in Java but you can overload static, final or private method in Java.

4) Overloaded method in Java is bonded by static binding and overridden methods are subject to dynamic binding.

5) Private and final method can also be not overridden in Java.

Exceptions handling while overloading and overriding method in Java

While overriding a method it can only throw checked exception declared by by overridden method or any subclass of it, means if overridden method throws IOExcpetion than overriding method can throw sub classes of IOExcpetion e.g. FileNotFoundException but not wider exception e.g. Exception or Throwable. This restriction is only for checked Exception for RuntimeException you can throw any RuntimeException. Overloaded method in Java doesn’t have such restriction and you are free to modify throws clause as per your need.

Example of method overloading and overriding in Java

Here is an example of both method overloading and method overriding in Java. In order to explain the concept we have create two classes Loan and PersonalLoan. createLoan() method is overloaded as it has different version with different signature, while toString() method which is original declared in Object class is overridden in both Loan and PersonalLoan class.

public class OverloadingOverridingTest {

public static void main(String[] args) {

// Example of method overloading in Java

Loan cheapLoan = Loan.createLoan("HSBC");

Loan veryCheapLoan = Loan.createLoan("Citibank");

// Example of method overriding in Java

Loan personalLoan = new PersonalLoan();

personalLoan.toString();

}

public class Loan {

private double interestRate;

private String customer;

private String lender;

public static Loan createLoan(String lender) {

Loan loan = new Loan();

loan.lender = lender;

return loan;

}

public static Loan createLoan(String lender, double interestRate) {

Loan loan = new Loan();

loan.lender = lender;

loan.interestRate = interestRate;

return loan;

}

@Override

public String toString() {

return "This is Loan by Citibank";

}

public class PersonalLoan extends Loan {

@Override

public String toString() {

return "This is Personal Loan by Citibank";

}

Summary

1) In case of method overloading method signature gets changed while in case of overriding signature remains same.

2) Return type is not part of method signature in Java.

3) Overloaded method can be subject to compile time binding but overridden method can only be bind at run-time.

4) Both overloaded and overridden method has same name in Java.

5) Static method can not be overridden in Java.

6) Since private method is also not visible outside of class, it can not be overridden and method binding happens during compile time.

7) From Java5 onwards you can use annotation in Java to declare overridden method just like we did with @override. @override annotation allows compiler, IDE like NetBeans and Eclipse to cross verify or check if this method is really overrides super class method or not.

Update: One of my reader Rajeev makes an interesting comment about one change related to return type of overriding method from Java 5 onwards, which enable to use subtype of return type of overridden method. This is really useful, when original method returns a general type like java.lang.Object. If you are overriding clone() method in Java then you can use this feature to return actual type, instead of returning java.lang.Object and can save caller from type-casting cloned object. Here is the actual comment from Rajeev:

Hi Javin,I visit your blog regularly and I found that you missed covariant return which is added in Java 5 in the case of method overriding. When a subclass wants to change the method implementation of an inherited method (an override), the subclass must define a method that matches the inherited version exactly. Or, as of Java 5, you're allowed to change the return type in the overriding method as long as the new return type is a subtype of the declared return type of the overridden (super class) method. Let's look at a covariant return in action:

class Alpha {

Alpha doStuff(char c) {

return new Alpha();

}

class Beta extends Alpha {

Beta doStuff(char c) { // legal override in Java 1.5

return new Beta();

}

AS I said one of the good example of this is overriding clone method and using return type as Actual type instead of java.lang.Object, which is suggested by Joshua Bloch in Effective Java as well. This in in-fact one of the Java best practices while implementing clone method in Java. By the way don't forget to follow these Java overloading best practices, when doing it in your project.

SoniJaiswal

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Friday 28 June 2013