Method overloading and method overriding in Java is an important concept to understand and even more important is difference between overloading and overriding in Java. Due to polymorphism in Java you can have two methods with same name, but since two duplicate methods can not reside in a class in Java you either overload a method or override a method. 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.
What are Method Overloading and Method 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 based 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 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 below example of method overloading. Binding of overloading method occurs during compile time. To overload a java method just changes its signature. Just remember in order to change signature you either need to change number of argument, or type or argument in Java. Since return type is not part of method signature simply changing return type will result in duplicate method and you will get compilation 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. How to override a method in Java
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. When you override a method in Java its signature remains exactly same including return type. JVM resolves correct overridden method to call based upon object at run-time. 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 in Java , 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.
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.
- Overriding method can not extend method access means if original or overridden method is protected access than overriding method can not make it public.
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. Overloading of method in Java doesn’t have such restriction and you are free to modify throws clause as per your need.
Difference between method overloading and overriding in Java
Overloading vs Overriding in Java is one of the popular java interview questions at many companies and 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) In Case of method overloading in Java, Signature of method changes while in case of overriding it remain same.
2) You can overload method in one class but overriding can only be done on subclass.
3) Static method can not be overridden as they are not associated with object.
4) Overloaded method 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.
Example of method overloading and overriding in Java
Here is an example of both method overloading and method overriding. 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 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();
}
}
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";
}
}
class PersonalLoan extends Loan{
@Override
public String toString() {
return "This is PersonalLoan 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.
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4 comments:
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what about the exceptions while overriding and overloading
Good point, I have added one section regarding exception handling for overloading and overriding methods in java. thanks for pointing this.
Hi Javin,I visit your blog regularly and I found that you missed covariant return which is added in Java5 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 (superclass) 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();
}
}
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