Method overloading and method overriding ( in other words, polymorphism) concepts are often tested in java interviews. Let us understand their differences with easy-to-follow examples.
It is worth remembering that method overloading happens within the same class, and method overriding happens when a Child class extends a Parent class.
1. Method Overloading
Method overloading happens when we define two methods of the same name in the same class. Here are a few rules that we should keep in mind while overloading any method:
1.1. Method Arguments must be Different
First and most important rule to overload a method is to change the method signature. The method signature comprises the number of method arguments, type of arguments and order of arguments if there are multiple arguments.
In the following example, the method sum() is defined two times. Both methods accept two arguments. The first method takes arguments of type Integer, and the second method takes arguments of type Float and Integer. Here the number of method arguments is same, but their types are different.
public class Calculator {
public Integer sum(Integer a, Integer b) {
return a + b;
}
public Integer sum(Float a, Integer b) {
return a.intValue() + b;
}
}1.2. Method Return Types are Not Considered
The return type of the method is not part of the method signature, so only changing the return type of the method does not amount to method overloading.
In the following example, the methods accept similar arguments and their return types differ. It is an invalid code and gives a compiler error “‘sum(Integer, Integer)’ is already defined in ‘Calculator’“.
public class Calculator {
public Integer sum(Integer a, Integer b) {
return a + b;
}
public Double sum(Integer a, Integer b) {
return new Double(a + b);
}
}
1.3. Thrown Exceptions are Not Considered
The exceptions thrown from methods are also not considered when overloading a method. So if the overloaded method throws the same exception, a different exception, or does not throw any exception, there is no effect at all on method overloading.
public class Calculator {
public Integer sum(Integer a, Integer b) throws NullPointerException{
return a + b;
}
public Integer sum(Integer a, Integer b) throws IllegalArgumentException{
return a + b;
}
}2. Method Overriding
Method overriding happens when a child class overrides a method from the parent class. Always remember these rules when overriding a method in java.
2.1. Method Arguments must be Exactly Same
The method argument list in overridden and overriding methods must be exactly the same. If they don’t match, we have created a different method.
In the following example, the Parent and the Child classes have methods with the same name and exact same parameters list. It is a valid method overriding.
class Parent {
public Integer sum(Integer a, Integer b) {
return a + b;
}
}
class Child extends Parent {
public Integer sum(Integer a, Integer b) {
return a + b;
}
}2.2. Method Return Type can be Subtype in Child Class
The return type of the overriding method can be the child class of the return type declared in the overridden method.
In the following example, the method return type in Parent is Number, and in the Child class, it is Integer. It is a valid return type and thus considered the valid method overriding.
class Parent {
public Number sum(Integer a, Integer b) {
return a + b;
}
}
class Child extends Parent {
public Integer sum(Integer a, Integer b) {
return a + b;
}
}If we use the incompatible return type in the Child class, we will get the compiler error.
class Parent {
public Number sum(Integer a, Integer b) {
return a + b;
}
}
class Child extends Parent {
public String sum(Integer a, Integer b) {
return a.toString() + b.toString();
}
}'sum(Integer, Integer)' in 'Child' clashes with 'sum(Integer, Integer)' in 'Parent'; attempting to use incompatible return type2.3. Thrown Exception can be Subtype in Child Class
The overriding method can not throw a checked exception higher in the hierarchy than thrown by the overridden method.
For example, the overridden method in Parent class throws FileNotFoundException, the overriding method in Child class can throw FileNotFoundException; but it is not allowed to throw IOException or Exception, because IOException or Exception are higher in the hierarchy.
class Parent {
public String readFile(String file) throws FileNotFoundException {
//...
return null;
}
}
class Child extends Parent {
public String readFile(String file) throws IOException {
//...
return null;
}
}In above code, we will get the compiler error:
'readFile(String)' in 'Child' clashes with 'readFile(String)' in 'Parent'; overridden method does not throw 'java.io.IOException'Note that we can omit the exception declaration from the overriding method. It’s allowed and perfectly valid. Also, overriding method can throw any unchecked (runtime) exception, regardless of whether the overridden method declares that exception or not.
In following example, the overriding method omits the FileNotFoundException and declares the RuntimeException which is unchecked exception. This is valid method overriding.
class Parent {
public String readFile(String file) throws FileNotFoundException {
//...
return null;
}
}
class Child extends Parent {
public String readFile(String file) throws RuntimeException {
//...
return null;
}
}2.4. The private, static and final methods can not be Overridden
The private, static and final methods can not be overridden in java in any way.
- The private modifier restricts the method to class.
- The static members belong to the class object.
- The final keyword is used wth members that must not be overridden.
In the following example, Parent and Child classes declare the method with same name and parameters, But it is not method overriding. The readFile() is never accessible outside the Parent class.
class Parent {
private String readFile(String file){
//...
return null;
}
}
class Child extends Parent {
public String readFile(String file) {
//...
return null;
}
}2.5. Overriding Method can not Reduce Access Scope
Also note that the overriding method can not reduce the access scope of the overridden method.
For example, if the overridden method in Parent class is protected, then the overriding method in Child class can not be private. It must be either protected (same access) or public (wider access).
class Parent {
protected String readFile(String file){
//...
return null;
}
}
class Child extends Parent {
public String readFile(String file) {
//...
return null;
}
}3. How to Verify if Method Overriding is Correct
To verify that we are correctly overriding a method or not, simply use the annotation @Override on overriding method in the Child class. This will verify all the method overriding rules. If there is any issue in overriding, it will result in compile time error.
class Parent {
protected String readFile(String file){
//...
return null;
}
}
class Child extends Parent {
@Override
public String readFile(String file) {
//...
return null;
}
}That’s all for this simple yet important concept to brush your basics in core java and object-oriented programming.
Happy Learning !!
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