5 Curious cases of Overloading & Generics
If you are preparing for an Interview you can expect questions on generics in java interview for sure , whether it is an position for Team lead or experience developer or Junior. But Generics gets more complicated when it mix with overloading in java. Let see the different scenarios when Generic mix with overloading.
What is generics in Java ?
Generics is technique which facilitates to declare a generic type(T, Irrespective of any Datatype in Java) in methods or class, which will be resolved later when caller of the method or class provide the actual Data Type(Integer, Boolean, String or ant Custom class) and ensure compile time safety but at the run-time it removes the type safety, we call this process as Type erasure.
Benefits of Generics:
By generics, we can achieve some level of the generic type declaration. To be specific Java is a static type language which means when we declare properties or methods we have to provide the type of the parameter or return type at the time of declaration.
But by generics we can defer the process we can declare methods or properties using generic syntax and later on caller can decide what data type pass to it, but remember it is not same as dynamic type language like javascript where var declaration means it represents any data types, on other hand Generics introduce the Type Inference from a bounded context, from Java perspective which is the first step towards functional programming(lambda) adaptation.
Challenges :
The most tricky part of generics is by Type erasure, java removes the bounded type at runtime, so in java runtime generics and non-generics method/property declaration both are same no difference at all, to maintain backward compatibility but generics ensure compile time safety so if you declare generics with certain type it only bounds to that certain type if you want to pass any other type compiler will complain instantly.
But as run time its type vanishes so in the case of Overloading we have to think what the method looks after type erasure erases the generic type, to do a perfect overloading unless compiler will complain.
In this Article, we will see 5 such scenarios which can occur often if I miss any please write in the comment section so I can add them to the Article.
Hope you are familiar with the generic syntax and how to use it.
Scenario 1 :
public Integer add(Integer a, Integer b) {
System.out.println(a+b);
return a+b;
}
/* (non-Javadoc)
* @see com.example.generics.ICalculate#add(java.lang.Object, java.lang.Object)
*/
public <T> T add(T a, T b) {
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return (T) resut;
}
See the two versions of add method can you tell me is it a valid overloading?
Yes this is a valid over loading as after type erasing public <T> T add(T a, T b) this method signature converts to , public Object add(Object a, Object b) clearly it is different than
public Integer add(Integer a, Integer b) .
Scenario 2:
Now see the following version of overloading
public <T> T add(T a, T b) {
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return (T) resut;
}
public Object add(Object a,Object b) {
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return resut;
}
Can you tell me is it a valid overloading?
Probably you guess the answer it is not as I told you in previous example public <T> T add(T a, T b) this method signature converts to , public Object add(Object a, Object b) after type erasing so now both signatures looks same so compiler will complain.
Scenario 3:
Now take the following version
public <T> T add(T a, T b) {
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return (T) resut;
}
public <T extends Number> T add(T a, T b) {
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return (T) resut;
}
Is the above a valid Overloading?
Yes it is a valid overloading as after type erasing public <T> T add(T a, T b) changed to public Object add(Object a,Object b) but public <T extends Number> T add(T a, T b) changed to
public Number add(Number a, Number b) as T extends Number means any type which extends Number so after Type erasing it will take Number as infer type.
Exercise :
Can you tell me Is it valid overloading with explanation -- without paste that code in editor?
public Integer add(Integer a, Integer b) {
System.out.println(a+b);
return a+b;
}
public <T extends Number> T add(T a, T b) {
System.out.println(a.getClass().getName());
Integer a1 = (Integer)a;
Integer b1 = (Integer)b;
Integer resut = a1+b1;
System.out.println(" Result is" + resut);
return (T) resut;
}
Now let take a slightly different example where I try to pass a data type to a collection which will be the bounded context for that collection.
Scenario 4:
public void add(List<Integer> list) {
System.out.println("add list of Integers");
}
public void add(List<?> list) {
System.out.println("add/concat list of any type");
}
Is it a valid overloading?
The Answer is, unfortunately, no, because after type erasing generics will lose its type and would become same as our old list.
So both signatures public void add(List<Integer> list) and public void add(List<?> list) change to public void add(List list) and public void add(List list) so compiler will complain for sure.
Scenario 5 :
public void add(List<Integer> list) {
System.out.println("add list of Integers");
}
public void add(List<Double> list) {
System.out.println("add list of Integers");
}
As previous reason it is also not a perfect overloading.
So always pay attention of your generics signature while extending a class or overload method.
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