[Solved] Why is it giving me the error “method ArrayList.add(String) is not applicable”?

Question

Problem:

ArrayList<String> means you want an array-backed list that can hold String objects. This restricts the add method to only accept strings. The mistake you are making is that you are passing other non-String objects into the add method.

Bad Answer 1:

The easy way out is to change it to ArrayList<Object>, but this defeats the purpose of generics. The good thing is, you can now pass any object to the add method. The bad thing is, you can now pass any object to the add method.

Bad Answer 2:

You can create three ArrayList objects instead:

ArrayList<Education> eduCategories [...]
ArrayList<Games> gameCategories [...]
ArrayList<Medical> medCategories [...]

But this is also bad, because now you have three lists.

Good Answer:

You only need one list. For this, you have to create a proper hierarchy:

Create an abstract class called Category:
```
abstract class Category
```

Create three classes Education, Games and Medical, all extending Category:

class Education extends Category
class Games extends Category
class Medical extends Category

Then you can create your list as:

ArrayList<Category> categories = new ArrayList<Category>();

categories.add( new Education() );
categories.add( new Games() ) ;
categories.add( new Medical() ) ;

In this case, you’re restricting the add method to accept only a Category object. Since Education, Games and Medical are all (by extension) Category objects, they will be accepted. But the following will fail:

categories.add( "some string" ); //compilation error: String is not a Category

So now we are able to take full advantage of generics (compile-time type protection) while allowing different sub-classes to be accepted by the same list.

Accepted Answer