CODEWITHSUNDEEP
javagenerics
WIZARDELF
WIZARDELF

Reputation: 3895

Why would you use a generic type specifier when extending a Java class?

I just wonder what usage the following code has:

public class Sub extends java.util.ArrayList<String> {...}

There is no any compiling restriction on the generic constraint java.util.ArrayList<String>.

Upvotes: 0

Views: 1046

Answers (4)

Saurabh Gokhale
Saurabh Gokhale

Reputation: 46415

You can extend class ArrayList, but it is not something that you should normally do.
Only ever say "extends" when you can truthfully say "this class IS-A that class."

Remember, Its not a good practise to extend the standard classes

Why not use like this ? 

public class Sub {
    List<String> s = new ArrayList<String>();
    // .. 
    // ...
}

Upvotes: 1

Mike Samuel
Mike Samuel

Reputation: 120566

The compiler does place restrictions on other code based on the type parameter in this case.

This will compile

public class Sub extends java.util.ArrayList<String> {
  void addTwice(String s) { this.add(s); this.add(s); }
}

but this will not

public class Sub extends java.util.ArrayList<String> {
  void addTwice(Object x) { this.add(x); this.add(x); }
}

Upvotes: 3

Marcelo
Marcelo

Reputation: 11308

If you do that you can add to the basic functionality of an ArrayList or even change its normal functionality.

For example, you can override the add() method so that it will only add emails to the list.

Upvotes: 0

Jeffrey
Jeffrey

Reputation: 44808

Let's say you were making an index for a book, but you don't know how many indices you will need. You could make a class BookIndex extends ArrayList<String> or if you want to get really picky: BookIndex extends ArrayList<IndexEntry>.

/e1 Also, when a one Class extends a generic Class like ArrayList<String> you can grab the String out from the generic declaration, unlike if you had a class ArrayList<T>. In ArrayList<T> you would never be able to figure out what the T is.

Upvotes: 1

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