Java undefined generic type

Question

I have a generic class which extends another generic class.

The abstract class has 2 type parameters, but I need only one in my functions.

Is it save to just assign it a random like String type, or are there any drawbacks to this?

public abstract class AbstractFoo<T, B>
{
   public abstract void read(T item);
}

public class LittleFoo extends AbstractFoo<byte[], String>
{
   @Override
   public void read(byte[] item)
   {
      // work here
   }
}

If you want just one generic parameter, then why you have declared 2 at the first place? — Rohit Jain
– Rohit Jain, Commented Nov 20, 2012 at 18:26
Because I have multiple classes inheriting from this one by design. Only a few special classes only need 1 parameter. — John Frost
– John Frost, Commented Nov 20, 2012 at 18:31

Ioan · Accepted Answer · 2012-11-20 18:43:49Z

5

Or you can use java.lang.Void instead of String.

answered Nov 20, 2012 at 18:43

Ioan

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Comments

arshajii · Accepted Answer · 2012-11-20 18:52:15Z

3

No there really isn't any drawback to this, and it seems like a simple way to do it in my opinion.

Also, you might want to consider using composition instead of extension. For example, take a look at the HashSet implementation from java.util:

87   public class HashSet<E>
88       extends AbstractSet<E>
89       implements Set<E>, Cloneable, java.io.Serializable
90   {
91       static final long serialVersionUID = -5024744406713321676L;
92   
93       private transient HashMap<E,Object> map;
94   
95       // Dummy value to associate with an Object in the backing Map
96       private static final Object PRESENT = new Object();

A HashSet is basically a HashMap, but only considers the set of keys (hence the dummy variable PRESENT, which is just a placeholder). You could perhaps do something similar.

edited Nov 20, 2012 at 18:52

answered Nov 20, 2012 at 18:40

arshajii

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1 Comment

John Frost Over a year ago

Very informative answer, thanks. Actually I'm using both, my classes have other children of AbstractFoo as fields.

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