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Generic Collections

Martin Allison
Greenhorn

Joined: Feb 28, 2009
Posts: 1
Hi

What is the difference between

Set<myClass> s1 = new TreeSet<myClass>();
and
Set<myClass> s2 = new TreeSet();

s2 seems to have the same restricions as s1, so why is there a warning on s2?

Marty
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18896
    
  40


For backward compatability, you are allowed to declare references to classes with generics, and instantiate classes with generics, without specifying the generic type.... However, it will generate a warning error.

Henry


Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Rob Spoor
Sheriff

Joined: Oct 27, 2005
Posts: 19722
    
  20

Consider the following code:

The above code will throw a ClassCastException when iterating over stringSet, because it will cast the Integer object to a String.

By warning you the compiler tells you that the code may cause problems at some stage. Therefore, in an ideal situation, you make sure there are no warnings. Sometimes you can't however; in those cases it is vital that you document why the warning is safe to ignore. If you can't then it isn't safe and you should prevent the warning.

An example of a warning you may ignore:


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subject: Generic Collections