Java - Generics with lists

Question

I wrote a function for my cache to retrieve a specific object. This way I don't need to cast it .

@SuppressWarnings("unchecked") public static <T> T inCache(Class<T> obj, String token) { Object cacheObj = Cache.get(token); if (cacheObj != null) { if (obj.isAssignableFrom(cacheObj.getClass())) { return (T) cacheObj; } } return null; }

I am using it like this

String s = inCache(String.class, title);

But now I have a list of Strings in my cache and I can't use it like this

List<String> ipList = Util.inCache(List<String>.class, title);

The problem is the List<String>.class . I am very new to java, how do I have to write it?

as a side note, you don't need that suppresswarnings, use return obj.cast(cacheObj) instead. — jtahlborn
– jtahlborn, Commented Aug 17, 2012 at 16:13

Raghav Sood · Accepted Answer · 2012-08-17 16:29:47Z

There is a concept in java called type erasure. Due to legacy reasons, something like List is just a list. It doesn't remember that it is a list of string at run time. You should just write List.class.

You can then specify the type of object in the List when iterating through it.

ice · Accepted Answer · 2012-08-17 16:14:24Z

4

You can't get class of List<String>, in your case the only way is:

List<String> ipList = (List<String>)Util.inCache(List.class, title);

answered Aug 17, 2012 at 16:14

ice

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1 Comment

jtahlborn Over a year ago

with an unavoidable SuppressWarnings.

Harmeet Singh · Accepted Answer · 2012-08-17 16:24:24Z

1

You can try :

List<String> ipList = Util.inCache(List.class, title);

answered Aug 17, 2012 at 16:24

Harmeet Singh

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Comments

Subhrajyoti Majumder · Accepted Answer · 2012-08-17 16:41:18Z

Try this-

List<String> inList = (List<String>)Test.inCache(List.class, title);

And you can do also -

List<String> inList = Test.inCache((Class<? extends List<String>>)List.class, token);

Shrulik · Accepted Answer · 2012-08-17 19:03:01Z

Just to clarify Joe's answer ( I don't have enough reputation to comment), at runtime there is no difference between a List <String> and List<Integer> or any other type of List, generics aren't kept at runtime.

Meaning, List<String>.class is completely identical to List<Integer>.class and is actually List.class. This is a weakness of the Java type system. I'm not familiar with a simple way to implement what you wish for.

A code proof for the heck of it :

// It is true that List<String> stringList = new ArrayList<String>(); List<Integer> integerList = new ArrayList<Integer>(); System.out.println( stringList.getClass() == integerList.getClass() ); // And that ... List objectList = new ArrayList(); System.out.println( stringList.getClass() == objectList.getClass() ); //However, the following is false because a different implementation is used ( I wanted a false case) List objectLinkedList = new LinkedList(); System.out.println( objectLinkedList.getClass() == objectList.getClass() );

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