How to define the type of an Collection at run time

[akhil achuthan]

how to define the type of a collection at runtime.

For example,

if i have an Arraylist which has to be created based on the type, may be integer sometimes, or String or any object. I will know the type only at runtime.

I cannot declare the arraylist like ArrayList<String> arrList = new ArrayList<String>(), as this will not work for any other object type. I do not want to declare the arraylist raw as well.

Please suggest......

[Paul Clapham]

You can't specify the generic type at run time.

[Rob Spoor]

Correct. Search for "type erasure" to find out why.

[salvin francis]

not exactly "run time", but this can help you:

      final int TYPE_STRING=0;
      final int TYPE_INT=1;
      final int TYPE_BOOLEAN=2;
      
      
      List<?> list;
      int desiredListType = getDesiredListType();//decides the desired type
      switch(desiredListType){
          case TYPE_STRING:
              list = new ArrayList<String>();
          case TYPE_INT:
              list = new ArrayList<Integer>();
          case TYPE_BOOLEAN:
              list = new ArrayList<Boolean>();
      };

My knowledge about generics is bad, someone please correct me if wrong...

[Prabhat Jha]

   List<?> list 

means anything and everything is excepted by the list.....

You wouldn't like to do this your type safety is gone for a toss unless you have very good reason to do so

[salvin francis]

Prabhat Jha wrote:

   List<?> list 

means anything and everything is excepted by the list.....

You wouldn't like to do this your type safety is gone for a toss unless you have very good reason to do so

ye, i agree with that. I do not know a situation where i do not know what my collection will hold.

[Jesper de Jong]

You cannot specify the type of a collection at runtime, and it is not useful to do so.

Java is a statically typed language. That means that the compiler needs to know all the types of the variables when you compile the program. If you don't know the type at compile time, you can't make use of Java's static type checking. If you know the type only at runtime, then that's too late for type checking by the compiler - because when your program is running, it has already been compiled.

So, there is no way to do this. It's like you want to eat the cake before you bake it.

[Rob Spoor]

Prabhat Jha wrote:

   List<?> list 

means anything and everything is excepted by the list.....

Actually, no. It means it can return anything and everything but you cannot add anything at all to the list. After all, the ? could in reality be String, Integer, Foo, ... The compiler can never know that what you add is allowed so it simply disallows everything.