Genrics in ReturnType

[Chandra shekar M]

Hi All

     class a{
    
       public static <T> getInstance(Enum instanceType){
             B<T>  instane;
              switch(instanceType)
              case 
              instance = new C<T>();
              case
               instance = new D<T>();
       
          return instance;
      }
 
       in my client code

         class testGenric{

              public void genrateInstance(){
                  a.getInstance(enum);   // How can i specify the T here because the Generic Type is required by the returnType and not arguments
                                                        // Then Hown can i specify T here ?  
           }
    

Thanks

[Ankit Garg]

Chandra your code has multiple compilation issues which makes it impossible to answer your query. And also can you show us the code of class B, C and D...

[Chandra shekar M]

import java.util.ArrayList;
import java.util.List;


public class Factory {

	
	public static <T> List<T> getInstance(int size){
		List<T> instance=null;
		instance = new ArrayList<T>(size);
		return instance;
	}
	
	public static void main(String...strings){
		List<String> ls=Factory.getInstance(5);
		
            //When i am making Factory.getInstance(5) this call
	   // I have not specified T for List<T> in return Type
	   // Is there a way to specify the T in this case ?	
	}
}

DONE.

[Chandra shekar M]

Hi All,

Any Luck on this.

[Rachid Aourich]

I really don't understand what you want to do
Doing something like that is allowed.

public class Factory {

	
	public static <T> List<T> getInstance(T elt,int size){
		List<T> instance=null;
		
		instance = new ArrayList<T>(size);
		instance.add(elt);
		return instance;
	}
	
	public static void main(String...strings){
		List<String> ls=Factory.getInstance(new String("dd"),5);
		System.out.println(ls);
	}
}

[Tomasz Sochanski]

Chandra shekar M wrote:

import java.util.ArrayList;
import java.util.List;

public class Factory {

	public static <T> List<T> getInstance(int size){
		List<T> instance=null;
		instance = new ArrayList<T>(size);
		return instance;
	}
	
	public static void main(String...strings){
		List<String> ls=Factory.getInstance(5);
		
       // When i am making Factory.getInstance(5) this call
	   // I have not specified T for List<T> in return Type
	   // Is there a way to specify the T in this case ?	
	}
}

This is really interesting, it seems that declaring list as List<String> will cause method getInstance(int) to create ArrayList for String objects, and declaring it as List<BigDecimal> will do this list for BigDecimals (in runtime both of them are just Lists of Objects with proper casting added to methods operating on those elements). Anybody could clarify what happens here?

[Ankit Garg]

The compiler is automatically able to find the type of <T> in your case but there is a way to specify it if you want
List<String> ls=Factory.<String>getInstance(5); //<T> is String now

[Chandra shekar M]

Many Thanks to all. This was a very good solution any good material for further reading on Generics.

Thanks
Chandra