Difference between the code snippets

[indra negi]

Please explain the difference between the following two code snippets and the outputs produced by them too.

public <T> List<?> meth(List<T> type)
{
   System.out.println(type); // 1
   return new ArrayList<String>(); // 2
}

public <T> List<T> meth(List<?> type)
{
   System.out.println(type);       // 1
   return new ArrayList<String>(); // 2
}    

[Harsh Pensi]

List<?> can be assigned a List of any type.
But List<T> must be assigned a List of type T. Since we do not know if at runtime, type T will be String or any other type, we cannot return ArrayList<String> from a method that is declared to return List<T>.

[indra negi]

Not clear. Please explain the output and also why it results in that output. Also one more code snippet has to be added to the above list :

public <T> List<?> meth(List<?> type)
{
   System.out.println(type); // 1
   return new ArrayList<?>();  // 2
}    

Please can someone explain the output of these three code snippets alongwith proper explanation. I am quite confused although I think I am clear about the last one but still please.

[Harsh Pensi]

This code does not produce any output, because there is no main method

[indra negi]

Well caught.

Actually I wan't to know whether the codes will compile or throw runtime exceptions. If compiler reports error then at which line and why.

[Salil Vverma]

  
	public <T> List<T> meth1(List<?> type) {
		System.out.println(type);
		return new ArrayList<String>();
	}

The above code will give compilation error "Type mismatch: cannot convert from ArrayList<String> to List<T>" reason being T might vary at run time and

new  ArrayList<String>() 

would not be equal to return type List<T> always, the correct syntax would be as mentioned below

 
	public <T> List<T> meth2(List<?> type) {
		System.out.println(type);
		return new ArrayList<T>();
	}

  

Regards
Salil Verma

[Salil Vverma]

   

public <T> List<?> meth3(List<?> type) {
		System.out.println(type); // 1
		return new ArrayList<?>(); // 2
	}

 

This code will give compilation error "Cannot instantiate the type ArrayList<?>" reason being. if meth3 is called from any function,by no means wild character (?) in the return statement would be replaced by a valid object. As we know that wild character (?) does not represents any specific object, the compilation error completely make sense.
The possible correction of this might be as follows -

   

public <T> List<?> meth3(List<?> type) {
		System.out.println(type); // 1
		return new ArrayList<T>(); // 2
	}

 

Regards
Salil Verma

[Salil Vverma]

  
public <T> List<?> meth(List<T> type) {
		System.out.println(type);
		return new ArrayList<String>();
	}

  

This code is perfect, runs without any error so I hope it does not require any explanation

Regards
Salil Verma

[Bert Bates]

As my good friend Jim Yingst used to say:

"Hmmm, if only there was an inexpensive way to find out on my own if a certain bit of code would compile, and if so what the output would be... if only I could find out for myself..."