ArrayList

[Ankur kothari]

  import java.util.*;
class ArrayTest
{
public static void main(String a[])
{
List<Integer> l=new ArrayList<Integer>();// why does this compile but List l=new ArrayList(); does not rest all being same?
l.add(42);
System.out.println(l.size());
l.add(34.44);
}
}  

has List l=new ArrayList(); been totally removed...nothing seems to be working with it?

[Lorand Komaromi]

Without the generic parameters from the compiler's point of view the list will look as follows:

List<Object> l = new ArrayList<Object>();

List.add() will accept an Object as parameter.

Floating point literals are doubles by default, and the compiler can box it into a Double, which is a descendant of Object that the add method accepts.

With the Integer generic parameter, however, the parameter of List.add() is an Integer, and there is no rule in the language for converting a double or Double to Integer when matching parameter types, so the compiler won't accept it.

[Ankur kothari]

whether it is a double or an int there is a compile error...

[Abhay Agarwal]

Hi Ankur

I ran below code

public class Testa {

	public static void main(String a[])   
	{   
	List l=new ArrayList();   
	l.add(42);   
    l.add(34.44);
    System.out.println(l.size());   
	}
}

It compiled fine and gave me output as 2.

If change code to add generics part as shown in below code

public class Testa {
	public static void main(String a[])   
	{   
	List<Integer> l=new ArrayList<Integer>();   
	l.add(42);   
    l.add(34.44);
    System.out.println(l.size());   
	}
}

here i get compile time error which is expected as we are trying to add double value in a list that takes only Integer

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The method add(Integer) in the type List<Integer> is not applicable for the arguments (double)

Hope it clears

Check which Java version are you using to compile your code as Autoboxing feature was not present before Java 5 version. You might getting error because you may be running your code on lesser version of Java (Java 1.4,1.3 etc)

[Ankur kothari]

  import java.util.*;
public class Testa {

	public static void main(String a[])   
	{   
	List l=new ArrayList();   
	l.add(42);   
    l.add(34.44);
    System.out.println(l.size());   
	}
}
  

oh am sorry...actually when i compile the program, it compiles but gives a warning that program uses unchecked operations.....i thought this was a compile error but it is a warning because when i ran it i got the output 2

Thanks everyone for your efforts..