What is the purpose of type arguments in constructor call following new?

Question

In the Java specification (http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.9), new has the following form:

ClassInstanceCreationExpressio         


        

Answer 1

Constructors can declare type parameters too

public class Main {     
    public static class Foo {
        public  Foo(T object, E object2) {

        }
    }
    public static void main(String[] args) throws Exception {
        Foo foo = new  Foo(1, "hello");           
    }    
}

That's what the preceding the constructor call is for. It is the type argument for the constructor.

The following

Foo foo = new  Foo(1, new Object());

fails with

The parameterized constructor Foo(Integer, String) of type Main.Foo is not applicable for the arguments (Integer, Object)

In your last

Foo t5 = new  Foo();  // fails -- NotDefined is undefined

NotDefined is just not a type that is found during compilation. If it was, it would just give you a warning that it is unused.