Java – what is the purpose of calling type parameters in the new constructor?

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

ClassInstanceCreationExpression ::=
| new TypeArguments_opt TypeDeclSpecifier TypeArgumentsOrDiamond_opt
    ( ArgumentListopt ) ClassBodyopt
| Primary . new TypeArguments_opt Identifier TypeArgumentsOrDiamond_opt
    ( ArgumentListopt ) ClassBodyopt

What is the purpose of the new first optional type parameter list? I can't read section 15.9 (all references to the type parameter list seem to refer to the list after the type / Identifier) Testing random bits on the standard Java compiler can produce confusing results:

public class Foo<T> { }
// ...
Foo<Integer> t1 = new <Integer> Foo<Integer>();  // works
Foo<Integer> t2 = new <Integer> Foo();           // works -- unchecked warning missing the type arg after Foo
Foo<Integer> t3 = new <Boolean> Foo<Integer>();  // works
Foo<Integer> t4 = new <Float,Boolean> Foo<Integer>();  // works
Foo<Integer> t5 = new <NotDefined> Foo<Integer>();  // fails -- NotDefined is undefined

In these simple examples, it seems that this first parameter list has no meaning, although it parses and checks the validity of its parameters

Solution

Constructors can also declare type parameters

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

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

This is what < string > is for before the constructor call It is the type parameter of the constructor

following

Foo<Integer> foo = new <String> Foo<Integer>(1,new Object());

failed

In your last

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

Notdefined is not a type found during compilation If so, it will only give you a warning that it is not used