Huston, we have a problem !

"Zdeněk Troníček" tronicek at
Sun Mar 6 03:23:26 PST 2011

My answer is here (I also post it on my blog at

As you may know, one of new features of upcoming Java 7 will be the
diamond operator. Purpose of the diamond operator is to simplify
instantiation of generic classes. For example, instead of
List<Integer> p = new ArrayList<Integer>();

with the diamond operator we can write only
List<Integer> p = new ArrayList<>();

and let compiler infer the value of type argument. Nice simplification.
But do we really need to write <>? Isn't new ArrayList() enough? In this
article, I will describe the arguments of the <> proponents and explain
why I think that these arguments are not very strong. However, I also
describe arguments why we need <>.

In Java 1.4, we had raw types only:
List p = new ArrayList();

Java 5 introduced generics:
List<Integer> p = new ArrayList<Integer>();

Many types in Java API were generified and even though we can still use
generic types as raw types, there is no reason for this in Java 5 or
newer. When generics were introduced, raw types were allowed for backward
compatibility so that we could gradually and smoothly adopt generics. For
example, code in Java 1.4 can be combined with new generic code because
raw and generic types are allowed together. This is also expressed in the
JLS (4.8 Raw Types):

"The use of raw types is allowed only as a concession to compatibility of
legacy code. The use of raw types in code written after the introduction
of genericity into the Java programming language is strongly discouraged.
It is possible that future versions of the Java programming language will
disallow the use of raw types."

Now let's go back to the diamond operator and ask again: "Do we really
need <>?". The proponents of the <> syntax say that we need <> to preserve
backward compatibility. Let's look at an example from the coin-dev

class Foo<X> {
   Foo(X x) { }
   Foo<X> get(X x) { return this; }

class Test {
   void test() {
      Foo<?> f1 = new Foo(1).get(""); //ok - can pass String where Object
is expected
      Foo<?> f2 = new Foo<>(1).get(""); //fail - cannot pass String where
Integer is expected

This shows the difference between new Foo(1) and new Foo<>(1). Clearly,
these two are different and if we changed the semantics of new Foo(1), it
would break backward compatibility. But wait. Backward compatibility with
what? Isn't line
Foo<?> f1 = new Foo(1).get("");

a little suspicious? It uses generic type in the left part and raw type in
the right part. Although it is legal, it is probably either omission or
malpractice. And its legality is probably only a side effect of "a
concession to compatibility of legacy code".

Let's go further and look at another example from the coin-dev conference.
It shows the difference between raw type and parameterized type with the

public class X<T> {
   public X(T t) { }
   public T get() { return null; }

   public static int f(String s) { return 1; }
   public static int f(Object o) { return 2; }

   public static void main(String[] args) {
      System.out.println(f(new X<>("").get()));
      System.out.println(f(new X("").get()));

Let's play with the code a bit. Let's assume that there was a library with
the X class:

public class X {
   public X(Object o) { }
   public Object get() { return null; }

and some code that compiled against this library:

public class Client {
   static int f(String s) { return 1; }
   static int f(Object o) { return 2; }

   public static void main(String[] args) {
      System.out.println(f(new X("").get()));

Then, the library was generified:

public class X<T> {
   public X(T t) { }
   public T get() { return null; }

and we compiled the client project against the generified version. Now, if
we changed the semantics of new X("") to new X<String>("") (or new X<>("")
with the diamond syntax), the code would behave differently. So, the
answer to the title question is 'yes'. If we want to stay backward
compatible, we need <> and we cannot put new X("") semantically equal to
new X<>("").

Another questions are how long can Java evolve and remain compatible with
concessions to compatibility and whether newcomers to Java will appreciate

Zdenek Tronicek
FIT CTU in Prague

Maurizio Cimadamore napsal(a):
> On 18/02/11 15:46, "Zdeněk Troníček" wrote:
>> Maurizio, this is not good example because the first call is not correct
>> here. So, if it does not compile it should not be wrong even if it
>> compiled under previous version.
> What is wrong about the following code?
> Foo f1 = new Foo(1).get("");
> Maurizio
>> And hopefully you do not want to drive the language design by a
>> requirement to be source compatible with sources which are apparently
>> wrong.
>> Z.

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