How does "object.new" work? (Does Java have a .new operator?)

Question

I came across this code today whilst reading Accelerated GWT (Gupta) - page 151.

public static void getListOfBooks(String category, BookStore bookStore) {
    serviceInstance.getBooks(category, bookStore.new BookListUpdaterCallback());
}
public static void storeOrder(List books, String userName, BookStore bookStore) {
    serviceInstance.storeOrder(books, userName,    bookStore.new StoreOrderCallback());
}

What are those new operators doing there? I've never seen such syntax, can anyone explain?

Does anyone know where to find this in the java spec?

I knew about inner classes, but I use to create them as: `new outer.SomeClass()` — OscarRyz, May 20 '10 at 23:19
@Ash Kim inner classes can not be static . The correct terminology is static nested class. — Inquisitive, Jul 02 '12 at 11:38

score 67 · Accepted Answer · edited Jul 21 '14 at 13:46

67

They're inner (nested non-static) classes:

public class Outer {
  public class Inner { public void foo() { ... } }
}

You can do:

Outer outer = new Outer();
outer.new Inner().foo();

or simply:

new Outer().new Inner().foo();

The reason for this is that Inner has a reference to a specific instance of the outer class. Let me give you a more detailed example of this:

public class Outer {
  private final String message;

  Outer(String message) {
    this.message = message;
  }

  public class Inner {
    private final String message;

    public Inner(String message) {
       this.message = message;
    }

    public void foo() {
      System.out.printf("%s %s%n", Outer.this.message, message);
    }
  }
}

and run:

new Outer("Hello").new Inner("World").foo();

Outputs:

Hello World

Note: nested classes can be static too. If so, they have no implicit this reference to the outer class:

public class Outer {
  public static class Nested {
    public void foo() { System.out.println("Foo"); }
  }
}

new Outer.Nested.foo();

More often than not, static nested classes are private as they tend to be implementation details and a neat way of encapsulating part of a problem without polluting the public namespace.

edited Jul 21 '14 at 13:46

Bloke

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answered May 19 '10 at 05:50

cletus

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It's all spelled out here if anyone's hazy on this: http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html – chickeninabiscuit May 19 '10 at 06:19
and also a good discussion here: http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class – chickeninabiscuit May 19 '10 at 06:21
1

@cleutus "More often than not, static inner classes are private" . This is a wrong statement . inner classes can't be static . This should be called as "static nested class". See the link http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class – Inquisitive Jul 02 '12 at 11:32
shouldn't the last code line read `new Outer.Nested().foo();`? – AplusKminus Aug 03 '16 at 10:08
In my experience, the last paragraph is backwards; inner classes are often private (because they're implementing some interface like `Iterator` that needs privileged access to the top-level class), and static nested classes are data types like command objects or exceptions that apply to one specific context. – chrylis -cautiouslyoptimistic- Jan 07 '19 at 05:44

Greg Case · Answer 2 · 2016-03-21T22:49:45.537

4

BookListUpdaterCallback and StoreOrderCallback are inner classes of BookStore.

See The Java Tutorial - http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html and http://docs.oracle.com/javase/tutorial/java/javaOO/innerclasses.html

edited Mar 21 '16 at 22:49

answered May 19 '10 at 05:51

Greg Case

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score 1 · Answer 3 · answered May 19 '10 at 05:49

1

I haven't seen this syntax before either, but I think it will create an inner class of BookStore.

answered May 19 '10 at 05:49

tangens

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How does "object.new" work? (Does Java have a .new operator?)

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