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?

Answer 1

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.

Answer 2

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