java11-intersection-type

preview

Since Java 8 we have a new type: intersection type that is an intersection (or subtype) of multiple types.

For more specific info please take a look at: https://docs.oracle.com/javase/specs/jls/se11/html/jls-4.html#jls-4.9 (quite complex and obscure description).

project description

We will show easy example where intersection types are very handy:

1. Suppose we have classes from third-party libraries (we can't modify them):

   class Impl1 implements AutoCloseable, Runnable {
   
       @Override
       public void close() {
           System.out.println("closed1");
       }
   
       @Override
       public void run() {
           System.out.println("run1");
       }
   }
   

   class Impl2 implements AutoCloseable, Runnable, Serializable {
   
       @Override
       public void close() {
           System.out.println("closed2");
       }
   
       @Override
       public void run() {
           System.out.println("run2");
       }
   }
   

2. And we want to write a method that will execute action and close the resource, something like this:

   static <X> void runAndClose(X action) {
       try (y) {
           y.run();
       } catch (Exception e) {
           // handle
       }
   }
   

   • where X is: AutoCloseable and Runnable
3. We may try to go through additional interface:
   • additional interface:

     interface RunnableAndAutoCloseable extends AutoCloseable, Runnable { }
     

   • and the method:

     static void runAndClose2(RunnableAndAutoCloseable y) {
         try (y) {
             y.run();
         } catch (Exception e) {
             // handle
         }
     }
     

   • but note that we cannot modify Impl1 and Impl2 classes, so it is a road to nowhere
4. We may use intersection type:

   static <I extends Runnable & AutoCloseable> void runAndClose(I y) {
       try (y) {
           y.run();
       } catch (Exception e) {
           // handle
       }
   }
   

   and tests:

   Runner.runAndClose(new Impl1());
   Runner.runAndClose(new Impl2());
   

   will print:

   run1
   closed1
   run2
   closed2
   

serializing lambda

Note that using type inference and intersection types you can easily make lambda serializable:

var serializable = (Serializable & Supplier<String>)() -> "a";