MPS Java compatibility

Also, dont forget to set java compliance level in the build scripts of your project. It should be the same as the project bytecode version.

In the MPS modules pool you can find the JDK solution, which holds the classes of the running Java. So when you start MPS under JDK 1.8, the latest Java Platform classes will be available in the JDK solution.

You can also use any external Java classes, compiled under JDK 1.8 by adding them as Java stubs.

Default interface methods

Java 8 introduced also default methods. These are methods implemented directly in the interface. You can read about default methods here: http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html

These methods can be called just like the usual instance methods. Sometimes, however, you need to call the default method directly from an interface that your class is implementing. For example, in case of multiple inheritance when a class implements several interfaces, each containing a default method with the same signature.

public interface A {
  public default void foo() {
     System.out.println("A");
  }
}

public interface B {
  public default void foo() {
     System.out.println("B");
  }
}

In that case foo() can be called explicitly on one of the interfaces via a SuperInterfaceMethodCall construction, which is newly located in the jetbrains.mps.baseLanguage.jdk8 language.

The jetbrains.mps.baseLanguage.jdk8 language allows users to create the 'default' methods in BaseLanguage interfaces.

The 'default' keyword is implemented by the DefaultModifier concept which extends the Modifier concept in the BaseLanguage.

The transformation/substitute menu + some cosy intentions are also supplied.

Java 8 introduced lambda expressions, of which you can learn more here: http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html.

Here is the example of an interaction with the new JDK 8 Collections API:

The forEach() method is the new default method of java.lang.Iterable. It takes a Consumer interface as a parameter. Consumer is a functional interface as it only has one method. In Java 8 it would be possible to pass a lambda expression to forEach(). In MPS you can pass the MPS closure. A closure knows the type of the parameter taken by forEach() while generating and it will be generated exactly to the correct instance of the Consumer.

The MPS closures are by default generated into Java classes, however, if a closure meets four compatibility rules, MPS will generate it into a Java lambda expression. Incompatibilities include the usage of:

• Yield operations

• “Functional” abstract classes

• Annotations

• Local variables conflicting with parent scope

Some issues may arise from this change:

• Ambiguous calls: while undetected in MPS, the Java compiler will report such issues with closures generated as lambdas.

• Closure instances: as a lambda, a closure will create a single instance at runtime, as opposed to anonymous classes that create a new instance every time they are called (see screenshot below).