Java Interop

General Syntax

Java Wrapper Types general syntax.

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data X = X @[class-name]
  deriving Class

• [class-name] should be the fully qualified Java class name and X should be the Eta name you would use to refer to the corresponding Java class in foreign imports. Note that [class-name] can also be converted to an array type by appending [].
• The Class typeclass is a built-in typeclass that is a marker for a JWT. Make sure all your JWTs derive a Class instance.

Examples of JWT

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data JInteger = JInteger @java.lang.Integer
  deriving Class

data JIntegerArray = JIntegerArray @java.lang.Integer[]
  deriving Class

In this example, we’re declaring JWTs for the java.lang.Integer class and the java.lang.Integer[] array (which is technically a class on its own).

Deriving Standard Typeclass Instances

General Syntax

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data X = X @[class-name]
  deriving (Class, Eq, Show)

Currently, deriving the Class, Eq, and Show instances for JWTs is supported. You should derive these instances based on the need of the application. The Eq instance will use the underlying Object.equals() method and the Show instance will use Object.toString().

Marshalling Between Java and Eta Types

When writing FFI declarations, you are essentially specifying the type of a function whose arguments will be translated from Eta types to Java types and whose result will be translated from Java types to Eta types. This translation process is called marshalling. A Java Wrapper Type will marshal to an object of the class given in the definition.

The following table shows a couple of Eta types which aren't JWTs, but still marshal to a Java class or return type:

Java Primitives

The following table lists the mapping from primitive Java types to Eta types.

The CLASS Pragma

In some older code you might see the following JWT definition syntax:

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data {-# CLASS "[class-name]" #-} X = X (Object# X) deriving Class

This is the same as the following:

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data X = X @[class-name] deriving Class

The older syntax is not recommended.

Next Section

We will now proceed with Java Foreign Import Declarations. We will also learn how to handle Java Generics, Enums, Interfaces, Covariances and Contravariance.