# `java`

The library Java entities define a minimal abstraction for calling Java from Logtalk. This abstraction makes use of Logtalk parametric objects and allows creating Java objects, accessing Java class fields, and calling Java class and object methods using syntax closer to Logtalk. It also gives access to some Java utility predicates.

This abstraction was developed primarily to work with XVM (with its `jni` plug-in installed) or the JPL library bundled with SWI-Prolog and YAP. However, it’s expected to be implementable with alternative Java interfaces found in other backend Prolog compilers. Currently, a preliminary implementation is also available for JIProlog.

The main idea in this abstraction layer is to use parametric objects where the first parameter holds the Java reference (usually to a class or object) and an optional second parameter holds the return value. Together with a forward message handler, this allows the use of Java messages with the same functor and number of arguments as found in the relevant JavaDocs.

## API documentation

Open the ../../apis/library_index.html#java link in a web browser.

## Loading

To load all entities in this library, load the `loader.lgt` file:

    | ?- logtalk_load(java(loader)).

## Testing

To test this library predicates, load the `tester.lgt` file:

    | ?- logtalk_load(java(tester)).

## Usage

The two main objects in this library are `java(Reference, ReturnValue)` and `java(Reference)`. Use the latter if you want to ignore the return value or when calling a void Java method.

The `java` object implements utility predicates. For some backend Java interfaces such as JPL (available in SWI-Prolog, XVM, and YAP) there is also a `java_hook` hook object for removing any overhead when using this library abstraction.

For usage examples and additional tests, see the `clustering`, `document_converter`, `jpl`, and `neo4j` examples.

## Known issues

When running Java GUI examples on the macOS Terminal application, you may get a Java error saying that the AWT cannot be started. In alternative, try to run the example from within the SWI-Prolog macOS application instead of using the shell integration script. This issue is due to a macOS Java issue that’s orthogonal to both Prolog backends and Logtalk.