Add an example of calling Mercury libraries from Java.

In particular, demonstrate how to invoke any module-local finalisers and
retrieve the exit status.

samples/java_interface/standalone_java/Makefile:
samples/java_interface/standalone_java/JavaMain.java:
samples/java_interface/standalone_java/mercury_lib.m:
	As above.

samples/java_interface/README:
	Describe the new directory.

samples/java_interface/standalone_java/JavaMain.java

@@ -0,0 +1,53 @@
+// vim: ts=4 sw=4 et
+
+// The JavaInternal class in the jmercury.runtime package provides various
+// Mercury runtime services that we may require.
+// All Mercury runtime and generated Java code lives in the jmercury package.
+//
+import jmercury.runtime.JavaInternal;
+
+// The mercury_lib class is generated by the compiler when we build
+// mercury_lib library.
+//
+import jmercury.mercury_lib;
+
+import static java.lang.System.out;
+
+public class JavaMain {
+
+    public static void main(String[] args)
+    {
+        // We do not need to do anything to initialise the Java version of the
+        // Mercury runtime.  It will be automatically initialised as the
+        // relevant classes are loaded by the JVM.
+
+        out.println("JavaMain: start main");
+
+        // This is a call to an exported Mercury procedure that does some I/O.
+        // The mercury_lib class contains a static method for each procedure
+        // that is foreign exported to Java.
+        //
+        mercury_lib.writeHello();
+
+        // This is a call to an exported Mercury function.
+        //
+        out.println("3^3 = " + mercury_lib.cube(3));
+
+        // When we have finished calling Mercury procedures then we need to
+        // invoke any finalisers specified using ':- finalise' declarations in
+        // the set of Mercury libraries we are using.
+        // The static method run_finalisers() in the JavaInternal class does
+        // this.  It will also perform any Mercury runtime finalisation that
+        // may be needed.
+        //
+        JavaInternal.run_finalisers();
+
+        // The Mercury exit status (as set by io.set_exit_status/1) may be read
+        // from the static field 'exit_status' in the JavaInternal class.
+        //
+        out.println("JavaMain: Mercury exit status = "
+            + JavaInternal.exit_status);
+
+        out.println("JavaMain: end main");
+   }
+}