This is what you have to do to interface from Java with a Haxe interface, as they all implement IHxObject for no particular reason.

ImplementsHaxe.java

import java.lang.Object;
import java.lang.String;

public class JavaClass implements HaxeInterface {
	// ...
	// Dummy IHxObject implementation:
	public boolean __hx_deleteField(String f) { return false; }
	public Object __hx_lookupField(String f, boolean e, boolean c) { return null; }
	public double __hx_lookupField_f(String f, boolean e) { return 0.0; }
	public Object __hx_lookupSetField(String f, Object v) { return null; }
	public double __hx_lookupSetField_f(String f, double v) { return 0.0; }
	public Object __hx_setField(String f, Object v, boolean p) { return null; }
	public double __hx_setField_f(String f, double v, boolean p) { return 0.0; }
	public Object __hx_getField(String f, boolean e, boolean c, boolean p) { return null; }
	public double __hx_getField_f(String f, boolean e, boolean p) { return 0.0; }
	public Object __hx_invokeField(String f, haxe.root.Array m) { return null; }
	public void __hx_getFields(haxe.root.Array<String> m) { }
}