java:reify

(java:reify "fully.qualified.Interface" "method" function ...)

Creates a host instance of the given interface whose methods are implemented one by one. Each "method" names one method of the interface, and the function after it is called with that method's arguments -- unlike java:proxy, without the method's name. The function's value is converted to the method's return type (a void method ignores it). Part of the JVM-only java interop package: the interpreter and JVM-compiled classes, not the WASM backend. See the Java interop guide.

Naming the method

A name several methods of the interface share is tagged with the parameter types, as a java:call method name is: "append(char)", "append(CharSequence,int,int)". A name that matches more than one method, or none, is an error.

toString, equals and hashCode can be named too. Otherwise equals and hashCode are identity, and toString answers #<java-reify fully.qualified.Interface>.

Methods no name designates

An abstract method is not implemented: calling it throws UnsupportedOperationException (java:reify: no implementation of java.util.Iterator.next()). A default method keeps the interface's body, so combinators work:

The value a function returns

It is converted as an argument is: an integer to int, a list to an array or a List, nil to false or null, and so on. A function is not made a proxy on the way back -- return a java:reify or java:proxy object where an interface is expected. A value that does not convert is an error, for example java:reify: cannot return "x" as int from java.util.function.IntSupplier.getAsInt.

In a compiled program

A java:reify whose interface and method names are literal strings, and whose interface the compile can see, is a class generated at compile time (Prog$Reify0.class beside the program). Nothing reflects, so the program compiles under --java-static and builds into a GraalVM native image with no configuration. The object prints as #<java Prog$Reify0> compiled and as the name of a java.lang.reflect.Proxy class interpreted. A java:reify whose names are computed is implemented when it runs, through the reflection bridge.