Calling Java methods from Scheme

You can call a Java method as if it were a Scheme procedure using various mechanisms.

Using colon notation

The general form of the colon notation is this:

container:part-name

This evaluates to the “part” named part-name in a given container. If the container is a class (i.e. a java.lang.Class or a gnu.bytecode.ClassType), then the “parts” of the class are its static methods and fields.

The simplest case is when the class is a fully-qualified class name:

(java.lang.Math:sqrt 9.0) ⇒ 3.0

This is only allowed when the name is of a class that exists and is accessible both at compile-time and run-time, and the name is not otherwise lexically bound.

You can also for the container use an expression that evalutes to a type or class. Most commonly you'd use a class identifier:

(<java.lang.Math>:sqrt 9.0) ⇒ 3.0

or even (though Kawa does not promise to compile this efficiently):

(let ((math <java.lang.Math>)) math:sqrt 9.0) ⇒ 3.0

Using a namespace prefix

You can also use define-namespace to define an alias for a Java class:

(define-namespace Int32 "class:java.lang.Integer")

In this example the name Int32 is a namespace alias for the namespace whose full name is "class:java.lang.Integer". The full name should be the 6 characters "class:" followed by the fully-qualified name of a Java class.

Instead of a vamespace-uri you can use a variable that names a class, usually of the form <classname>. The following is equivalent to the above:

(define-namespace Int32 <java.lang.Integer>)

However, there is one important difference: The <classname> is first searched in the lexical scope. It may resolve to a class defined in the current compilation unit (perhaps defined using define-simple-class), or imported from another module, or an alias (such as from define-alias). Only if <classname> is not found in the current scope is it tried as the class name classname.

You can name a method using a qualified name containing a colon. The part of the name before the colon is a namespace alias (in this case Int32), and the part of the name after the colon is the method name. For example:

(Int32:toHexString 255) ⇒ ff

This invokes the static method toHexString in the Java class java.lang.Integer, passing it the argument 255, and returning the Java String "ff". (Note this is not the same as a Scheme string!)

The general syntax is

(prefix:method-name arg ...)

This invokes the method named method-name in the class corresponding to prefix, and the args are the method arguments.

You can use the method name new to construct new objects:

(Int32:new '|255|)

This is equivalent to the Java expression new Integer("255"). You can also write:

(Int32:new "255")

Kawa is smart enough to convert the Kawa string to a Java String.

Invoking non-static methods

To invoke a non-static method you can use a special prefix *:

(*:toString (list 9 8 7))

The general syntax is

(*:method-name instance arg ...)

This invokes the method named method-name with instance as the target object, and the args are the method arguments.

Alternatively, you can invoke a non-static method using an explicit prefix:

(prefix:method-name instance arg ...)

Assuming that prefix is bound to class then the above is equivalent to:

(*:method-name (as class instance) arg ...)

For example:

(Int32:doubleValue (Int32:new "00255"))

This returns the double value 255.0.

As a shorthand, you can use the name of a Java class instead of a namespace alias:

(java.lang.Integer:toHexString 255)
(java.lang.Object:toString some-value)

If Kawa sees a qualified name with a prefix that is not defined and that matches the name of a known class, then Kawa will automatically treat the prefix as a nickname for namespace uri like class:java.lang.Integer. Both conditions should be true at both compile-time and run-time. However, using an explicit define-namespace is recommended.

As a final shorthand you can use an identifier in handle brackets, such as an existing type alias like <list>. The following are all equivalent:

(<list>:list3 'a 'b 'c)

This is equivalent to:

(define-namespace prefix <list>
(prefix:list3 'a 'b 'c)

for some otherwise-unused prefix.

Invoking a method with the `invoke` function

If you prefer, you can instead use the following functions. (There is also an older deprecated lower-level interface (see Low-level Method invocation.)

Function: invoke-static class name args ...

The class can be a <java.lang.Class>, a <gnu.bytecode.ClassType>, or a <symbol> or <string> that names a Java class. The name can be <symbol> or <string> that names one or more methods in the Java class.

Any accessible methods (static or instance) in the specified class (or its super-classes) that match "name" or "name$V" collectively form a generic procedure. When the procedure is applied to the argument list, the most specific applicable method is chosen depending on the argument list; that method is then called with the given arguments. Iff the method is an instance method, the first actual argument is used as the this argument. If there are no applicable methods (or no methods at all!), or there is no "best" method, WrongType is thrown.

An example (derived from the Skij FAQ):
(invoke-static <java.lang.Thread> 'sleep 100)
The behavior of interpreted code and compiled code is not identical, though you should get the same result either way unless you have designed the classes rather strangely. The details will be nailed down later, but the basic idea is that the compiler will "inline" the invoke-static call if it can pick a single "best" matching method.

Function: invoke object name args ...

The name can be <symbol> or <string> that names one or more methods in the Java class.

Any accessible methods (static or instance) in the specified class (or its super-classes) that match "name" or "name$V" collectively form a generic procedure. When the procedure is applied to the argument list, the most specific applicable method is chosen depending on the argument list; that method is then called with the given arguments. Iff the method is an instance method, the object is used as the this argument; otherwise object is prepended to the args list. If there are no applicable methods (or no methods at all!), or there is no "best" method, WrongType is thrown.

The behavior of interpreted code and compiled code is not indentical, though you should get the same result either way unless you have designed the classes rather strangely. The details will be nailed down later, but the basic idea is that the compiler will "inline" the invoke-static call if it can pick a single "best" matching method.

If the compiler cannot determine the method to call (assuming the method name is constant), the compiler has to generate code at run-time to find the correct method. This is much slower, so the compiler will print a warning. To avoid a waning, you can use a type declaration, or insert a cast:
(invoke (as <java.util.Date> my-date) 'setDate cur-date)
or
(let ((my-date :: <java.util.Date> (calculate-date))
      (cur-date :: <int> (get-cur-date)))
  (invoke my-date 'setDate cur-date))

Function: invoke-special class receiver-object name arg ...

The class can be a <java.lang.Class>, a <gnu.bytecode.ClassType>, or a <symbol> or <string> that names a Java class. The name can be <symbol> or <string> that names one or more methods in the Java class.

This procedure is very similar to invoke and invoke-static and invokes the specified method, ignoring any methods in subclasses that might overide it. One interesting use is to invoke a method in your super-class like the Java language super keyword.

Any methods in the specified class that match "name" or "name$V" collectively form a generic procedure. That generic procedure is then applied as in invoke using the receiver-object and the arguments (if any).

The compiler must be able to inline this procedure (because you cannot force a specific method to be called using reflection). Therefore the class and name must resolve at compile-time to a specific method.
(define-simple-class <MyClass> (<java.util.Date>)
  ((get-year) :: <int>
   (+ (invoke-special <java.util.Date> (this) 'get-year)) 1900)
  ((set-year (year :: <int>)) :: <void>
   (invoke-special <java.util.Date> (this) 'set-year (- year 1900))))

Function: class-methods class name

Return a generic function containing those methods of class that match the name name, in the sense of invoke-static. Same as:
(lambda args (apply invoke-static (cons class (cons name args))))

Some examples using these functions are ‘vectors.scm’ and ‘characters.scm’ the directory ‘kawa/lib’ in the Kawa sources.

Method names

All of the above select a method to invoke base on specified method name and argments. If specified name is not a Java name, it is "mangled" (see Mapping Scheme names to Java names) into a valid Java name. All accessible methods whose names match are considered. Methods that match after appending $V or $X or $V$X are also considered. A $V suffix matches a variable number of arguments: any excess arguments are collect into an gnu.lists.LList or a Java array (depending on the final parameter type). A $X specifies that the method expects an extra implicit CallContext parameter. In that case the method's result is written to the CallContext, so the method result type must be void.

(Kawa compiles a procedure with a #!rest or keyword args whose name is fn to a method named fn$V. It adds an implicit parameter for the extra arguments. By default this extra extra parameter is a Scheme list, but you can specify a Java array type instead. In tha case the array element type must be compatible with all the extra arguments.)

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