Calling static methods using colon notation

The easiest way to invoke a static method is to use colon notation, specifically:

(class-expression:method-name argument ...)

The class-expression can be a class in the current lexical scope, such as a class defined using define-simple-class:

(define-simple-class MyClass ()
  ((add2 x y) allocation: 'static (+ x y)))
(MyClass:add2 3 4) ⇒ 7

Often class-expression 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 use a defined alias:

(define-alias jlMath java.lang.Math)
(jlMath:sqrt 16.0) ⇒ 4.0

You can even evaluate class-expression at run-time (in which case Kawa may have to use slower reflection):

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

Here java.lang.Math evaluates to a java.lang.Class instance for the named class (like Java’s java.lang.Class.class, again assuming the class exists and is accessible both at compile-time and run-time, and the name is not otherwise lexically bound.

Calling instance methods using colon notation

The syntax is:

(instance:method-name argument ...)

This invokes the method named method-name with the evaluated instance as the target object and the evaluated arguments as the method arguments.

For example:

((list 9 8 7):toString) ⇒ "(9 8 7)"
([5 6 7]:get 2) ⇒ 7

This older syntax is also available:

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

For example:

(*:toString (list 9 8 7))

You can also name the class explicitly:

(class-expression:method-name instance argument ...)

For example:

(java.util.List:get [5 6 7] 2) ⇒ 7

Using an explicit class is like coercing the instance:

(*:method-name (as class-expression instance )argument ...)

Note that for some special values, including java.lang.Class instances, you can’t use the compact form of colon notation where the instance is before the comma:

(java.lang.Integer:getDeclaredField "MAX_VALUE") ⇒ error

This is because in this case we look for a static member of java.lang.Integer (at least as currently defined and implemented), while we want an instance member of java.lang.Class. In those cases you can use one of these alternative forms, which all return the same java.lang.reflect.Field result:

(*:getDeclaredField java.lang.Integer "MAX_VALUE")
(java.lang.Class:getDeclaredField java.lang.Integer "MAX_VALUE")
(invoke java.lang.Integer 'getDeclaredField "MAX_VALUE")

Method names

The method to invoke is selected using the 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 may compile 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. You can specify a Java array type instead, in which case the method is named fn without the $V, and instead it is marked as a Java-5 varargs method. The array element type must be compatible with all the extra arguments.)

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.)

Procedure: 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:

(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.

Procedure: 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))

Procedure: 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))))

Procedure: 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.

Using a namespace prefix

This way of invoking a method is deprecated.

You can 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 String "ff".

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")

You can also call instance methods using a namespace prefix:

(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.