Accessing fields of Java objects

The recommmended way to access fields uses the namespace-colon syntax described in see Calling Java methods from Scheme. The difference is that instead of a method name following the colon there is field name a following the colon and a period.

To access an static field named field-name use this syntax

(prefix:.field-name instance)

The prefix can be as discussed in See Calling Java methods from Scheme. Here are 5 equivalent ways:

(java.lang.Integer:.MAX_VALUE)
(<java.lang.Integer>:.MAX_VALUE)
(define-namespace Int32 <java.lang.Integer>)
(Int32:.MAX_VALUE)
(define-namespace Integer "class:java.lang.Integer")
(Integer:.MAX_VALUE)
(define-alias <j.l.Integer> <java.lang.Integer>)
(<j.l.Integer>:.MAX_VALUE)

You can set a static field using this syntax:

(set! (prefix:.field-name) new-value)

The special field name class can be used to extract the java.lang.Class object for a class-type. For example:

(java.util.Vector:.class) ⇒ class java.util.Vector

To access a instance field named field-name use the following syntax. Note the period before the field-name.

(*:.field-name instance)

This syntax works with set! - to set the field use this syntax:

(set! (*:.field-name instance) new-value)

Here is an example:

(define p (list 3 4 5))
(*:.cdr p) ⇒ (4 5)
(set! (*:.cdr p) (list 6 7))
p ⇒ (3 6 7)

You can specify an explicit class:

(prefix:.field-name instance)

If prefix is bound to <class>, then the above is equivalent to:

(*:.field-name (as <class> instance))

Using field and static-field methods

Kawa has both a high-level interface and a low-level interface for accessing the fields of Java objects and static fields. The lower-level interfaces are macros that return functions. These functions can be inlined, producing efficient code. The higher-level functions are less verbose and more convenient.

Function: field object fieldname

Get the instance field with the given fieldname from the given Object. Returns the value of the field, which must be accessible. This procedure has a setter, and so can be used as the first operand to set!.

The field name is "mangled" (see Mapping Scheme names to Java names) into a valid Java name. If there is no accessible field whose name is "fieldname", we look for a no-argument method whose name is "getFieldname" (or "isFieldname" for a boolean property).

If object is a primitive Java array, then fieldname can only be 'length, and the result is the number of elements of the array.

Function: static-field class fieldname

Get the static field with the given fieldname from the given class. Returns the value of the field, which must be accessible. This procedure has a setter, and so can be used as the first operand to set!.

If the fieldname is the special name class, then it returns the java.lang.Class object corresponding to class (which is usually a gnu.bytecode.ClassType object).

Examples:

(static-field <java.lang.System> 'err)
;; Copy the car field of b into a.
(set! (field a 'car) (field b 'car))

Function: slot-ref object fieldname

A synonym for (field object fieldname).

Function: slot-set! object fieldname value

A synonym for (set! (field object fieldname) value).