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  <head>
    <title>Accessing object fields</title>
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    <section class="sect1" title="Accessing object fields" epub:type="subchapter" id="Field-operations">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both">Accessing object fields</h2>
          </div>
        </div>
      </div>
      <section class="sect2" title="Accessing static fields and properties" epub:type="division" id="idm139667870907808">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title">Accessing static fields and properties</h3>
            </div>
          </div>
        </div>
        <p>The recommmended way to access fields 
uses the <a class="link" href="Colon-notation.xhtml" title="Property access using colon notation">colon notation</a>.
For static fields and properties the following is recommended:
</p>
        <div class="literallayout">
          <p><em class="replaceable"><code>class-expression</code></em><code class="literal"><span class="bold"><strong>:</strong></span></code><em class="replaceable"><code>field-name</code></em><br/>
</p>
        </div>
        <p>For example:
</p>
        <pre class="screen">java.lang.Integer:MAX_VALUE
</pre>
        <p>A property with a <code class="literal">get</code> method is equivalent to a field.
The following are all equivalent:
</p>
        <pre class="screen">java.util.Currency:available-currencies
java.util.Currency:availableCurrencies
(java.util.Currency:getAvailableCurrencies)
</pre>
        <p>Just like for a method call, the <em class="replaceable"><code>class-expression</code></em>
can be a class in the current lexical scope,
a fully-qualified class name, or more generally an
expression that evaluates to a class.
</p>
      </section>
      <section class="sect2" title="Accessing instance fields and properties" epub:type="division" id="idm139667870901152">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title">Accessing instance fields and properties</h3>
            </div>
          </div>
        </div>
        <p>The syntax is:
</p>
        <div class="literallayout">
          <p><em class="replaceable"><code>instance</code></em><code class="literal"><span class="bold"><strong>:</strong></span></code><em class="replaceable"><code>field-name</code></em><br/>
</p>
        </div>
        <p>The <em class="replaceable"><code>field-name</code></em> can of course be the name of an actual
object field, but it can also be the name of a property with
a zero-argument <code class="literal">get</code> method.
For example, if <code class="literal">cal</code> is a <code class="literal">java.util-Calendar</code> instance,
then the following are all equivalent:
</p>
        <pre class="screen">cal:time-zone
cal:timeZone
(cal:getTimeZone)
(cal:get-time-zone)
</pre>
        <p>You can use colon notation to assign to a field:
</p>
        <pre class="screen">(set! cal:time-zone TimeZone:default)
</pre>
        <p>which is equivalent to:
</p>
        <pre class="screen">(cal:setTimeZone (TimeZone:getDefault))
</pre>
        <p>A Java array only has the <code class="literal">length</code> field, plus the <code class="literal">class</code> property:
</p>
        <pre class="screen">(int[] 4 5 6):length ⇒ 3
(int[] 4 5 6):class:name ⇒ "int[]"
</pre>
      </section>
      <section class="sect2" title="Using field and static-field methods" epub:type="division" id="idm139667870892384">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title">Using field and static-field methods</h3>
            </div>
          </div>
        </div>
        <p>The following methods are useful in cases where colon notation
is ambiguous, for example where there are both fields and methods
with the same name.
You might also prefer as a matter of style, to
emphasise that a field is being accessed.
</p>
        <p class="synopsis" kind="Procedure"><span class="kind">Procedure</span><span class="ignore">: </span><a id="idm139667870890656" class="indexterm"/> <code class="function">field</code> <em class="replaceable"><code>object</code></em> <em class="replaceable"><code>fieldname</code></em></p>
        <div class="blockquote">
          <blockquote class="blockquote">
            <p>Get the instance field with the given <em class="replaceable"><code>fieldname</code></em> from the given
<em class="replaceable"><code>Object</code></em>.  Returns the value of the field, which must be accessible.
This procedure has a <code class="literal">setter</code>, and so can be used as the first
operand to <code class="literal">set!</code>.
</p>
            <p>The field name is "mangled" (see <a class="link" href="Mangling.xhtml" title="Mapping Scheme names to Java names">Mangling</a>) into a valid Java name.
If there is no accessible field whose name is <code class="literal">"<em class="replaceable"><code>fieldname</code></em>"</code>,
we look for a no-argument method whose name is
<code class="literal">"get<em class="replaceable"><code>Fieldname</code></em>"</code> (or <code class="literal">"is<em class="replaceable"><code>Fieldname</code></em>"</code> for a
boolean property).
</p>
            <p>If <em class="replaceable"><code>object</code></em> is a primitive Java array, then <em class="replaceable"><code>fieldname</code></em> can only
be <code class="literal">'length</code>, and the result is the number of elements of the array.
</p>
          </blockquote>
        </div>
        <p class="synopsis" kind="Procedure"><span class="kind">Procedure</span><span class="ignore">: </span><a id="idm139667870879248" class="indexterm"/> <code class="function">static-field</code> <em class="replaceable"><code>class</code></em> <em class="replaceable"><code>fieldname</code></em></p>
        <div class="blockquote">
          <blockquote class="blockquote">
            <p>Get the static field with the given <em class="replaceable"><code>fieldname</code></em> from the given
<em class="replaceable"><code>class</code></em>.  Returns the value of the field, which must be accessible.
This procedure has a <code class="literal">setter</code>, and so can be used as the first
operand to <code class="literal">set!</code>.
</p>
            <p>If the <em class="replaceable"><code>fieldname</code></em> is the special name <code class="literal">class</code>,
then it returns the <code class="literal">java.lang.Class</code> object corresponding to
<em class="replaceable"><code>class</code></em> (which is usually a <code class="literal">gnu.bytecode.ClassType</code> object).
</p>
          </blockquote>
        </div>
        <p>Examples:
</p>
        <pre class="screen">(static-field java.lang.System 'err)
;; Copy the car field of b into a.
(set! (field a 'car) (field b 'car))
</pre>
        <p class="synopsis" kind="Procedure"><span class="kind">Procedure</span><span class="ignore">: </span><a id="idm139667870870112" class="indexterm"/> <code class="function">slot-ref</code> <em class="replaceable"><code>object</code></em> <em class="replaceable"><code>fieldname</code></em></p>
        <div class="blockquote">
          <blockquote class="blockquote">
            <p>A synonym for <code class="literal">(field <em class="replaceable"><code>object</code></em> <em class="replaceable"><code>fieldname</code></em>)</code>.
</p>
          </blockquote>
        </div>
        <p class="synopsis" kind="Procedure"><span class="kind">Procedure</span><span class="ignore">: </span><a id="idm139667870865072" class="indexterm"/> <code class="function">slot-set!</code> <em class="replaceable"><code>object</code></em> <em class="replaceable"><code>fieldname</code></em> <em class="replaceable"><code>value</code></em></p>
        <div class="blockquote">
          <blockquote class="blockquote">
            <p>A synonym for <code class="literal">(set! (field <em class="replaceable"><code>object</code></em> <em class="replaceable"><code>fieldname</code></em>) <em class="replaceable"><code>value</code></em>)</code>.
</p>
          </blockquote>
        </div>
      </section>
      <section class="sect2" title="Older colon-dot notation" epub:type="division" id="idm139667870859200">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title">Older colon-dot notation</h3>
            </div>
          </div>
        </div>
        <p>There is older syntax where following the colon
there is field name a following the colon <span class="emphasis"><em>and</em></span> a period.
</p>
        <p>To access an static field named <em class="replaceable"><code>field-name</code></em> use this syntax
</p>
        <pre class="screen">(<em class="replaceable"><code>prefix</code></em>:.<em class="replaceable"><code>field-name</code></em> <em class="replaceable"><code>instance</code></em>)
</pre>
        <p>The <em class="replaceable"><code>prefix</code></em> can be as discussed in See <a class="link" href="Method-operations.xhtml" title="Calling Java methods from Scheme">Method operations</a>.
Here are 5 equivalent ways:
</p>
        <pre class="screen">(java.lang.Integer:.MAX_VALUE)
(&lt;java.lang.Integer&gt;:.MAX_VALUE)
(define-namespace Int32 &lt;java.lang.Integer&gt;)
(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)
</pre>
        <p>You can set a static field using this syntax:
</p>
        <pre class="screen">(set! (<em class="replaceable"><code>prefix</code></em>:.<em class="replaceable"><code>field-name</code></em>) <em class="replaceable"><code>new-value</code></em>)
</pre>
        <p>The special field name <code class="literal">class</code> can be used to extract the
<code class="literal">java.lang.Class</code> object for a class-type.  For example:
</p>
        <pre class="screen">(java.util.Vector:.class) ⇒ class java.util.Vector
</pre>
        <p>To access a instance field named <em class="replaceable"><code>field-name</code></em> use the following syntax.
Note the period before the <em class="replaceable"><code>field-name</code></em>.
</p>
        <pre class="screen">(*:.<em class="replaceable"><code>field-name</code></em> <em class="replaceable"><code>instance</code></em>)
</pre>
        <p>This syntax works with <code class="literal">set!</code> - to set the field use this syntax:
</p>
        <pre class="screen">(set! (*:.<em class="replaceable"><code>field-name</code></em> <em class="replaceable"><code>instance</code></em>) <em class="replaceable"><code>new-value</code></em>)
</pre>
        <p>Here is an example:
</p>
        <pre class="screen">(define p (list 3 4 5))
(*:.cdr p) ⇒ (4 5)
(set! (*:.cdr p) (list 6 7))
p ⇒ (3 6 7)
</pre>
        <p>You can specify an explicit class:
</p>
        <pre class="screen">(<em class="replaceable"><code>prefix</code></em>:.<em class="replaceable"><code>field-name</code></em> <em class="replaceable"><code>instance</code></em>)
</pre>
        <p>If <em class="replaceable"><code>prefix</code></em> is bound to <code class="literal">&lt;<em class="replaceable"><code>class</code></em>&gt;</code>, then the above
is equivalent to:
</p>
        <pre class="screen">(*:.<em class="replaceable"><code>field-name</code></em> (as &lt;<em class="replaceable"><code>class</code></em>&gt; <em class="replaceable"><code>instance</code></em>))
</pre>
      </section>
    </section>
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              <a href="Field-operations.xhtml#idm139667870907808">Accessing static fields and properties</a>
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            <b class="toc">
              <a href="Field-operations.xhtml#idm139667870901152">Accessing instance fields and properties</a>
            </b>
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          <li>
            <b class="toc">
              <a href="Field-operations.xhtml#idm139667870892384">Using field and static-field methods</a>
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            <b class="toc">
              <a href="Field-operations.xhtml#idm139667870859200">Older colon-dot notation</a>
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