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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Arrays</title><link rel="stylesheet" href="core.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"/></head><body><div class="sect1" title="Arrays"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-4-SECT-7"/>Arrays</h1></div></div></div><p>An array is a special type of object that can hold an ordered
collection of elements. The type of the elements of the array is called
the <span class="emphasis"><em>base type</em></span> of the array; the number of elements it
holds is a fixed attribute called its <a id="I_indexterm4_id687163" class="indexterm"/><span class="emphasis"><em>length</em></span>. Java supports arrays of all
primitive and reference types.</p><p>The basic syntax of arrays looks much like that of C or C++. We
create an array of a specified length and access the elements with the
<a id="I_indexterm4_id687176" class="indexterm"/><span class="emphasis"><em>index</em></span> operator, <a id="I_indexterm4_id687184" class="indexterm"/><a id="I_indexterm4_id687194" class="indexterm"/><code class="literal">[]</code>. Unlike other
languages, however, arrays in Java are true, first-class objects. An array
is an instance of a special Java <a id="I_indexterm4_id687210" class="indexterm"/><code class="literal">array</code> class and has a
corresponding type in the type system. This means that to use an array, as
with any other object, we first declare a variable of the appropriate type
and then use the <code class="literal">new</code> operator to create
an instance of it.</p><p>Array objects differ from other objects in Java in three
respects:</p><div class="itemizedlist"><ul class="itemizedlist"><li class="listitem"><p>Java implicitly creates a special array class type for us
whenever we declare a new type of array. It’s not strictly necessary
to know about this process in order to use arrays, but it helps in
understanding their structure and their relationship to other objects
in Java later.</p></li><li class="listitem"><p>Java lets us use the <code class="literal">[]</code>
operator to access array elements so that arrays look as we expect. We
could implement our own classes that act like arrays, but we would
have to settle for having methods such as <code class="literal">get()</code> and <code class="literal">set()</code> instead of using the special <code class="literal">[]</code> notation.</p></li><li class="listitem"><p>Java provides a corresponding special form of the <code class="literal">new</code> operator that lets us construct an
instance of an array with a specified length with the <code class="literal">[]</code> notation or initialize it directly from a
structured list of values.</p></li></ul></div><div class="sect2" title="Array Types"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.1"/>Array Types</h2></div></div></div><p><a id="I_indexterm4_id687289" class="indexterm"/> <a id="idx10182" class="indexterm"/>An array type variable is denoted by a base type followed
by the empty brackets, <code class="literal">[]</code>.
Alternatively, Java accepts a C-style declaration with the brackets
placed after the array name.</p><p>The following are equivalent:</p><a id="I_4_tt198"/><pre class="programlisting"> <code class="kt">int</code> <code class="o">[]</code> <code class="n">arrayOfInts</code><code class="o">;</code> <code class="c1">// preferred</code>
<code class="kt">int</code> <code class="n">arrayOfInts</code> <code class="o">[];</code> <code class="c1">// C-style</code></pre><p>In each case, <code class="literal">arrayOfInts</code> is
declared as an array of integers. The size of the array is not yet an
issue because we are declaring only the array type variable. We have not
yet created an actual instance of the <code class="literal">array</code> class, with its associated storage. It’s
not even possible to specify the length of an array when declaring an
array type variable. The size is strictly a function of the array object
itself, not the reference to it.</p><p>An array of reference types can be created in the same
way:<a id="I_indexterm4_id687352" class="indexterm"/></p><a id="I_4_tt199"/><pre class="programlisting"> <code class="n">String</code> <code class="o">[]</code> <code class="n">someStrings</code><code class="o">;</code>
<code class="n">Button</code> <code class="o">[]</code> <code class="n">someButtons</code><code class="o">;</code></pre></div><div class="sect2" title="Array Creation and Initialization"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.2"/>Array Creation and Initialization</h2></div></div></div><p><a id="idx10132" class="indexterm"/> <a id="idx10180" class="indexterm"/>The <a id="I_indexterm4_id687403" class="indexterm"/><code class="literal">new</code> operator is used to
create an instance of an array. After the <code class="literal">new</code> operator, we specify the base type of the
array and its length with a bracketed integer expression:</p><a id="I_4_tt200"/><pre class="programlisting"> <code class="n">arrayOfInts</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">int</code> <code class="o">[</code><code class="mi">42</code><code class="o">];</code>
<code class="n">someStrings</code> <code class="o">=</code> <code class="k">new</code> <code class="n">String</code> <code class="o">[</code> <code class="n">number</code> <code class="o">+</code> <code class="mi">2</code> <code class="o">];</code></pre><p>We can, of course, combine the steps of declaring and allocating
the array:</p><a id="I_4_tt201"/><pre class="programlisting"> <code class="kt">double</code> <code class="o">[]</code> <code class="n">someNumbers</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">double</code> <code class="o">[</code><code class="mi">20</code><code class="o">];</code>
<code class="n">Component</code> <code class="o">[]</code> <code class="n">widgets</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Component</code> <code class="o">[</code><code class="mi">12</code><code class="o">];</code></pre><p>Array indices start with zero. Thus, the first element of <code class="literal">someNumbers[]</code> is <code class="literal">0</code>, and the last element is <code class="literal">19</code>. After creation, the array elements are
initialized to the default values for their type. For numeric types,
this means the elements are initially zero:</p><a id="I_4_tt202"/><pre class="programlisting"> <code class="kt">int</code> <code class="o">[]</code> <code class="n">grades</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">int</code> <code class="o">[</code><code class="mi">30</code><code class="o">];</code>
<code class="n">grades</code><code class="o">[</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="mi">99</code><code class="o">;</code>
<code class="n">grades</code><code class="o">[</code><code class="mi">1</code><code class="o">]</code> <code class="o">=</code> <code class="mi">72</code><code class="o">;</code>
<code class="c1">// grades[2] == 0</code></pre><p>The elements of an array of objects are references to the
objects—just like individual variables they point to—but do not actually
contain instances of the objects. The default value of each element is
therefore <code class="literal">null</code> until we assign
instances of appropriate objects:</p><a id="I_4_tt203"/><pre class="programlisting"> <code class="n">String</code> <code class="n">names</code> <code class="o">[]</code> <code class="o">=</code> <code class="k">new</code> <code class="n">String</code> <code class="o">[</code><code class="mi">4</code><code class="o">];</code>
<code class="n">names</code> <code class="o">[</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="n">String</code><code class="o">();</code>
<code class="n">names</code> <code class="o">[</code><code class="mi">1</code><code class="o">]</code> <code class="o">=</code> <code class="s">"Boofa"</code><code class="o">;</code>
<code class="n">names</code> <code class="o">[</code><code class="mi">2</code><code class="o">]</code> <code class="o">=</code> <code class="n">someObject</code><code class="o">.</code><code class="na">toString</code><code class="o">();</code>
<code class="c1">// names[3] == null</code></pre><p>This is an important distinction that can cause confusion. In many
other languages, the act of creating an array is the same as allocating
storage for its elements. In Java, a newly allocated array of objects
actually contains only reference variables, each with the value <code class="literal">null</code>.<sup>[<a id="learnjava3-CHP-4-FNOTE-5" href="#ftn.learnjava3-CHP-4-FNOTE-5" class="footnote">11</a>]</sup> That’s not to say that there is no memory associated with
an empty array; memory is needed to hold those references (the empty
“slots” in the array). <a class="xref" href="ch04s07.html#learnjava3-CHP-4-FIG-4" title="Figure 4-4. A Java array">Figure 4-4</a>
illustrates the <code class="literal">names</code> array of the
previous example.</p><div class="figure"><a id="learnjava3-CHP-4-FIG-4"/><div class="figure-contents"><div class="mediaobject"><a id="I_4_tt204"/><img src="httpatomoreillycomsourceoreillyimages1707616.png" alt="A Java array"/></div></div><p class="title">Figure 4-4. A Java array</p></div><p><code class="literal">names</code> is a variable of type
<code class="literal">String[]</code> (i.e., a string array). This
particular <code class="literal">String[]</code> object contains
four <code class="literal">String</code> type variables. We have
assigned <code class="literal">String</code> objects to the first
three array elements. The fourth has the default value <code class="literal">null</code>.</p><p>Java supports the C-style curly braces <a id="I_indexterm4_id687590" class="indexterm"/><a id="I_indexterm4_id687598" class="indexterm"/><code class="literal">{}</code> construct for
creating an array and initializing its elements:</p><a id="I_4_tt205"/><pre class="programlisting"> <code class="kt">int</code> <code class="o">[]</code> <code class="n">primes</code> <code class="o">=</code> <code class="o">{</code> <code class="mi">2</code><code class="o">,</code> <code class="mi">3</code><code class="o">,</code> <code class="mi">5</code><code class="o">,</code> <code class="mi">7</code><code class="o">,</code> <code class="mi">7</code><code class="o">+</code><code class="mi">4</code> <code class="o">};</code> <code class="c1">// e.g., primes[2] = 5</code></pre><p>An array object of the proper type and length is implicitly
created, and the values of the comma-separated list of expressions are
assigned to its elements. Note that we did not use the <code class="literal">new</code> keyword or the array type here. The type
of the array was inferred from the assignment.</p><p>We can use the <a id="I_indexterm4_id687632" class="indexterm"/><a id="I_indexterm4_id687639" class="indexterm"/><code class="literal">{}</code> syntax with an array
of objects. In this case, each expression must evaluate to an object
that can be assigned to a variable of the base type of the array or the
value <code class="literal">null</code>. Here are some
examples:</p><a id="I_4_tt206"/><pre class="programlisting"> <code class="n">String</code> <code class="o">[]</code> <code class="n">verbs</code> <code class="o">=</code> <code class="o">{</code> <code class="s">"run"</code><code class="o">,</code> <code class="s">"jump"</code><code class="o">,</code> <code class="n">someWord</code><code class="o">.</code><code class="na">toString</code><code class="o">()</code> <code class="o">};</code>
<code class="n">Button</code> <code class="o">[]</code> <code class="n">controls</code> <code class="o">=</code> <code class="o">{</code> <code class="n">stopButton</code><code class="o">,</code> <code class="k">new</code> <code class="n">Button</code><code class="o">(</code><code class="s">"Forwards"</code><code class="o">),</code>
<code class="k">new</code> <code class="nf">Button</code><code class="o">(</code><code class="s">"Backwards"</code><code class="o">)</code> <code class="o">};</code>
<code class="c1">// All types are subtypes of Object</code>
<code class="n">Object</code> <code class="o">[]</code> <code class="n">objects</code> <code class="o">=</code> <code class="o">{</code> <code class="n">stopButton</code><code class="o">,</code> <code class="s">"A word"</code><code class="o">,</code> <code class="kc">null</code> <code class="o">};</code></pre><p>The following are equivalent:<a id="I_indexterm4_id687672" class="indexterm"/><a id="I_indexterm4_id687679" class="indexterm"/></p><a id="I_4_tt207"/><pre class="programlisting"> <code class="n">Button</code> <code class="o">[]</code> <code class="n">threeButtons</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Button</code> <code class="o">[</code><code class="mi">3</code><code class="o">];</code>
<code class="n">Button</code> <code class="o">[]</code> <code class="n">threeButtons</code> <code class="o">=</code> <code class="o">{</code> <code class="kc">null</code><code class="o">,</code> <code class="kc">null</code><code class="o">,</code> <code class="kc">null</code> <code class="o">};</code></pre></div><div class="sect2" title="Using Arrays"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.3"/>Using Arrays</h2></div></div></div><p><a id="idx10134" class="indexterm"/> <a id="idx10183" class="indexterm"/>The size of an array object is available in the public
variable <code class="literal">length</code>:</p><a id="I_4_tt208"/><pre class="programlisting"> <code class="kt">char</code> <code class="o">[]</code> <code class="n">alphabet</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">char</code> <code class="o">[</code><code class="mi">26</code><code class="o">];</code>
<code class="kt">int</code> <code class="n">alphaLen</code> <code class="o">=</code> <code class="n">alphabet</code><code class="o">.</code><code class="na">length</code><code class="o">;</code> <code class="c1">// alphaLen == 26</code>
<code class="n">String</code> <code class="o">[]</code> <code class="n">musketeers</code> <code class="o">=</code> <code class="o">{</code> <code class="s">"one"</code><code class="o">,</code> <code class="s">"two"</code><code class="o">,</code> <code class="s">"three"</code> <code class="o">};</code>
<code class="kt">int</code> <code class="n">num</code> <code class="o">=</code> <code class="n">musketeers</code><code class="o">.</code><code class="na">length</code><code class="o">;</code> <code class="c1">// num == 3</code></pre><p><a id="I_indexterm4_id687746" class="indexterm"/> <code class="literal">length</code> is the only
accessible field of an array; it is a variable, not a method. (Don’t
worry; the compiler tells you when you accidentally use parentheses as
if it were a method, as everyone does now and then.)</p><p>Array access in Java is just like array access in other languages;
you access an element by putting an integer-valued expression between
brackets after the name of the array. The following example creates an
array of <code class="literal">Button</code> objects called
<code class="literal">keyPad</code> and then fills the array with
<code class="literal">Button</code> objects:</p><a id="I_4_tt209"/><pre class="programlisting"> <code class="n">Button</code> <code class="o">[]</code> <code class="n">keyPad</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Button</code> <code class="o">[</code> <code class="mi">10</code> <code class="o">];</code>
<code class="k">for</code> <code class="o">(</code> <code class="kt">int</code> <code class="n">i</code><code class="o">=</code><code class="mi">0</code><code class="o">;</code> <code class="n">i</code> <code class="o"><</code> <code class="n">keyPad</code><code class="o">.</code><code class="na">length</code><code class="o">;</code> <code class="n">i</code><code class="o">++</code> <code class="o">)</code>
<code class="n">keyPad</code><code class="o">[</code> <code class="n">i</code> <code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Button</code><code class="o">(</code> <code class="n">Integer</code><code class="o">.</code><code class="na">toString</code><code class="o">(</code> <code class="n">i</code> <code class="o">)</code> <code class="o">);</code></pre><p>Remember that we can also use the enhanced <code class="literal">for</code> loop to iterate over array values. Here
we’ll use it to print all the values we just assigned:</p><a id="I_4_tt210"/><pre class="programlisting"> <code class="k">for</code> <code class="o">(</code><code class="n">Button</code> <code class="n">b</code> <code class="o">:</code> <code class="n">keyPad</code><code class="o">)</code>
<code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="n">b</code><code class="o">);</code></pre><p>Attempting to access an element that is outside the range of the
array generates an <a id="I_indexterm4_id687815" class="indexterm"/><code class="literal">ArrayIndexOutOfBoundsException</code>. This is a type
of <code class="literal">RuntimeException</code>, so you can
either catch and handle it yourself if you really expect it, or ignore
it, as we’ve already discussed:</p><a id="I_4_tt211"/><pre class="programlisting"> <code class="n">String</code> <code class="o">[]</code> <code class="n">states</code> <code class="o">=</code> <code class="k">new</code> <code class="n">String</code> <code class="o">[</code><code class="mi">50</code><code class="o">];</code>
<code class="k">try</code> <code class="o">{</code>
<code class="n">states</code><code class="o">[</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="s">"California"</code><code class="o">;</code>
<code class="n">states</code><code class="o">[</code><code class="mi">1</code><code class="o">]</code> <code class="o">=</code> <code class="s">"Oregon"</code><code class="o">;</code>
<code class="o">...</code>
<code class="n">states</code><code class="o">[</code><code class="mi">50</code><code class="o">]</code> <code class="o">=</code> <code class="s">"McDonald's Land"</code><code class="o">;</code> <code class="c1">// Error: array out of bounds</code>
<code class="o">}</code>
<code class="k">catch</code> <code class="o">(</code> <code class="n">ArrayIndexOutOfBoundsException</code> <code class="n">err</code> <code class="o">)</code> <code class="o">{</code>
<code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code> <code class="s">"Handled error: "</code> <code class="o">+</code> <code class="n">err</code><code class="o">.</code><code class="na">getMessage</code><code class="o">()</code> <code class="o">);</code>
<code class="o">}</code></pre><p>It’s a common task to copy a range of elements from one array into
another. One way to copy arrays is to use the low-level <a id="I_indexterm4_id687850" class="indexterm"/><code class="literal">arraycopy()</code> method of
the <code class="literal">System</code> class:</p><a id="I_4_tt212"/><pre class="programlisting"> <code class="n">System</code><code class="o">.</code><code class="na">arraycopy</code><code class="o">(</code> <code class="n">source</code><code class="o">,</code> <code class="n">sourceStart</code><code class="o">,</code> <code class="n">destination</code><code class="o">,</code> <code class="n">destStart</code><code class="o">,</code> <code class="n">length</code> <code class="o">);</code></pre><p>The following example doubles the size of the <code class="literal">names</code> array from an earlier example:</p><a id="I_4_tt213"/><pre class="programlisting"> <code class="n">String</code> <code class="o">[]</code> <code class="n">tmpVar</code> <code class="o">=</code> <code class="k">new</code> <code class="n">String</code> <code class="o">[</code> <code class="mi">2</code> <code class="o">*</code> <code class="n">names</code><code class="o">.</code><code class="na">length</code> <code class="o">];</code>
<code class="n">System</code><code class="o">.</code><code class="na">arraycopy</code><code class="o">(</code> <code class="n">names</code><code class="o">,</code> <code class="mi">0</code><code class="o">,</code> <code class="n">tmpVar</code><code class="o">,</code> <code class="mi">0</code><code class="o">,</code> <code class="n">names</code><code class="o">.</code><code class="na">length</code> <code class="o">);</code>
<code class="n">names</code> <code class="o">=</code> <code class="n">tmpVar</code><code class="o">;</code></pre><p>A new array, twice the size of <code class="literal">names</code>, is allocated and assigned to a
temporary variable, <code class="literal">tmpVar</code>. The
<code class="literal">arraycopy()</code> method is then used to
copy the elements of <code class="literal">names</code> to the new
array. Finally, the new array is assigned to <code class="literal">names</code>. If there are no remaining references to
the old array object after <code class="literal">names</code> has
been copied, it is garbage-collected on the next pass.</p><p>An easier way is to use the <code class="literal">java.util.Arrays</code><a id="I_indexterm4_id687937" class="indexterm"/><code class="literal">copyOf()</code> and <a id="I_indexterm4_id687947" class="indexterm"/><code class="literal">copyOfRange()</code>
methods:</p><a id="I_programlisting4_id687958"/><pre class="programlisting"> <code class="kt">byte</code> <code class="o">[]</code> <code class="n">bar</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">byte</code><code class="o">[]</code> <code class="o">{</code> <code class="mi">1</code><code class="o">,</code> <code class="mi">2</code><code class="o">,</code> <code class="mi">3</code><code class="o">,</code> <code class="mi">4</code><code class="o">,</code> <code class="mi">5</code> <code class="o">};</code>
<code class="kt">byte</code> <code class="o">[]</code> <code class="n">barCopy</code> <code class="o">=</code> <code class="n">Arrays</code><code class="o">.</code><code class="na">copyOf</code><code class="o">(</code> <code class="n">bar</code><code class="o">,</code> <code class="n">bar</code><code class="o">.</code><code class="na">length</code> <code class="o">);</code>
<code class="c1">// { 1, 2, 3, 4, 5 }</code>
<code class="kt">byte</code> <code class="o">[]</code> <code class="n">expanded</code> <code class="o">=</code> <code class="n">Arrays</code><code class="o">.</code><code class="na">copyOf</code><code class="o">(</code> <code class="n">bar</code><code class="o">,</code> <code class="n">bar</code><code class="o">.</code><code class="na">length</code><code class="o">+</code><code class="mi">2</code> <code class="o">);</code>
<code class="c1">// { 1, 2, 3, 4, 5, 0, 0 }</code>
<code class="kt">byte</code> <code class="o">[]</code> <code class="n">firstThree</code> <code class="o">=</code> <code class="n">Arrays</code><code class="o">.</code><code class="na">copyOfRange</code><code class="o">(</code> <code class="n">bar</code><code class="o">,</code> <code class="mi">0</code><code class="o">,</code> <code class="mi">3</code> <code class="o">);</code>
<code class="c1">// { 1, 2, 3 }</code>
<code class="kt">byte</code> <code class="o">[]</code> <code class="n">lastThree</code> <code class="o">=</code> <code class="n">Arrays</code><code class="o">.</code><code class="na">copyOfRange</code><code class="o">(</code> <code class="n">bar</code><code class="o">,</code> <code class="mi">2</code><code class="o">,</code> <code class="n">bar</code><code class="o">.</code><code class="na">length</code> <code class="o">);</code>
<code class="c1">// { 3, 4, 5 }</code>
<code class="kt">byte</code> <code class="o">[]</code> <code class="n">lastThreePlusTwo</code> <code class="o">=</code> <code class="n">Arrays</code><code class="o">.</code><code class="na">copyOfRange</code><code class="o">(</code> <code class="n">bar</code><code class="o">,</code> <code class="mi">2</code><code class="o">,</code> <code class="n">bar</code><code class="o">.</code><code class="na">length</code><code class="o">+</code><code class="mi">2</code> <code class="o">);</code>
<code class="c1">// { 3, 4, 5, 0, 0 }</code></pre><p>The <code class="literal">copyOf()</code> method takes the
original array and a target length. If the target length is larger than
the original array length, then the new array is padded (with zeros or
nulls) to the desired length. The <code class="literal">copyOfRange()</code> takes a starting index
(inclusive) and an ending index (exclusive) and a desired length, which
will also be padded if necessary.<a id="I_indexterm4_id687984" class="indexterm"/><a id="I_indexterm4_id687992" class="indexterm"/></p></div><div class="sect2" title="Anonymous Arrays"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.4"/>Anonymous Arrays</h2></div></div></div><p><a id="I_indexterm4_id688006" class="indexterm"/> <a id="idx10131" class="indexterm"/> <a id="I_indexterm4_id688024" class="indexterm"/>Often it is convenient to create “throwaway” arrays,
arrays that are used in one place and never referenced anywhere else.
Such arrays don’t need a name because you never need to refer to them
again in that context. For example, you may want to create a collection
of objects to pass as an argument to some method. It’s easy enough to
create a normal, named array, but if you don’t actually work with the
array (if you use the array only as a holder for some collection), you
shouldn’t need to do this. Java makes it easy to create “anonymous”
(i.e., unnamed) arrays.</p><p>Let’s say you need to call a method named <code class="literal">setPets()</code>, which takes an array of <code class="literal">Animal</code> objects as arguments. Provided <code class="literal">Cat</code> and <code class="literal">Dog</code>
are subclasses of <code class="literal">Animal</code>, here’s how
to call <code class="literal">setPets()</code> using an anonymous
array:</p><a id="I_4_tt214"/><pre class="programlisting"> <code class="n">Dog</code> <code class="n">pokey</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Dog</code> <code class="o">(</code><code class="s">"gray"</code><code class="o">);</code>
<code class="n">Cat</code> <code class="n">boojum</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Cat</code> <code class="o">(</code><code class="s">"grey"</code><code class="o">);</code>
<code class="n">Cat</code> <code class="n">simon</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Cat</code> <code class="o">(</code><code class="s">"orange"</code><code class="o">);</code>
<code class="n">setPets</code> <code class="o">(</code> <code class="k">new</code> <code class="n">Animal</code> <code class="o">[]</code> <code class="o">{</code> <code class="n">pokey</code><code class="o">,</code> <code class="n">boojum</code><code class="o">,</code> <code class="n">simon</code> <code class="o">});</code></pre><p>The syntax looks similar to the initialization of an array in a
variable declaration. We implicitly define the size of the array and
fill in its elements using the curly-brace notation. However, because
this is not a variable declaration, we have to explicitly use the
<code class="literal">new</code> operator and the array type to
create the array object.</p><p>Anonymous arrays were sometimes used as a substitute for
variable-length argument lists to methods, which are discussed in <a class="xref" href="ch05.html" title="Chapter 5. Objects in Java">Chapter 5</a>. With the introduction of variable-length
argument lists in Java, the usefulness of anonymous arrays has
diminished.</p></div><div class="sect2" title="Multidimensional Arrays"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.5"/>Multidimensional Arrays</h2></div></div></div><p><a id="idx10133" class="indexterm"/> <a id="idx10166" class="indexterm"/> <a id="idx10181" class="indexterm"/>Java supports multidimensional arrays in the form of
arrays of array type objects. You create a multidimensional array with
C-like syntax, using multiple bracket pairs, one for each dimension. You
also use this syntax to access elements at various positions within the
array. Here’s an example of a multidimensional array that represents a
chess board:</p><a id="I_4_tt215"/><pre class="programlisting"> <code class="n">ChessPiece</code> <code class="o">[][]</code> <code class="n">chessBoard</code><code class="o">;</code>
<code class="n">chessBoard</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ChessPiece</code> <code class="o">[</code><code class="mi">8</code><code class="o">][</code><code class="mi">8</code><code class="o">];</code>
<code class="n">chessBoard</code><code class="o">[</code><code class="mi">0</code><code class="o">][</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">.</code><code class="na">Rook</code><code class="o">;</code>
<code class="n">chessBoard</code><code class="o">[</code><code class="mi">1</code><code class="o">][</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">.</code><code class="na">Pawn</code><code class="o">;</code>
<code class="o">...</code></pre><p>Here, <code class="literal">chessBoard</code> is declared as
a variable of type <code class="literal">ChessPiece[][]</code>
(i.e., an array of <code class="literal">ChessPiece</code>
arrays). This declaration implicitly creates the type <code class="literal">ChessPiece[]</code> as well. The example illustrates the special
form of the <code class="literal">new</code> operator used to
create a multidimensional array.
It creates an array of <code class="literal">ChessPiece[]</code>
objects and then, in turn, makes each element into an array of <code class="literal">ChessPiece</code> objects. We then index <code class="literal">chessBoard</code> to specify values for particular
<code class="literal">ChessPiece</code> elements. (We’ll neglect
the color of the pieces here.)</p><p>Of course, you can create arrays with more than two dimensions.
Here’s a slightly impractical example:</p><a id="I_4_tt216"/><pre class="programlisting"> <code class="n">Color</code> <code class="o">[][][]</code> <code class="n">rgbCube</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Color</code> <code class="o">[</code><code class="mi">256</code><code class="o">][</code><code class="mi">256</code><code class="o">][</code><code class="mi">256</code><code class="o">];</code>
<code class="n">rgbCube</code><code class="o">[</code><code class="mi">0</code><code class="o">][</code><code class="mi">0</code><code class="o">][</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="n">Color</code><code class="o">.</code><code class="na">black</code><code class="o">;</code>
<code class="n">rgbCube</code><code class="o">[</code><code class="mi">255</code><code class="o">][</code><code class="mi">255</code><code class="o">][</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="n">Color</code><code class="o">.</code><code class="na">yellow</code><code class="o">;</code>
<code class="o">...</code></pre><p>We can specify a partial index of a multidimensional array to get
a subarray of array type objects with fewer dimensions. In our example,
the variable <code class="literal">chessBoard</code> is of type
<code class="literal">ChessPiece[][]</code>. The expression
<code class="literal">chessBoard[0]</code> is valid and refers to
the first element of <code class="literal">chessBoard</code>,
which, in Java, is of type <code class="literal">ChessPiece[]</code>. For example, we can populate our
chess board one row at a time:</p><a id="I_4_tt217"/><pre class="programlisting"> <code class="n">ChessPiece</code> <code class="o">[]</code> <code class="n">homeRow</code> <code class="o">=</code> <code class="o">{</code>
<code class="k">new</code> <code class="nf">ChessPiece</code><code class="o">(</code><code class="s">"Rook"</code><code class="o">),</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">(</code><code class="s">"Knight"</code><code class="o">),</code>
<code class="k">new</code> <code class="nf">ChessPiece</code><code class="o">(</code><code class="s">"Bishop"</code><code class="o">),</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">(</code><code class="s">"King"</code><code class="o">),</code>
<code class="k">new</code> <code class="nf">ChessPiece</code><code class="o">(</code><code class="s">"Queen"</code><code class="o">),</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">(</code><code class="s">"Bishop"</code><code class="o">),</code>
<code class="k">new</code> <code class="nf">ChessPiece</code><code class="o">(</code><code class="s">"Knight"</code><code class="o">),</code> <code class="k">new</code> <code class="n">ChessPiece</code><code class="o">(</code><code class="s">"Rook"</code><code class="o">)</code>
<code class="o">};</code>
<code class="n">chessBoard</code><code class="o">[</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="n">homeRow</code><code class="o">;</code></pre><p>We don’t necessarily have to specify the dimension sizes of a
multidimensional array with a single <a id="I_indexterm4_id688293" class="indexterm"/><code class="literal">new</code> operation. The
syntax of the <code class="literal">new</code> operator lets us
leave the sizes of some dimensions unspecified. The size of at least the
first dimension (the most significant dimension of the array) has to be
specified, but the sizes of any number of trailing, less significant
array dimensions may be left undefined. We can assign appropriate
array-type values later.</p><p>We can create a checkerboard of Boolean values (which is not quite
sufficient for a real game of checkers either) using this
technique:</p><a id="I_4_tt218"/><pre class="programlisting"> <code class="kt">boolean</code> <code class="o">[][]</code> <code class="n">checkerBoard</code><code class="o">;</code>
<code class="n">checkerBoard</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">8</code><code class="o">][];</code></pre><p>Here, <code class="literal">checkerBoard</code> is declared
and created, but its elements, the eight <code class="literal">boolean[]</code> objects of the next level, are left
empty. Thus, for example, <code class="literal">checkerBoard[0]</code> is <code class="literal">null</code> until we explicitly create an array and
assign it, as follows:</p><a id="I_4_tt219"/><pre class="programlisting"> <code class="n">checkerBoard</code><code class="o">[</code><code class="mi">0</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">8</code><code class="o">];</code>
<code class="n">checkerBoard</code><code class="o">[</code><code class="mi">1</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">8</code><code class="o">];</code>
<code class="o">...</code>
<code class="n">checkerBoard</code><code class="o">[</code><code class="mi">7</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">8</code><code class="o">];</code></pre><p>The code of the previous two examples is equivalent to:</p><a id="I_4_tt220"/><pre class="programlisting"> <code class="kt">boolean</code> <code class="o">[][]</code> <code class="n">checkerBoard</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">8</code><code class="o">][</code><code class="mi">8</code><code class="o">];</code></pre><p>One reason we might want to leave dimensions of an array
unspecified is so that we can store arrays given to us by another
method.</p><p>Note that because the length of the array is not part of its type,
the arrays in the checkerboard do not necessarily have to be of the same
length; that is, multidimensional arrays don’t have to be rectangular.
Here’s a defective (but perfectly legal in Java) checkerboard:</p><a id="I_4_tt221"/><pre class="programlisting"> <code class="n">checkerBoard</code><code class="o">[</code><code class="mi">2</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">3</code><code class="o">];</code>
<code class="n">checkerBoard</code><code class="o">[</code><code class="mi">3</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">boolean</code> <code class="o">[</code><code class="mi">10</code><code class="o">];</code></pre><p>And here’s how you could create and initialize a triangular
array:<a id="I_indexterm4_id688401" class="indexterm"/><a id="I_indexterm4_id688408" class="indexterm"/><a id="I_indexterm4_id688416" class="indexterm"/></p><a id="I_4_tt222"/><pre class="programlisting"> <code class="kt">int</code> <code class="o">[][]</code> <code class="n">triangle</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">int</code> <code class="o">[</code><code class="mi">5</code><code class="o">][];</code>
<code class="k">for</code> <code class="o">(</code><code class="kt">int</code> <code class="n">i</code> <code class="o">=</code> <code class="mi">0</code><code class="o">;</code> <code class="n">i</code> <code class="o"><</code> <code class="n">triangle</code><code class="o">.</code><code class="na">length</code><code class="o">;</code> <code class="n">i</code><code class="o">++)</code> <code class="o">{</code>
<code class="n">triangle</code><code class="o">[</code><code class="n">i</code><code class="o">]</code> <code class="o">=</code> <code class="k">new</code> <code class="kt">int</code> <code class="o">[</code><code class="n">i</code> <code class="o">+</code> <code class="mi">1</code><code class="o">];</code>
<code class="k">for</code> <code class="o">(</code><code class="kt">int</code> <code class="n">j</code> <code class="o">=</code> <code class="mi">0</code><code class="o">;</code> <code class="n">j</code> <code class="o"><</code> <code class="n">i</code> <code class="o">+</code> <code class="mi">1</code><code class="o">;</code> <code class="n">j</code><code class="o">++)</code>
<code class="n">triangle</code><code class="o">[</code><code class="n">i</code><code class="o">][</code><code class="n">j</code><code class="o">]</code> <code class="o">=</code> <code class="n">i</code> <code class="o">+</code> <code class="n">j</code><code class="o">;</code>
<code class="o">}</code></pre></div><div class="sect2" title="Inside Arrays"><div class="titlepage"><div><div><h2 class="title"><a id="learnjava3-CHP-4-SECT-7.6"/>Inside Arrays</h2></div></div></div><p>We said earlier that arrays are instances of special array classes
in the Java language. If arrays have classes, where do they fit into the
class hierarchy and how are they related? These are good questions, but
we need to talk more about the object-oriented aspects of Java before
answering them. That’s the subject of the next chapter. For now, take it
on faith that arrays fit into the class hierarchy.</p></div><div class="footnotes"><br/><hr/><div class="footnote"><p><sup>[<a id="ftn.learnjava3-CHP-4-FNOTE-5" href="#learnjava3-CHP-4-FNOTE-5" class="para">11</a>] </sup>The analog in C or C++ is an array of pointers to objects.
However, pointers in C or C++ are themselves two- or four-byte
values. Allocating an array of pointers is, in actuality, allocating
the storage for some number of those pointer objects. An array of
references is conceptually similar, although references are not
themselves objects. We can’t manipulate references or parts of
references other than by assignment, and their storage requirements
(or lack thereof) are not part of the high-level Java language
specification.</p></div></div></div></body></html>