Hash code

Doc/Sd1/coreClasses.xml

@@ -2,6 +2,7 @@
 <chapter annotations="slide" version="5.1" xml:id="sw1ChapterCoreClasses"
          xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"
+         xmlns:xila="http://www.w3.org/2001/XInclude/local-attributes"
          xmlns:xi="http://www.w3.org/2001/XInclude"
          xmlns:svg="http://www.w3.org/2000/svg"
          xmlns:ns="http://docbook.org/ns/transclusion"
@@ -75,13 +76,131 @@
 String s2 = new String("Kate");
 
 System.out.println("s1 == s2: " + (s1 == s2));
-System.out.println("s1.equals(s2): " + s1.equals(s2));
-</programlisting>
+System.out.println("s1.equals(s2): " + s1.equals(s2));</programlisting>
 
     <screen>s1 == s2: false
 s1.equals(s2): true</screen>
   </figure>
 
+  <figure xml:id="sd1_coreclasses_fig_hashPrinciple">
+    <title>Hashing principle</title>
+
+    <informaltable border="0">
+      <tr>
+        <td valign="top"><mediaobject>
+            <imageobject>
+              <imagedata fileref="Ref/CoreClasses/Hash/iceCreamPrices.jpg"/>
+            </imageobject>
+          </mediaobject></td>
+
+        <td valign="middle"><quote>I want the 12p one</quote></td>
+      </tr>
+    </informaltable>
+  </figure>
+
+  <figure xml:id="sd1_coreclasses_fig_hashExample">
+    <title><classname>Rectangle</classname>
+    <methodname>equals(...)</methodname> and
+    <methodname>hashCode()</methodname></title>
+
+    <programlisting language="java">public class Rectangle {
+    int width, height;
+    @Override public boolean equals(Object o) {
+        if (o instanceof  Rectangle) {
+            return width == ((Rectangle) o).width
+                    &amp;&amp; height == ((Rectangle) o).height;
+        } else {
+            return false;
+        }
+    }
+    @Override public int hashCode() {
+        return width + height;
+    }
+}</programlisting>
+  </figure>
+
+  <figure xml:id="sd1_coreclasses_fig_hashRectangleValues">
+    <title><classname>Rectangle</classname> hash values</title>
+
+    <informaltable border="1">
+      <tr>
+        <th>width</th>
+
+        <th>height</th>
+
+        <th>hash value</th>
+      </tr>
+
+      <tr>
+        <td valign="top">1</td>
+
+        <td valign="top">3</td>
+
+        <td valign="top">4</td>
+      </tr>
+
+      <tr>
+        <td valign="top">2</td>
+
+        <td valign="top">2</td>
+
+        <td valign="top">4</td>
+      </tr>
+
+      <tr>
+        <td valign="top">5</td>
+
+        <td valign="top">5</td>
+
+        <td valign="top">10</td>
+      </tr>
+
+      <tr>
+        <td valign="top">2</td>
+
+        <td valign="top">7</td>
+
+        <td valign="top">9</td>
+      </tr>
+
+      <tr>
+        <td valign="top">4</td>
+
+        <td valign="top">9</td>
+
+        <td valign="top">13</td>
+      </tr>
+    </informaltable>
+  </figure>
+
+  <figure xml:id="sd1_coreclasses_fig_hashRectangleBetter">
+    <title><classname>Rectangle</classname>
+    <methodname>equals(...)</methodname> and
+    <methodname>hashCode()</methodname></title>
+
+    <programlisting language="java">public class Rectangle {
+    int width, height;
+...
+    @Override public int hashCode() {
+        return width + <emphasis role="red">13 * height</emphasis>;
+    }
+}</programlisting>
+  </figure>
+
+  <figure xml:id="sd1_coreclasses_fig_hashEqualsContract">
+    <title><classname>Contract
+    </classname><methodname>equals(...)</methodname> and
+    <methodname>hashCode()</methodname></title>
+
+    <para>If <methodname>a.equals(b)</methodname> ==&gt; <code>a.hashCode() ==
+    b.hashCode()</code>. <emphasis role="red">The reverse does not
+    apply!</emphasis></para>
+
+    <para>Consequence: <methodname>equals()</methodname> and
+    <methodname>hashCode()</methodname> must be <emphasis role="red">redefined
+    simultaneously</emphasis>!</para>
+  </figure>
+
   <section xml:id="sd1SectVarargsFormat">
     <title>Reconsidering System.out.format().</title>