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When an overridden method is called by a reference, Java determines which version of that method to execute based on the type of object it refer to. This is also know as runtime polymorphism.</p> <p>We will see this through an example.</p> <h2 id="dynamic-method-dispatch-example-code"><a href="#dynamic-method-dispatch-example-code" class="header-anchor">#</a> Dynamic Method Dispatch - Example Code</h2> <blockquote></blockquote> <p>Abstract Class :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">base</span><span class="token punctuation">;</span>
<span class="token comment">/*
Abstract classes cannot be instantiated, but they can be subclassed
*/</span>
<span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">ClsVirusScanner</span> <span class="token punctuation">{</span>
<span class="token comment">//With One Abstract method</span>
<span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">fnStartScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">fnCheckForUpdateVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Perform Virus Scanner Version Check"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">fnBootTimeScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Perform BootTime Scan"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">fnInternetSecutiry</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Scan for Internet Security"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">fnRealTimeScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Perform RealTime Scan"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">fnVirusMalwareScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Detect Virus & Malware"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><blockquote></blockquote> <p>Overriding Abstract Method in Child Class :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">import</span> <span class="token namespace">base<span class="token punctuation">.</span></span><span class="token class-name">ClsVirusScanner</span><span class="token punctuation">;</span>
<span class="token comment">//All the 3 child classes inherits the base class ClsVirusScanner</span>
<span class="token comment">//Child Class 1</span>
<span class="token keyword">class</span> <span class="token class-name">ClsPaidVersion</span> <span class="token keyword">extends</span> <span class="token class-name">ClsVirusScanner</span><span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">fnStartScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnCheckForUpdateVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnBootTimeScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnInternetSecutiry</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnRealTimeScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnVirusMalwareScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">//ClsPaidVersion IS-A ClsVirusScanner</span>
<span class="token comment">//Child Class 2</span>
<span class="token keyword">class</span> <span class="token class-name">ClsTrialVersion</span> <span class="token keyword">extends</span> <span class="token class-name">ClsVirusScanner</span><span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">fnStartScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnInternetSecutiry</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnVirusMalwareScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">//ClsTrialVersion IS-A ClsVirusScanner</span>
<span class="token comment">//Child Class 3</span>
<span class="token keyword">class</span> <span class="token class-name">ClsFreeVersion</span> <span class="token keyword">extends</span> <span class="token class-name">ClsVirusScanner</span><span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">fnStartScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">fnVirusMalwareScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span> <span class="token comment">//ClsTrialVersion IS-A ClsVirusScanner </span>
</code></pre></div><blockquote></blockquote> <p>Dynamic/Late Binding leads to Dynamic method dispatch :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token comment">//Calling Class</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ClsRunTheApplication</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">final</span> <span class="token class-name">String</span> VIRUS_SCANNER_VERSION <span class="token operator">=</span> <span class="token string">"TRIAL_VERSION"</span><span class="token punctuation">;</span>
<span class="token comment">//Parent Refers Null</span>
<span class="token class-name">ClsVirusScanner</span> objVS<span class="token operator">=</span><span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token comment">//String Cases Supported from Java SE 7</span>
<span class="token keyword">switch</span> <span class="token punctuation">(</span>VIRUS_SCANNER_VERSION<span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token keyword">case</span> <span class="token string">"FREE_VERSION"</span><span class="token operator">:</span>
<span class="token comment">//Parent Refers Child Object 3</span>
<span class="token comment">//ClsFreeVersion IS-A ClsVirusScanner</span>
objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsFreeVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Dynamic or Runtime Binding</span>
<span class="token keyword">break</span><span class="token punctuation">;</span>
<span class="token keyword">case</span> <span class="token string">"PAID_VERSION"</span><span class="token operator">:</span>
<span class="token comment">//Parent Refers Child Object 1</span>
<span class="token comment">//ClsPaidVersion IS-A ClsVirusScanner</span>
objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsPaidVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Dynamic or Runtime Binding</span>
<span class="token keyword">break</span><span class="token punctuation">;</span>
<span class="token keyword">case</span> <span class="token string">"TRIAL_VERSION"</span><span class="token operator">:</span>
<span class="token comment">//Parent Refers Child Object 2</span>
objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsTrialVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">//Dynamic or Runtime Binding</span>
<span class="token keyword">break</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">//Method fnStartScan() is the Version of ClsTrialVersion()</span>
objVS<span class="token punctuation">.</span><span class="token function">fnStartScan</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><blockquote></blockquote> <p>Result :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Scan</span> <span class="token keyword">for</span> <span class="token class-name">Internet</span> <span class="token class-name">Security</span>
<span class="token class-name">Detect</span> <span class="token class-name">Virus</span> <span class="token operator">&</span> <span class="token class-name">Malware</span>
</code></pre></div><blockquote></blockquote> <p>Upcasting :</p> <div class="language-java extra-class"><pre class="language-java"><code>objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsFreeVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsPaidVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
objVS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClsTrialVersion</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
</code></pre></div><h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <ul><li>Dynamic Binding = Late binding</li> <li>Abstract classes cannot be instantiated, but they can be sub-classed (Base for a child class)</li> <li>An abstract method is a method that is declared without an implementation</li> <li>Abstract class may contain a mix of methods declared with or without an implementation</li> <li>When an abstract class is sub-classed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract</li> <li>Dynamic method dispatch is a mechanism by which a call to an overridden method is resolved at runtime. This is how java implements runtime polymorphism.</li> <li>Upcasting : Casting a subtype to a supertype, upward to the inheritance tree.</li> <li>Runtime Polymorphism = Dynamic Polymorphism</li></ul></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/dynamic-method-dispatch.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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