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    <p>Each Java class is associated with a <em>virtual method table</em> that contains "links" to the bytecode of each method of a class. That table is inherited from the superclass of a particular class and extended with regard to the new methods of a subclass. E.g.,</p> <pre class="lang-java prettyprint-override"><code>class BaseClass { public void method1() { } public void method2() { } public void method3() { } } class NextClass extends BaseClass { public void method2() { } // overridden from BaseClass public void method4() { } } </code></pre> <p>results in the tables</p> <pre>BaseClass 1. BaseClass/method1() 2. BaseClass/method2() 3. BaseClass/method3() NextClass 1. BaseClass/method1() 2. NextClass/method2() 3. BaseClass/method3() 4. NextClass/method4()</pre> <p>Note, how the virtual method table of <code>NextClass</code> retains the order of entries of the table of <code>BaseClass</code> and just overwrites the "link" of <code>method2()</code> which it overrides.</p> <p>An implementation of the JVM can thus optimize a call to <code>invokevirtual</code> by remembering that <code>BaseClass/method3()</code> will always be the third entry in the virtual method table of any object this method will ever be invoked on.</p> <p>With <code>invokeinterface</code> this optimization is not possible. E.g.,</p> <pre class="lang-java prettyprint-override"><code>interface MyInterface { void ifaceMethod(); } class AnotherClass extends NextClass implements MyInterface { public void method4() { } // overridden from NextClass public void ifaceMethod() { } } class MyClass implements MyInterface { public void method5() { } public void ifaceMethod() { } } </code></pre> <p>This class hierarchy results in the virtual method tables</p> <pre>AnotherClass 1. BaseClass/method1() 2. NextClass/method2() 3. BaseClass/method3() 4. AnotherClass/method4() 5. MyInterface/ifaceMethod() MyClass 1. MyClass/method5() 2. MyInterface/ifaceMethod()</pre> <p>As you can see, <code>AnotherClass</code> contains the interface's method in its fifth entry and <code>MyClass</code> contains it in its second entry. To actually find the correct entry in the virtual method table, a call to a method with <code>invokeinterface</code> will always have to search the complete table without a chance for the style of optimization that <code>invokevirtual</code> does.</p> <p>There are additional differences like the fact, that <code>invokeinterface</code> can be used together with object references that do not actually implement the interface. Therefore, <code>invokeinterface</code> will have to check at runtime whether a method exists in the table and potentially throw an exception. If you want to dive deeper into the topic, I suggest, e.g., <a href="http://www.research.ibm.com/people/d/dgrove/papers/oopsla01.html" rel="noreferrer">"Efficient Implementation of Java Interfaces: Invokeinterface Considered Harmless"</a>.</p>
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    1. POWhat is the point of invokeinterface?
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    1. POWhat is the point of invokeinterface?
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    1. CO"always have to search the complete table without a chance for the style of optimization that invokevirtual does" -- It should be noted that it doesn't *always* have to search the table, since it can do a different style of optimization. See the linked "considered harmless" paper for more details.
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      1. USBrandon Bloom
 

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