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copied!<blockquote> <p><em>Why do I get the error with "this" but not with "TestFilter"?</em></p> </blockquote> <ul> <li><p><code>this</code> is used to refer to "instance" attributes or method ( among others ). Instance means a new object exist and each object ( instance ) have a copy of the given attribute.</p></li> <li><p>The <code>class name</code> ( in your case <code>TestFilter</code> ) is used to refer to "class" attributes or methods ( those who do not require an instance to extist.</p></li> </ul> <p>So, in your first line you're declaring <code>filterFiles</code> as a class attribute ( you don't require an instance for that.</p> <p>See:</p> <pre><code>private static final FileFilter filterFiles; </code></pre> <p>This means, you declare <strong>class</strong> attribute named: <code>filterFiles</code> of type <code>FileFilter</code> which is <code>private</code> and whose reference can't be changed ( because it is <code>final</code>).</p> <p>Since it is a <strong>class</strong> attribute you may access it in the <code>main</code> method ( which is a class level method ). This both will work:</p> <pre><code>for(File f : file.listFiles(TestFilter.filterFiles)){ </code></pre> <p>and </p> <pre><code>for(File f : file.listFiles(filterFiles)){ </code></pre> <p>But </p> <pre><code>for(File f : file.listFiles(this.filterFiles)){ </code></pre> <p>Won't, because <code>this</code> refers to the current instance, but since you're in a class level method ( main ) there is no instance, so, there is no <code>this</code> or in compiler words: <em>non-static variable this cannot be referenced from a static context</em></p> <p>Instance attributes are unique per instance. Class level attribute are unique per class. </p> <p>Consider the following class:</p> <pre><code>import static java.lang.System.out; class Employee { // class level counter. It exist regardless of the instances created. public static int employeeCount = 0; // instance attribute. Each one is different from others instances private String employeeName; // Class level method, can be invoked without instance. public static Employee createEmployee( String withName ) { Employee e = new Employee(); // Set the name to the instance e.employeeName = withName; // Increments the class counter Employee.employeeCount++; return e; } // Object constructor. public Employee() { out.println("Constructor invoked!!! A new object has born, yeah!"); } // Instance method "toString()" public String toString() { // Uses "this" to refer instance level method return this.emploeeName; } // Test public static void main( String [] args ) { // The counter already exist out.printf("Employees created %d%n", Employee.employeeCount ); // Create employee a Employee a = Employee.createEmployee("Oscar"); // Its name now exists out.printf("Employee name: %s %nEmployees created %d%n", a.employeeName, Employee.employeeCount ); // Create employee b with a new name Employee b = Employee.createEmployee("HH"); out.printf("Employee name: %s %nEmployees created %d%n", b.employeeName, Employee.employeeCount ); // Now both employees exist, each one with a name out.printf("a=%s, b=%s%n, a, b );// invoke toString method which in turn uses "this" } } </code></pre> <p>I hope this sample make everything clear.</p>

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