BAEL-1936 Use of FilenameFilter (#4520)

* Added tests for FilenameFilter demo

-added a test to show FilenameFilter implementation
-added another test to show similar functionality using Predicate

* refactored code to get directory at a single location

* fixing formatting

* changed test class name to conform to custom rule

UnitTestNamingConventionRule lists the allowed test class names. Added ManualTest at the end to conform to the rule.

core-java-io/src/test/java/com/baeldung/file/FilenameFilterManualTest.java

@@ -0,0 +1,48 @@
+package com.baeldung.file;
+
+import java.io.File;
+import java.io.FilenameFilter;
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class FilenameFilterManualTest {
+
+    private static File directory;
+
+    @BeforeClass
+    public static void setupClass() {
+        directory = new File(FilenameFilterManualTest.class.getClassLoader()
+            .getResource("testFolder")
+            .getFile());
+    }
+
+    @Test
+    public void whenFilteringFilesEndingWithJson_thenEqualExpectedFiles() {
+        FilenameFilter filter = (dir, name) -> name.endsWith(".json");
+
+        String[] expectedFiles = { "people.json", "students.json" };
+        String[] actualFiles = directory.list(filter);
+
+        Assert.assertArrayEquals(expectedFiles, actualFiles);
+    }
+
+    @Test
+    public void whenFilteringFilesEndingWithXml_thenEqualExpectedFiles() {
+        Predicate<String> predicate = (name) -> name.endsWith(".xml");
+
+        String[] expectedFiles = { "teachers.xml", "workers.xml" };
+        List<String> files = Arrays.stream(directory.list())
+            .filter(predicate)
+            .collect(Collectors.toList());
+        String[] actualFiles = files.toArray(new String[files.size()]);
+
+        Assert.assertArrayEquals(expectedFiles, actualFiles);
+    }
+
+}

core-java-io/src/test/resources/testFolder/people.json

@@ -0,0 +1 @@
+{}

core-java-io/src/test/resources/testFolder/students.json

@@ -0,0 +1 @@
+{}

core-java-io/src/test/resources/testFolder/teachers.xml

@@ -0,0 +1 @@
+<xml></xml>

core-java-io/src/test/resources/testFolder/workers.xml

@@ -0,0 +1 @@
+<xml></xml>