BAEL-4391: Check if a file exists in Java (#9748)

* Check if a file exists in Java

* Simple Assertions

core-java-modules/core-java-io-3/.gitignore

@@ -0,0 +1,2 @@
+test-link*
+0.*

core-java-modules/core-java-io-3/src/test/java/com/baeldung/existence/ExistenceUnitTest.java

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+package com.baeldung.existence;
+
+import org.junit.Test;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.LinkOption;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.concurrent.ThreadLocalRandom;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class ExistenceUnitTest {
+
+    @Test
+    public void givenFile_whenDoesNotExist_thenFilesReturnsFalse() {
+        Path path = Paths.get("does-not-exist.txt");
+
+        assertFalse(Files.exists(path));
+        assertTrue(Files.notExists(path));
+    }
+
+    @Test
+    public void givenFile_whenExists_thenFilesShouldReturnTrue() throws IOException {
+        Path tempFile = Files.createTempFile("baeldung", "exist-nio");
+        assertTrue(Files.exists(tempFile));
+        assertFalse(Files.notExists(tempFile));
+
+        Path tempDirectory = Files.createTempDirectory("baeldung-exists");
+        assertTrue(Files.exists(tempDirectory));
+        assertFalse(Files.notExists(tempDirectory));
+
+        assertTrue(Files.isDirectory(tempDirectory));
+        assertFalse(Files.isDirectory(tempFile));
+        assertTrue(Files.isRegularFile(tempFile));
+
+        assertTrue(Files.isReadable(tempFile));
+
+        Files.deleteIfExists(tempFile);
+        Files.deleteIfExists(tempDirectory);
+    }
+
+    @Test
+    public void givenSymbolicLink_whenTargetDoesNotExists_thenFollowOrNotBasedOnTheOptions() throws IOException {
+        Path target = Files.createTempFile("baeldung", "target");
+        Path symbol = Paths.get("test-link-" + ThreadLocalRandom.current().nextInt());
+
+        Path symbolicLink = Files.createSymbolicLink(symbol, target);
+        assertTrue(Files.exists(symbolicLink));
+        assertTrue(Files.isSymbolicLink(symbolicLink));
+        assertFalse(Files.isSymbolicLink(target));
+
+        Files.deleteIfExists(target);
+        assertFalse(Files.exists(symbolicLink));
+        assertTrue(Files.exists(symbolicLink, LinkOption.NOFOLLOW_LINKS));
+
+        Files.deleteIfExists(symbolicLink);
+    }
+
+    @Test
+    public void givenFile_whenDoesNotExist_thenFileReturnsFalse() {
+        assertFalse(new File("invalid").exists());
+        assertFalse(new File("invalid").isFile());
+    }
+
+    @Test
+    public void givenFile_whenExist_thenShouldReturnTrue() throws IOException {
+        Path tempFilePath = Files.createTempFile("baeldung", "exist-io");
+        Path tempDirectoryPath = Files.createTempDirectory("baeldung-exists-io");
+
+        File tempFile = new File(tempFilePath.toString());
+        File tempDirectory = new File(tempDirectoryPath.toString());
+
+        assertTrue(tempFile.exists());
+        assertTrue(tempDirectory.exists());
+
+        assertTrue(tempFile.isFile());
+        assertFalse(tempDirectory.isFile());
+
+        assertTrue(tempDirectory.isDirectory());
+        assertFalse(tempFile.isDirectory());
+
+        assertTrue(tempFile.canRead());
+
+        Files.deleteIfExists(tempFilePath);
+        Files.deleteIfExists(tempDirectoryPath);
+    }
+}