Add isNotEmpty assertion for File

Fixes #1638.

src/main/java/org/assertj/core/api/AbstractFileAssert.java

@@ -990,4 +990,55 @@ public SELF isNotEmptyDirectory() {
     files.assertIsNotEmptyDirectory(info, actual);
     return myself;
   }
+
+  /**
+   * Verify that the actual {@code File} is empty (i.e. size is equal to zero bytes).
+   * <p>
+   * Example:
+   * <pre><code class='java'> File file = File.createTempFile(&quot;tmp&quot;, &quot;txt&quot;);
+   *
+   * // assertion will pass
+   * assertThat(file).isEmpty();
+   *
+   * Files.write(file.toPath(), new byte[]{1, 1});
+   *
+   * // assertion will fail
+   * assertThat(file).isEmpty();</code></pre>
+   *
+   * @return {@code this} assertion object.
+   * @throws AssertionError if the actual {@code File} is {@code null}.
+   * @throws AssertionError if the actual {@code File} does not exist.
+   * @throws AssertionError if the actual {@code File} is not empty.
+   * @since 3.14.0
+   */
+  public SELF isEmpty() {
+    files.assertIsEmptyFile(info, actual);
+    return myself;
+  }
+
+  /**
+   * Verify that the actual {@code File} is not empty (i.e. size is greater than zero bytes).
+   * <p>
+   * Example:
+   * <pre><code class='java'> File file = File.createTempFile(&quot;tmp&quot;, &quot;txt&quot;);
+   * Files.write(file.toPath(), new byte[]{1, 1});
+   *
+   * // assertion will pass
+   * assertThat(file).isNotEmpty();
+   *
+   * file = File.createTempFile(&quot;tmp&quot;, &quot;txt&quot;);
+   *
+   * // assertion will fail
+   * assertThat(file).isNotEmpty();</code></pre>
+   *
+   * @return {@code this} assertion object.
+   * @throws AssertionError if the actual {@code File} is {@code null}.
+   * @throws AssertionError if the actual {@code File} does not exist.
+   * @throws AssertionError if the actual {@code File} is empty.
+   * @since 3.14.0
+   */
+  public SELF isNotEmpty() {
+    files.assertIsNotEmptyFile(info, actual);
+    return myself;
+  }
 }

src/main/java/org/assertj/core/error/ShouldBeEmpty.java

@@ -12,9 +12,11 @@
  */
 package org.assertj.core.error;
 
+import java.io.File;
+
 /**
  * Creates an error message indicating that an assertion that verifies a group of elements is empty failed. A group of elements
- * can be a collection, an array or a {@code String}.
+ * can be a collection, an array, {@code String} or a {@code File}.
  * 
  * @author Alex Ruiz
  */
@@ -26,10 +28,19 @@ public class ShouldBeEmpty extends BasicErrorMessageFactory {
    * @return the created {@code ErrorMessageFactory}.
    */
   public static ErrorMessageFactory shouldBeEmpty(Object actual) {
-    return new ShouldBeEmpty(actual);
+    return new ShouldBeEmpty("%nExpecting empty but was:<%s>", actual);
+  }
+
+  /**
+   * Creates a new <code>{@link ShouldBeEmpty}</code>.
+   * @param actual the actual file in the failed assertion.
+   * @return the created {@code ErrorMessageFactory}.
+   */
+  public static ErrorMessageFactory shouldBeEmpty(File actual) {
+    return new ShouldBeEmpty("%nExpecting file <%s> to be empty", actual);
   }
 
-  private ShouldBeEmpty(Object actual) {
-    super("%nExpecting empty but was:<%s>", actual);
+  private ShouldBeEmpty(String format, Object... arguments) {
+    super(format, arguments);
   }
 }

src/main/java/org/assertj/core/error/ShouldNotBeEmpty.java

@@ -12,16 +12,17 @@
  */
 package org.assertj.core.error;
 
+import java.io.File;
 
 /**
  * Creates an error message indicating that an assertion that verifies a group of elements is not empty failed. A group of
- * elements can be a collection, an array or a {@code String}.
+ * elements can be a collection, an array, {@code String} or a {@code File}.
  * 
  * @author Alex Ruiz
  */
 public class ShouldNotBeEmpty extends BasicErrorMessageFactory {
 
-  private static final ShouldNotBeEmpty INSTANCE = new ShouldNotBeEmpty();
+  private static final ShouldNotBeEmpty INSTANCE = new ShouldNotBeEmpty("%nExpecting actual not to be empty");
 
   /**
    * Returns the singleton instance of this class.
@@ -31,8 +32,16 @@ public static ErrorMessageFactory shouldNotBeEmpty() {
     return INSTANCE;
   }
 
-  private ShouldNotBeEmpty() {
-    super("%nExpecting actual not to be empty");
+   /**
+   * Creates a new <code>{@link ShouldNotBeEmpty}</code>.
+   * @param actual the actual file in the failed assertion.
+   * @return the created {@code ErrorMessageFactory}.
+   */
+  public static ErrorMessageFactory shouldNotBeEmpty(File actual) {
+    return new ShouldNotBeEmpty("%nExpecting file <%s> not to be empty", actual);
   }
 
+  private ShouldNotBeEmpty(String format, Object... arguments) {
+    super(format, arguments);
+  }
 }

src/main/java/org/assertj/core/internal/Files.java

@@ -17,6 +17,7 @@
 import static java.util.stream.Collectors.toList;
 import static org.assertj.core.error.ShouldBeAbsolutePath.shouldBeAbsolutePath;
 import static org.assertj.core.error.ShouldBeDirectory.shouldBeDirectory;
+import static org.assertj.core.error.ShouldBeEmpty.shouldBeEmpty;
 import static org.assertj.core.error.ShouldBeEmptyDirectory.shouldBeEmptyDirectory;
 import static org.assertj.core.error.ShouldBeFile.shouldBeFile;
 import static org.assertj.core.error.ShouldBeReadable.shouldBeReadable;
@@ -275,18 +276,46 @@ public void assertDoesNotExist(AssertionInfo info, File actual) {
   }
 
   /**
-  * Asserts that the given file can be modified by the application.
-  * @param info contains information about the assertion.
-  * @param actual the given file.
-  * @throws AssertionError if the given file is {@code null}.
-  * @throws AssertionError if the given file can not be modified.
-  */
+   * Asserts that the given file can be modified by the application.
+   * @param info contains information about the assertion.
+   * @param actual the given file.
+   * @throws AssertionError if the given file is {@code null}.
+   * @throws AssertionError if the given file can not be modified.
+   */
   public void assertCanWrite(AssertionInfo info, File actual) {
     assertNotNull(info, actual);
     if (actual.canWrite()) return;
     throw failures.failure(info, shouldBeWritable(actual));
   }
 
+  /**
+   * Asserts that the given {@code File} is empty (i.e. size is equal to zero bytes).
+   * @param info contains information about the assertion.
+   * @param actual the given file.
+   * @throws AssertionError if the given {@code File} is {@code null}.
+   * @throws AssertionError if the given {@code File} does not exist.
+   * @throws AssertionError if the given {@code File} is not empty.
+   */
+  public void assertIsEmptyFile(AssertionInfo info, File actual) {
+    assertIsFile(info, actual);
+    if (actual.length() == 0) return;
+    throw failures.failure(info, shouldBeEmpty(actual));
+  }
+
+  /**
+   * Asserts that the given {@code File} is not empty (i.e. size is greater than zero bytes).
+   * @param info contains information about the assertion.
+   * @param actual the given file.
+   * @throws AssertionError if the given {@code File} is {@code null}.
+   * @throws AssertionError if the given {@code File} does not exist.
+   * @throws AssertionError if the given {@code File} is empty.
+   */
+  public void assertIsNotEmptyFile(AssertionInfo info, File actual) {
+    assertIsFile(info, actual);
+    if (actual.length() > 0) return;
+    throw failures.failure(info, shouldNotBeEmpty(actual));
+  }
+
   /**
    * Asserts that the given file can be read by the application.
    * @param info contains information about the assertion.

src/test/java/org/assertj/core/api/file/FileAssert_isEmptyFile_Test.java

@@ -0,0 +1,36 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ *
+ * Copyright 2012-2019 the original author or authors.
+ */
+package org.assertj.core.api.file;
+
+import org.assertj.core.api.FileAssert;
+import org.assertj.core.api.FileAssertBaseTest;
+import org.junit.jupiter.api.DisplayName;
+
+import static org.mockito.Mockito.verify;
+
+/**
+ * Tests for <code>{@link FileAssert#isEmpty()}</code>.
+ */
+@DisplayName("FileAssert isEmptyFile")
+public class FileAssert_isEmptyFile_Test extends FileAssertBaseTest {
+
+  @Override
+  protected FileAssert invoke_api_method() {
+    return assertions.isEmpty();
+  }
+
+  @Override
+  protected void verify_internal_effects() {
+    verify(files).assertIsEmptyFile(getInfo(assertions), getActual(assertions));
+  }
+}

src/test/java/org/assertj/core/api/file/FileAssert_isNotEmptyFile_Test.java

@@ -0,0 +1,36 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ *
+ * Copyright 2012-2019 the original author or authors.
+ */
+package org.assertj.core.api.file;
+
+import org.assertj.core.api.FileAssert;
+import org.assertj.core.api.FileAssertBaseTest;
+import org.junit.jupiter.api.DisplayName;
+
+import static org.mockito.Mockito.verify;
+
+/**
+ * Tests for <code>{@link FileAssert#isNotEmpty()}</code>.
+ */
+@DisplayName("FileAssert isNotEmptyFile")
+public class FileAssert_isNotEmptyFile_Test extends FileAssertBaseTest {
+
+  @Override
+  protected FileAssert invoke_api_method() {
+    return assertions.isNotEmpty();
+  }
+
+  @Override
+  protected void verify_internal_effects() {
+    verify(files).assertIsNotEmptyFile(getInfo(assertions), getActual(assertions));
+  }
+}

src/test/java/org/assertj/core/error/ShouldBeEmpty_create_Test.java

@@ -12,34 +12,44 @@
  */
 package org.assertj.core.error;
 
-import static org.assertj.core.api.Assertions.assertThat;
+import static java.lang.String.format;
+import static org.assertj.core.api.BDDAssertions.then;
 import static org.assertj.core.error.ShouldBeEmpty.shouldBeEmpty;
+import static org.assertj.core.presentation.StandardRepresentation.STANDARD_REPRESENTATION;
 import static org.assertj.core.util.Lists.newArrayList;
 
-import org.assertj.core.description.Description;
+import java.io.File;
+
 import org.assertj.core.internal.TestDescription;
-import org.assertj.core.presentation.StandardRepresentation;
-import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
 /**
- * Tests for <code>{@link ShouldBeEmpty#create(Description, org.assertj.core.presentation.Representation)}</code>.
- * 
+ * Tests for <code>{@link ShouldBeEmpty#create((org.assertj.core.description.Description, org.assertj.core.presentation.Representation)}</code>.
+ *
  * @author Alex Ruiz
  * @author Yvonne Wang
  */
-public class ShouldBeEmpty_create_Test {
+@DisplayName("ShouldBeEmpty create")
+class ShouldBeEmpty_create_Test {
 
-  private ErrorMessageFactory factory;
-
-  @BeforeEach
-  public void setUp() {
-    factory = shouldBeEmpty(newArrayList("Luke", "Yoda"));
+  @Test
+  void should_create_error_message() {
+    // GIVEN
+    ErrorMessageFactory factory = shouldBeEmpty(newArrayList("Luke", "Yoda"));
+    // WHEN
+    String message = factory.create(new TestDescription("Test"), STANDARD_REPRESENTATION);
+    // THEN
+    then(message).isEqualTo(format("[Test] %nExpecting empty but was:<[\"Luke\", \"Yoda\"]>"));
   }
 
   @Test
-  public void should_create_error_message() {
-    String message = factory.create(new TestDescription("Test"), new StandardRepresentation());
-    assertThat(message).isEqualTo(String.format("[Test] %nExpecting empty but was:<[\"Luke\", \"Yoda\"]>"));
+  void should_create_error_message_for_File() {
+    // GIVEN
+    ErrorMessageFactory factory = shouldBeEmpty(new File("/te%st.txt"));
+    // WHEN
+    String message = factory.create(new TestDescription("Test"), STANDARD_REPRESENTATION);
+    // THEN
+    then(message).isEqualTo(format("[Test] %nExpecting file </te%%st.txt> to be empty"));
   }
 }

src/test/java/org/assertj/core/error/ShouldNotBeEmpty_create_Test.java

@@ -12,32 +12,44 @@
  */
 package org.assertj.core.error;
 
-import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.BDDAssertions.then;
 import static org.assertj.core.error.ShouldNotBeEmpty.shouldNotBeEmpty;
 
 import org.assertj.core.internal.TestDescription;
 import org.assertj.core.presentation.StandardRepresentation;
-import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
+import java.io.File;
+
 /**
  * Tests for <code>{@link ShouldNotBeEmpty#create(org.assertj.core.description.Description, org.assertj.core.presentation.Representation)}</code>.
- * 
+ *
  * @author Alex Ruiz
  * @author Yvonne Wang
  */
-public class ShouldNotBeEmpty_create_Test {
+@DisplayName("ShouldNotBeEmpty create")
+class ShouldNotBeEmpty_create_Test {
 
   private ErrorMessageFactory factory;
 
-  @BeforeEach
-  public void setUp() {
+  @Test
+  void should_create_error_message() {
+    // GIVEN
     factory = shouldNotBeEmpty();
+    // WHEN
+    String message = factory.create(new TestDescription("Test"), new StandardRepresentation());
+    // THEN
+    then(message).isEqualTo(String.format("[Test] %nExpecting actual not to be empty"));
   }
 
   @Test
-  public void should_create_error_message() {
+  void should_create_error_message_for_File() {
+    // GIVEN
+    factory = shouldNotBeEmpty(new File("/test.txt"));
+    // WHEN
     String message = factory.create(new TestDescription("Test"), new StandardRepresentation());
-    assertThat(message).isEqualTo(String.format("[Test] %nExpecting actual not to be empty"));
+    // THEN
+    then(message).isEqualTo(String.format("[Test] %nExpecting file </test.txt> not to be empty"));
   }
 }