Fixes assertj#308 : Add asString() to switch to String assertions

Author: pimterry

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

@@ -361,6 +361,13 @@ public S isNotOfAnyClassIn(Class<?>... types) {
     return Assertions.assertThat((List<Object>) actual);
   }
 
+  /** {@inheritDoc} */
+  @Override
+  public AbstractCharSequenceAssert<?, String> asString() {
+    objects.assertIsInstanceOf(info, actual, String.class);
+    return Assertions.assertThat((String) actual);
+  }
+
   /**
    * The description of this assertion set with {@link #describedAs(String, Object...)} or {@link #describedAs(Description)}.
    * 

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

@@ -259,6 +259,23 @@ public interface Assert<S extends Assert<S, A>, A> extends Descriptable<S>, Exte
    */
   AbstractListAssert asList();
 
+  /**
+   * Verifies that the actual value is an instance of String,
+   * and returns a String assertion, to allow chaining of String-specific
+   * assertions from this call.
+   * <p>
+   * Example :
+   *
+   * <pre><code class='java'>
+   * Object stringAsObject = "hello world";
+   *
+   * assertThat(stringAsObject).asString().contains("hello");
+   * </code></pre>
+   *
+   * @return a string assertion object
+   */
+  AbstractCharSequenceAssert<?, String> asString();
+
   /**
    * @deprecated
    * Throws <code>{@link UnsupportedOperationException}</code> if called. It is easy to accidentally call

src/test/java/org/assertj/core/api/Assertions_assertThat_asString.java

@@ -0,0 +1,46 @@
+/**
+ * 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 2012-2014 the original author or authors.
+ */
+package org.assertj.core.api;
+
+import org.assertj.core.test.ExpectedException;
+import org.junit.Rule;
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.test.ExpectedException.none;
+
+/**
+ * Tests for Assert.asString() methods
+ */
+public class Assertions_assertThat_asString {
+
+    @Rule
+    public ExpectedException thrown = none();
+
+    @Test
+    public void should_pass_string_asserts_on_string_objects_with_asString() {
+        Object stringAsObject = "hello world";
+        assertThat(stringAsObject).asString().contains("hello");
+    }
+
+    @Test
+    public void should_fail_string_asserts_on_non_string_objects_even_with_asString() {
+        Object nonString = new Object();
+
+        thrown.expectAssertionError("an instance of:\n <java.lang.String>\nbut was instance of:\n <java.lang.Object>");
+        assertThat(nonString).asString().contains("hello");
+    }
+
+}