Enabled fluent String assertions for non-String objects with asString…

…() (#1307).

This is a breaking change for those that depended on asString() to
assert that actual is an instance of String.  It now only asserts that
actual is not null.  Those needing that functionality can use
`assertThat(actual).isInstanceOf(String.class).asString()`.

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

@@ -456,8 +456,8 @@ public AbstractListAssert<?, List<? extends Object>, Object, ObjectAssert<Object
   @Override
   @CheckReturnValue
   public AbstractStringAssert<?> asString() {
-    objects.assertIsInstanceOf(info, actual, String.class);
-    return Assertions.assertThat((String) actual);
+    objects.assertNotNull(info, actual);
+    return Assertions.assertThat(actual.toString());
   }
 
   /**

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

@@ -559,9 +559,8 @@ public interface Assert<SELF extends Assert<SELF, ACTUAL>, ACTUAL> extends Descr
   AbstractListAssert<?, List<? extends Object>, Object, ObjectAssert<Object>> 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.
+   * Returns a String assertion for the <code>toString()</code> of the actual
+   * value, to allow chaining of String-specific assertions from this call.
    * <p>
    * Example :
    * <pre><code class='java'> Object stringAsObject = "hello world";

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

@@ -12,7 +12,6 @@
  */
 package org.assertj.core.api;
 
-import static java.lang.String.format;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
@@ -25,16 +24,28 @@ public class Assertions_assertThat_asString_Test {
 
   @Test
   public void should_pass_string_asserts_on_string_objects_with_asString() {
-	Object stringAsObject = "hello world";
-	assertThat(stringAsObject).asString().contains("hello");
+    Object stringAsObject = "hello world";
+    assertThat(stringAsObject).asString().contains("hello");
   }
 
   @Test
-  public void should_fail_string_asserts_on_non_string_objects_even_with_asString() {
+  public void should_pass_string_asserts_on_non_string_objects_with_asString() {
     Object nonString = new Object();
+    assertThat(nonString).asString().isEqualTo(nonString.toString());
+  }
+
+  @Test
+  public void should_fail_string_asserts_on_non_string_objects_with_asString() {
+    Object nonString = new Object();
+    assertThatExceptionOfType(AssertionError.class).isThrownBy(() -> assertThat(nonString).asString()
+                                                                                          .contains("probably not this"));
+  }
 
-    assertThatExceptionOfType(AssertionError.class).isThrownBy(() -> assertThat(nonString).asString().contains("hello"))
-                                                   .withMessageContaining(format("an instance of:%n  <java.lang.String>%nbut was instance of:%n  <java.lang.Object>"));
+  @Test
+  public void should_fail_if_actual_is_null_with_asString() {
+    Object nullObject = null;
+    assertThatExceptionOfType(AssertionError.class).isThrownBy(() -> assertThat(nullObject).asString()
+                                                                                           .isEqualTo("never gonna happen"));
   }
 
 }