Allow .named() to have a varargs/format construction.

Fixes github issue #218

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130320000

core/src/main/java/com/google/common/truth/AbstractArraySubject.java

@@ -57,11 +57,6 @@ public void hasLength(int length) {
     }
   }
 
-  @Override
-  public S named(String name) {
-    return (S) super.named(name);
-  }
-
   abstract String underlyingType();
 
   /** Returns a List representation suitable for displaying in a string. */

core/src/main/java/com/google/common/truth/MultimapSubject.java

@@ -50,13 +50,9 @@ public class MultimapSubject extends Subject<MultimapSubject, Multimap<?, ?>> {
     super(failureStrategy, multimap);
   }
 
-  /**
-   * Renames the subject so that this name appears in the error messages in place of string
-   * representations of the subject.
-   */
   @Override
-  public MultimapSubject named(String name) {
-    super.named(name);
+  public MultimapSubject named(String format, Object... args) {
+    super.named(format, args);
     return this;
   }
 

core/src/main/java/com/google/common/truth/MultisetSubject.java

@@ -31,13 +31,9 @@ public final class MultisetSubject extends IterableSubject {
     super(failureStrategy, multiset);
   }
 
-  /**
-   * Renames the subject so that this name appears in the error messages in place of string
-   * representations of the subject.
-   */
   @Override
-  public MultisetSubject named(String name) {
-    super.named(name);
+  public MultisetSubject named(String format, Object... args) {
+    super.named(format, args);
     return this;
   }
 

core/src/main/java/com/google/common/truth/Subject.java

@@ -49,16 +49,22 @@ protected String internalCustomName() {
   }
 
   /**
-   * Adds a prefix to the subject, when it is displayed in error messages.  This is especially
-   * useful in the context of types that have no helpful {@code toString()} representation,
-   * e.g. boolean.  Writing {@code assertThat(foo).named("foo").isTrue();} then results in a
-   * more reasonable error message.
+   * Adds a prefix to the subject, when it is displayed in error messages. This is especially useful
+   * in the context of types that have no helpful {@code toString()} representation, e.g. boolean.
+   * Writing {@code assertThat(foo).named("foo").isTrue();} then results in a more reasonable error
+   * message.
+   *
+   * <p>{@code named()} takes a format template and argument objects which will be substituted into
+   * the template, similar to {@link String#format(String, Object...)}, the chief difference being
+   * that extra parameters (for which there are no template variables) will be appended to the
+   * resulting string in brackets. Additionally, this only supports the {@code %s} template variable
+   * type.
    */
   @SuppressWarnings("unchecked")
   @CanIgnoreReturnValue
-  public S named(String name) {
-    // TODO: use check().withFailureMessage... here?
-    this.customName = checkNotNull(name, "Name passed to named() cannot be null.");
+  public S named(String format, Object... args) {
+    checkNotNull(format, "Name passed to named() cannot be null.");
+    this.customName = StringUtil.format(format, args);
     return (S) this;
   }