Fix NPE in MapSubject.containsExactly et al when a key is present wit…

…h the wrong value, and the actual or expected value is null.

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201697457

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

@@ -348,7 +348,8 @@ private static class ValueDifference<A, E> {
       new Function<ValueDifference<Object, Object>, String>() {
         @Override
         public String apply(ValueDifference<Object, Object> values) {
-          boolean includeTypes = values.actual.toString().equals(values.expected.toString());
+          boolean includeTypes =
+              String.valueOf(values.actual).equals(String.valueOf(values.expected));
           return lenientFormat(
               "(expected %s but got %s)",
               includeTypes ? new TypedToStringWrapper(values.expected) : values.expected,

core/src/test/java/com/google/common/truth/MapSubjectTest.java

@@ -192,6 +192,19 @@ public void containsExactlyWrongValue() {
                 + "{march=(expected 33 but got 3)}");
   }
 
+  @Test
+  public void containsExactlyWrongValueWithNull() {
+    // Test for https://github.com/google/truth/issues/468
+    ImmutableMap<String, Integer> actual = ImmutableMap.of("jan", 1, "feb", 2, "march", 3);
+    expectFailureWhenTestingThat(actual).containsExactly("jan", 1, "march", null, "feb", 2);
+    assertThat(expectFailure.getFailure())
+        .hasMessageThat()
+        .isEqualTo(
+            "Not true that <{jan=1, feb=2, march=3}> contains exactly "
+                + "<{jan=1, march=null, feb=2}>. It has the following entries with matching keys "
+                + "but different values: {march=(expected null but got 3)}");
+  }
+
   @Test
   public void containsExactlyExtraKeyAndMissingKey() {
     ImmutableMap<String, Integer> actual = ImmutableMap.of("jan", 1, "march", 3);